1H and 13C-NMR chemical shifts (ppm) observed and calculated for the repeating unit and the aliphatic end group polyester PTOBDME-choline.
We report the synthesis of six multifunctional cationic cholesteric liquid crystals polyesters functionalized with choline, amine, and amide groups to obtain new chemical formulations involving macromolecular features with new properties added to those of precursor chiral cholesteric polyesters. They are designed as PTOBDME-choline [(C34H36O8)n─C5H13N]; PTOBEE-choline [(C26H20O8)n─C5H13N]; PTOBDME-ammonium [(C34H36O8)n─C5H13N]; PTOBEE-ammonium [(C26H20O8)n─C5H13N]; PTOBUME-amide (C33H33O9N)n; and PTOBEE-amide (C26H19O9N)n. Structural characterization is performed by NMR. Thermal behavior is studied by thermogravimetry (TG) and differential scanning calorimetry (DSC), showing all the polymers endothermic transition from crystal phase to liquid crystal mesophase. Chirality is determined by optical rotatory dispersion (ORD). The cationic cholesteric liquid crystal polymers described here have proved to act as nonviral vectors in gene therapy, transfecting DNA to the nucleus cell.
- cholesteric LC
- cationic polymers
- chiral polyesters
Cholesteric liquid crystal polyesters have received much attention in the last few years for their interesting chemical, optical, mechanical, and biological properties. Due to their anisotropic formulation and amphiphilic nature, their molecules are able to self-associate and/or aggregate in blocks to form species with supramolecular ordered structure, which presents desirable material properties.
Two cholesteric liquid crystal polyesters, named PTOBDME and PTOBEE in Figure 1, were obtained by polycondensation reaction. Although only racemic materials were used in their synthesis, a cholesteric, chiral morphology, theoretically unexpected, was found. Evidence of this was obtained when a white solid, recrystallized, as the second fraction, from toluene mother liquor after the filtration of the polymer, was identified as ─PTOBDME, with [α]25589 = −1.43 (1.538g/100 ml, toluene) [1, 2] and ─PTOBEE, with a value of [α]25589 = −2.33 (0.0056 mol/l, toluene) , respectively. The synthetic method , based on the previously reported by Bilibin , leads to obtain two or more fractions of different kinetic rates, with different enantiomeric excess. Not always, the enantiomer in excess is the same.
We are interested in the molecular design and chemical modifications of these multifunctional cholesteric liquid crystals to obtain new chemical formulations involving macromolecular features with new properties added. Our main interest being to introduce cationic charge, hence favoring the creation of hydrogen bonds, through intra and intermolecular interactions, giving secondary structures with long-range supramolecular order, and enabling to interact with molecules of interest, such as biological molecules (lipids, DNA, and oligonucleotides) and metal surfaces. The functional groups selected to be introduced at the end of the main chains were Choline [─CH2─CH2─N─(CH3)3] and ammonium [─CH2─CH2─CH2─NH─(CH3)2] and amide groups (─CONH2) at the end of the lateral hydrophobic chains.
The new synthetized cationic polymers reported here have proved to be able to interact with negatively charged DNA, forming polyplexes, which are able to condense and successfully transfect the new DNA into the nucleus cell, protecting it from damage during the transfection process, acting as nonviral vectors in Gene Therapy [6, 7]. Besides, they are sensitive to pH changes, acting as polycationic efficient transfection agents possessing substantial buffering capacity below physiological pH. These vectors have shown to deliver genes as well as oligonucleotides, both
The new cholesteric liquid crystal polymers so designed have been synthesized as follows: PTOBDME-choline [(C34H36O8)n─C5H13N]; PTOBEE-choline [(C26H20O8)n─C5H13N]; PTOBDME-ammonium [(C34H36O8)n─C5H13N]; PTOBEE-ammonium [(C26H20O8)n─C5H13N]; PTOBUME-amide [(C33H33O9N)n; and PTOBEE-amide (C26H19O9N)n.
2.1. Synthesis of cholesteric PTOBDME-choline [(C34H36O8)n─C5H13N]
2.2. Synthesis of cholesteric PTOBEE-choline [(C26H20O8)n─C5H13N]
The structure of Poly[oxy(1,2-butane)oxycarbonyl-1,4-phenylene-oxy-1,4-terephthaloyl-oxy-1,4-phenylene-carbonyl]-oxy-
2.2.1. Preparation of PTOBDME-choline and PTOBEE-choline
The dichloride, TOBC, was obtained by reaction between thionyl chloride and 4,4′-(terephthaloyldioxydibenzoic) acid (TOBA), previously synthesized from terephthaloyl chloride and 4-hydroxybenzoic acid .
The polycondensation reaction between TOBC and the racemic mixture, the corresponding glycol, takes place in presence of 1/7 equimolecular choline chloride. The preparation of these compounds was performed on melting due to the insolubility of choline chloride in the solvents used in the synthesis of PTOBDME or PTOBEE precursors, diphenyl oxide, or chloronaphthalene.
A mixture of 0.0054 mol of the glycol, either DL-1,2-dodecanediol or DL-1,2-butanediol, from Flucka Chemie GmBH (Buchs, Switzerland) and 0.000775 mol of choline chloride from Sigma-Aldrich Chemie GmBH (Steinheim, Germany) were placed into a flask of 50 ml contained in a bath with a high-temperature transfer agent, while a current of dry nitrogen from Praxair (Madrid, Spain) was used to purge the system at room temperature and then maintained in the rest of the reaction. The mixture was stirred and heated to 110°C to whole dissolution of the choline chloride into diol. The bath was cooled to 80°C, and 0.0062 mol of TOBC was added; this temperature was maintained for 15 minutes. The bath was heated up to 190°C, the mixture was melted, and emission of HCl was observed. After 60 minutes, 15 ml of chloronaphthalene from Sigma-Aldrich Chemie GmBH (Steinheim, Germany) was added. The reaction mix was maintained into the solvent stirring at 190°C for 150 minutes. Then, it was poured into 150 ml of toluene from Merck KGaA (Darmstadt, Germany), decanting PTOBDME-choline or PTOBEE-choline, respectively, which was filtered, washed with ethanol, and vacuum dried.
2.3. Synthesis of cholesteric PTOBDME-ammonium [(C34H36O8)n─C5H13N]
The structure of Poly[oxy (1,2-dodecane)-oxy-carbonyl-1,4-phenylene-oxy-1,4-terephthaloyl-oxy-1,4-phenylene-carbonyl]-oxy-3-dimethyl amine-1-propyl choride is shown in
2.4. Synthesis of Cholesteric PTOBEE-ammonium [(C26H20O8)n─C5H13N]
The structure of Poly[oxy(1,2-butane)oxycarbonyl-1,4-phenylene-oxy-1,4-terephthaloyl -oxy-1,4-phenylene-carbonyl]-oxy-3-dimethylamine-1-propyl choride is shown in
2.4.1. Preparation of PTOBMDE-ammonium and PTOBEE-ammonium
Into a flask of 50 ml, TOBC (0.0079 mol) and 1,2-dodecanediol (0.0079 mol) from Flucka Chemie GmBH (Buchs, Switzerland) and diphenyl oxide (19.7 ml) of from Sigma-Aldrich Chemie GmBH (Steinheim, Germany) were mixed, while the system was purged with stream of dry nitrogen from Praxair (Madrid, Spain), for 30 min at room temperature. Then, while maintaining the gas current, the flask was transferred to a bath at 200°C for 2 hours; since the liberation of HCl is still observed, the temperature of the bath was descended to 160°C, the polycondensation was stopped and was not observed HCl formation. 3-Dimethylamino-1-propanol (0.2 ml, 0.00156 mol) was added to the reaction mix, and the liberation of HCl returned again. After 2 hours, the reaction finished. The result of the polycondensation reaction was poured into 200 ml of toluene from Merck KGaA (Darmstadt, Germany), decanting PTOBDME, which was filtered, washed with ethanol, and vacuum dried.
2.5. Synthesis of cholesteric PTOBUME-amide [(C33H33O9N)n
2.5.1. Preparation of undec-10-enoyl chloride (
IIin Figure 4)
To a stirred solution of 0.118 mol of undec-10-enoic acid in 100 ml of toluene, at 25°C, 0.078 mol of oxalyl chloride was added during 30 minutes. The solution was stirred for 30 minutes after emission of HCl gas had completed. The mixture reaction was concentrated to about half the initial volume by using a vacuum pump equipped with a sodium hydroxide trap. This solution was used directly to prepare undec-10-enamide (
2.5.2. Preparation of undec-10-enamide
A NH3 gas stream was used to purge the stirred solution of undec-10-enoyl chloride, cooled in a bath of dry ice/acetone. The NH3 stream was produced by boiling to reflux ammonia solution generated by reaction between 100 g of ClHN4 solved into 300 ml of H2O and 76 g of NaOH solved in 50 ml of water at 10°C. The reflux condenser and a NaOH trap were connected between the ammonia solution and the mixture reaction to prevent moisture. After 30 minutes of reaction, when a white solid had precipitated and HCl gas emission was not observed, the reaction flask was allowed to warm to room temperature. The mixture reaction was concentrated to a residue on a rotatory evaporator. The solid was partitioned between 10% aqueous sodium hydroxide and dichloromethane, and the aqueous phase was washed three times with additional dichloromethane. The combined dichloromethane extracts were washed with brine, dried with anhydrous sodium sulfate, and concentrated to a solid on a rotatory evaporator [12, 13, 14, 15]. The solid was recrystallized in a mix chloroform/hexane (1:1) to give pure undec-10-enamide—yield (80%) and melting point 87°C (
1H NMR (CDCl3 300 MHz, δ; (ppm)): δ 7.19, (dd, 2H)
13C NMR (CDCl3 100 MHz, δ; (ppm)): 175.2
2.5.3. Preparation of 10-11 epoxy undecanamide
To a stirred solution of undec-10-enamide (7 g;) in 108.4 ml of acetone, NaHCO3 (26.4 g) was added, and then, 5.2 ml of water was added carefully. The resultant thick mixture was strongly stirred, while a solution of 40.6 g of oxone in 158 ml of water was added dropwise during 45 min. The reaction was monitored by thin layer chromatography (TLC) using a mix of ethyl acetate/hexane 2:1. After the reaction was complete, the acetone was removed by evaporation. The remaining solution was acidified with HCl 10% to pH 2 at 10°C and followed rapid extraction with 250 ml of dichloromethane. The aqueous phase was washed three times with additional dichloromethane. The combined organic phase was washed with brine, dried with anhydrous sodium sulfate, and concentrated to a white solid on a rotatory evaporator (
1H NMR (CDCl3 300 MHz, δ; (ppm)): δ 5.44, (bs, 2H), δ 2.90, (m, 1H), δ 2.74, (dd, 1H J = 4.6 Hz), δ 2.46, (dd, 1H J = 5.0 Hz), δ 2.21, (t, 2H, J = 7.6 Hz); δ 1.62 (t, 2H, J = 7.4 Hz), δ 1.51, (m, 2H), δ 1.44 (m, 2H), δ 1.33-1.28, (bs, 8H); 13C NMR (CDCl3 100 MHz, δ; (ppm)): 173.8 (1C) (
2.5.4. Preparation of 10-11 of dihydroxyundecanamide
The previously obtained 10-11 epoxy undecanamide was stirred during 8 hours at 60°C in aqueous HCl 10%. The reaction was monitored by TLC using a mix of ethyl acetate/hexane 2:1. An oil, not miscible with water, was obtained. The mixture reaction was extracted with dichloromethane, and the aqueous phase was washed three times with additionally dichloromethane. The combined organic phase was washed with brine, dried with anhydrous sodium sulfate, and concentrated to a yellow oil on a rotatory evaporator. Yield (75%) (
1H NMR (CDCl3 300 MHz, δ; (ppm)): δ 5.44, (bs, 2H), δ 3.78, (m, 1H), δ 3.60, (dd, 1H J = 11.0 Hz), δ 3.48, (dd, 1H J = 2.21, (t, 2H, J = 7.6 Hz); δ 1.62 (t, 2H, J = 7.4 Hz);), δ 1.51, (m, 2H), δ 1.44 (m, 2H), δ 1.33-1.28, (bs, 8H);
13C NMR (CDCl3 100 MHz, δ; (ppm)): HRMS m/z calc. For C11H23NO3H3O+ [M + H3O] + 236.2; found 236.2.
2.5.5. Preparation of TOBC
In the course of 20 minutes, 20 g TOBA were added to 350 ml thionyl chloride from Sigma-Aldrich Chemie GmBH (Steinheim, Germany), while stirring rapidly at room temperature (
The solution was boiled with the reflux condenser. When the emission of HCl had finished and most of the sediment had dissolved, the hot solution was filtered and cooled down to 0°C for a day. The obtained product that separated out was filtered, vacuum dried, and recrystallized in chloroform, from SDS Votre Partenaire Chimie (Peypin, France).
Yield: 14 g (60%).
2.5.6. Preparation of PTOBUME-amide.
A mixture of TOBC (5.5 g; 0.012 mol), 10-11 of dihydroxyundecanamide (2.7 g; 0.012 mol) in 3 ml of diphenyl oxide from Sigma-Aldrich Chemie GmBH (Steinheim, Germany) was purged with dry nitrogen from Praxair (Madrid, Spain) for 25min at room temperature. Then, while maintaining the gas stream, the flask was transferred to a bath containing a high-temperature heat-transfer agent. The polycondensation was carried out for 360 minutes at 200°C. The reaction gets completed when emission of HCl had finished. The reaction mixture was poured into 300 ml of toluene from Merck KGaA (Darmstadt, Germany), decanting PTOBUME-amide. After 12 hours, it was filtered, washed with ethanol, and vacuum dried. After 3 weeks, a second fraction of polymer was precipitated of the toluene mother liquors, which was filtered, washed with ethanol, and vacuum dried.
Yield first fraction 3.0 g (38.5%); yield first and second fraction 0.1 g (40.0%).
2.6. Synthesis of Cholesteric PTOBEE-amide (C26H19O9N)n
2.6.1. Preparation of 3,4-dihydroxybutanoic acid
To a stirred mixture of 10 g of but-3-enoic acid solved in 130 ml acetone, 34 g. NaHCO3 in 65 ml mili-Q water was added carefully. The resultant mixture was strongly stirred, while a solution of 51.1 g oxone in 200 ml of water was added dropwise during 120 min. The reaction was monitored by thin layer chromatography (TLC) using a mix of ethyl acetate/diethyl ether 1:1. After the reaction was complete, the acetone was removed by evaporation. The remaining solution was acidified with HCl 10% to pH 2 at 10°C and followed of rapid extraction with 250 ml of ethyl acetate. The aqueous phase was washed three times with additional ethyl acetate. The combined organic phase was washed with brine, dried with anhydrous sodium sulfate, and concentrated to a white solid on a rotatory evaporator (
2.6.2. Preparation of 4-hydroxydihydrofuran-2(3H)-one
To 3,4-dihydroxybutanoic acid into a flask equipped with a Dean Stark adapter filled with a toluene column finally connected to a refrigerant, 0.5 ml trifluoroacetic acid was added in 100 ml toluene heating to 110°C, mixing for 3 hours. The reaction product was removed with ethanol, washed in ethyl acetate, and dried. The reaction was monitored by thin layer chromatography (TLC) using a mix of ethyl acetate/diethyl ether 1:1 (
2.6.3. Preparation of 3,4-dihydroxybutanamide
To the 4-hydroxydihydrofuran-2(3
2.6.4. Preparation of PTOBEE-amide
In a three-neck round-bottom flask, 0.2 g 3,4-dihydroxybutanamide was added dropwise to 1 g TOBC solved in 100 ml 1,1,2,2-Tetrachloroethane. The reaction was stirred at 90°C for 20 hours. The reaction product was filtered, washed in 50 ml ethanol, 100 ml water, 200 ml NaHCO3 (10%), 200 ml HCl (5%), 300 ml water, and 200 ml ethanol, and dried.
3. Characterization techniques
3.1. Conventional NMR techniques
The obtained polymers are characterized by 1H-NMR, 13C-NMR, COSY (Homonuclear Correlation Spectroscopy), TOCSY (Total Correlation Spectroscopy), NOESY (Nuclear Overhauser Effect Spectroscopy, through-space correlation method), HSQC (Heteronuclear Single-Quantum Correlation spectroscopy), and HMBC (Heteronuclear Multiple Bond Correlation) for correlations between carbons and protons that are separated by two, three, and sometimes four bonds, in conjugated systems. Direct one-bond correlations being suppressed.
The experiments were performed in a Bruker 300 MHz NMR spectrometer and VARIAN 400 and 500 MHz spectrometers. The solvents used were DMSO-d6 and CDCl3, from Merck KGaA (Darmstadt, Germany), at 25°C. 1H chemical shifts were referenced to the residual solvent signal at δ = 2.50 ppm (DMSO-d6) relative to tetramethylsilane (TMS). All the spectra were processed and analyzed with MestReNova v.11.0.4 software . Predicted 1H and 13C-NMR chemical shifts were calculated from the formula with ChemDraw Professional, v.18.104.22.168 .
3.2. Thermal behavior
Thermal stability was studied by Thermogravimetry on a Mettler TA4000-TG50 at heating rate of 10°C/min with nitrogen purge between 30 and 600°C. Thermal behavior was determined by differential scanning calorimetry (DSC) in a Mettler TA4000/DSC30/TC11 calorimeter, with series of heating/cooling cycles in a temperature range between 0 and 230°C.
3.3. The optical activity
The optical activity of the polymers was measured as optical rotatory dispersion (ORD) at 25°C in DMSO from Scharlau Chemie, in a Perkin Elmer 241 MC polarimeter with wavelengths: λNa = 589 nm, slit = 5 mm, integration time = 50 s; λHg = 574 nm, slit = 14 mm, integration time = 50s; λHg = 546 nm, slit = 30 mm, integration time = 50 s; λHg = 435 nm, slit = 5 mm, integration time = 50s; λHg = 365 nm, slit = 2.5 mm, integration time = 50 s.
4. Structural characterization by NMR
4.1. Structural characterization of PTOBDME-choline
The designation of the 1H and 13C-NMR chemical shifts, in DMSO-d6, of the monomeric unit and the end groups of Polyester PTOBDME-choline, is given in Table 1. All the spectra have been analyzed and interpreted the help of MestReNova . The predicted theoretical values, also in Table 1, have been calculated by ChemDraw . Similar notations as those assigned with precursor cholesteric liquid crystal polyesters PTOBDME [1, 2] have been used.
|Set of signal of system ( ‘ ) and ( “ )||Set of signal of system without apostrophe ( )||Calculated chemical shift|
|Ha’Hb’||4.63, 4.52||11’C||65.7||Ha, Hb||3.95, 3.89||11C||46.4||4.80, 4.55||67.5|
|Hf’, Hg’||1.45||9’C||24.6||Hf, Hg||1.35||9C||24.4||1.29||23.3|
|Hf”, Hg”||1.53, 1.44||9”C||25.6||1.29||23.1|
|8’H||1.24||8’C||28.9 m*||8H||1.24||8C||28.9 m*||1.29||29.6|
|7’H||1.24||7’C||28.9 m*||7H||1.24||7C||28.9 m*||1.29||29.6|
|6’H||1.24||6’C||28.9 m*||6H||1.24||6C||28.9 m*||1.26||29.6|
|5’H||1.24||5’C||28.9 m*||5H||1.24||5C||28.9 m*||1.26||29.6|
|4’H||1.24||4’C||28.6 m*||4H||1.24||4C||28.6 m*||1.26||29.3|
Considering the monomer structure, three zones can be differentiated in the 1H-NMR spectrum, corresponding to
The presence of two independent 1H-NMR sets of signals are observed in the spectrum, one marked with ( ‘ ) and the other without it ( ). They are attributed to two conformers
In the aromatic zone singlet at 8.36 ppm belongs to 20H and doublets at 7.50 and 8.08 ppm are assigned to 16’H and 15’H, respectively, and doublets at 7.55 and 8.15 ppm to 16H and 15H; similar assignation was previously carried out in precursor PTOBDME . In the spacer zone, multiplet at 5.45 ppm is interpreted due to Hc’, and the double doublets at 4.63 and 4.52 ppm correspond to Ha’ and Hb’. These peaks presented correlation signals in COSY and were related with other aliphatic signals Hd’He’ (1.83 ppm) and Hf’Hg’ (1.45 ppm) by TOCSY experiment. Multiplet at 5.26 ppm was assigned to Hc and double doublets at 3.95 and 3.89 ppm to Ha and Hb, and they showed COSY correlations and were related with signals at 1.77 ppm (Hd) and 1.35 ppm (Hf Hg) respectively by TOCSY experiment. The peaks assigned to Ha”, Hb”, and Hc” due to aliphatic end group are overlapped with Hb’ (4.52 ppm) and with 21H (4.54 ppm) (in Table 2), and they were assigned through TOCSY correlations observed for signal at 4.52 ppm (not observed for 4.63 ppm, Ha’), with the multiplet at 4.39 ppm (Hc”) and confirmed by HSQC. By this method, carbon C11’ (65.7 ppm) was correlated with signals at 4.63 (Ha’) and 4.52 (Hb’) and carbon 11”C (67.8 ppm) with 4.52 ppm (Ha”) and (Hb”). Signals at 1.83 and 1.45 ppm assigned to (Hd’) and (Hf’, Hg’) and correlated by COSY with Hc’(5.45 ppm). Peaks at 1.77 ppm (Hd He) and 1.35 ppm (Hf Hg) are correlated by COSY with Hc (5.26 ppm). Signals at 1.92, 1.78 ppm are related with Hc” (4.39 ppm) by COSY experiments, and they were assigned to Hd” and He”. They are also related with Hf” (1.53 ppm) and Hg”(1.44 ppm) by the same experiment.
|Observed chemical shifts||Calc. chemical shifts||Observed chemical shifts||Calc. chemical shifts|
|Atom 1H(ppm)||Atom 13C(ppm)||Atom 1H(ppm)||Atom 13C(ppm)|
|DMSO||DMSO||1H calc||13C calc||DMSO||DMSO||1H calc||13C calc|
|21H, 21’H||4.54, 4.76||21C 21’C||62.6 58.8||4.69||58.1||13H 13’H||13C 13’C||58.8 58.5||4.69||58.1|
|22H, 22’H||3.33 3.85||22C 22’C||55.6 64.0||3.70||66.5||14H 14’H||14C 14’C||55.0||3.70||66.5|
|23H, 23’H||2.74 3.21||23C 23’C||43.2 53.0||3.30||54.4||15H 15’H||2.74 3.22||15C 15’C||42.5 52.7||3.30||54.4|
Choline end group showed in Table 2, two set of signals probably due to conformational equilibrium: Multiplets assigned to 21H (4.54 ppm), 22H (3.33 ppm), and 23H (2.74 ppm), correlated in COSY, and another set was multiplets 21’H (4.76 ppm), 22’H (3.85 ppm) and a singlet 23’H (3.21 ppm).
The HSQC experiment allowed the direct allocation of carbon atoms linked to hydrogens, confirming the assignation of the proton signals overlapped in the 1HNMR experiment. The correlation of carbon atom 11’C (65.7 ppm) with Ha’ (4.63 ppm) and Hb’ (4.52 ppm); correlation between 11”C (67.8 ppm) and Ha” and Hb”; and correlation between carbon 21C at (62.6 ppm) and 21H at (4.54 ppm), are observed in Table 2.
4.2. Structural characterization of PTOBEE-choline
The assignment of the 1H and 13C-NMR chemical shifts, in CDCl3 and DMSO-d6, of the monomeric unit and the end groups of PTOBEE-choline are given in Table 3, with the predicted values calculated by ChemDraw Professional . Similar notations as those designated for precursor cholesteric liquid crystal PTOBEE  have been used.
|Set of signal of system (’) and (“)||Set of signal of system without apostrophe ( )||Calculated chemical shifts|
|8’H||7.36||7.48, 7.50||8’C||121.7||121.7||8H||7.34||7.53, 7.51||8C||121.7||122.0||7.26||121.5|
|7’H||8.18||8.08, 8.06||7’C||131.5||130.6||7H||8.16||8.11, 8.09||7C||131.5||130.6||8.13, 8.11||130.3|
|Ha’, Hb’||4.60, 4.53||4.63, 4.52||3’C||65.6||64.8||Ha, Hb||3.76, 3.74||3.94, 3.91||3C||45.23||45.6||4.80, 4.55||67.2|
|Ha”, Hb”||4.53||4.52 *||3”C||67.8||67.1||4.53, 4.28||70.5|
|Hd”, He”||1.88, 1.13||1.96, 1.80||2”C||*||1.48||26.8|
In the 1H-NMR experiment in CDCl3, observed chemical shifts are 12H singlet at (8.34 ppm), 8H doublet at (7.34 ppm), 7H doublet at (8.16 ppm), 8’H doublet at (7.36 ppm) and 7’H doublet at (8.18 ppm). Multiplets at 5.46 and at 5.25ppm are interpreted as Hc’ and Hc, respectively. The double doublet at 4.60 ppm is assigned to Ha’ and correlates in COSY with Hc‘ signal. An overlapped signal at 4.53 ppm is identified as Hb’, with COSY and TOCSY cross signal with Ha’. At 4.15, a weak multiplet is assigned to Hc”, it presented COSY correlation with signal 4.53 ppm, indicating the presence of Ha” and Hb”. The two double doublets at 3.76 and 3.74 ppm were identified as Ha and Hb and presented the expected COSY correlation with Hc (5.25 ppm). The overlapped signal at 1.90 ppm is identified as Hd‘, with cross signal with Hc’. A very weak COSY cross signal between 4.15 and Hd”(1.88 ppm) is observed. Triplet at 1.09 ppm is due to 1’H, with TOCSY correlation with Hc’, while triplet at (1.03 ppm) is 1H, with TOCSY correlation with Hc. The weak triplet at 1.13 ppm corresponded with 1”H. As in PTOBDME-choline, the choline end group shows, in Table 2, two set of signals due to conformational equilibrium. Multiplets 13H, 14H are observed at 4.56 and 3.32 ppm, respectively, and 15H at 2.74 ppm, correlated in COSY experiments, and another set was 13’H and 14’H multiplets at 4.75 and 3.84 ppm, respectively, singlet 15’H at 3.22 ppm.
HSQC experiment was performed to determine the chemical shift of carbons bonded to the assigned hydrogen. The complex signal at 4.53 in proton presented several correlations with carbons. Ha’(4.60 ppm) and Hb’ (4.53 ppm) showed correlation with carbon 3’C (65.6 ppm). Another correlation with overlapped signal at 4.53 ppm Ha”Hb” was observed with carbon 3”C (67.8 ppm). Signals corresponding to 1C, 2C, and 4C of aliphatic end group were not observed due to the low concentration. 13C-NMR experiment allowed the assignation of the carbons not attached to hydrogens matching the calculated model.
4.3. Structural characterization of PTOBDME-ammonium
The structure of PTOBDME-ammonium, as depicted
|Set of signal of system (‘) and (“)||Set of signal of system without apostrophe ( )||Calculated chemical shift|
|Ha’, Hb’||4.64, 4.48||11’C||66.0||Ha, Hb||3.94, 3.88||11C||46.4||4.80, 4.55||67.6|
|Ha”, Hb”||4.48||11”C||67.8||4.53, 4.28||70.9|
|Hd’, He’||1.81||10’C||30.3||Hd, He||1.77||10C||31.3||1.71||30.8|
|Hd”, He”||1.91 1.78||10’C||33.6||1.44||34.1|
|Hf’, Hg’||1.42||9’C||24.6||Hf, Hg||1.33||9C||24.3||1.29||23.4|
|8’H||1.22||8’C||28.6 m*||8H||1.22||8C||28.6 m*||1.29||29.7|
|7’H||1.22||7’C||28.6 m*||7H||1.22||7C||28.6 m*||1.29||29.7|
|6’H||1.22||6’C||28.6 m*||6H||1.22||6C||28.6 m*||1.26||29.7|
|5’H||1.22||5’C||28.5 m*||5H||1.22||5C||28.5 m*||1.26||29.7|
|4’H||1.22||4’C||28.1 m*||4H||1.22||4C||28.1 m*||1.26||29.4|
Ammonium end group shows 23H, 22H, 24H multiplets at 3.28 ppm, 2.17 ppm, and 2.79 ppm, respectively . 23 H at 3.28 ppm was overlapped with signal of H2O of the deuterated solvent, and it was assigned due to the COSY and TOCSY correlations with signals at 4.38 ppm (21H) and at 2.17 ppm (22H) and HSQC correlation with 23C at 54.0 ppm. 21H signal was overlapped at 4.38 ppm, and it was identified by COSY correlations with 2.17 ppm (22H) and TOCSY correlations with 3.28 ppm (21H). The polymer holds positive charge due to ammonium proton 25H observed at 10.33 ppm.
The HSQC experiment confirmed the direct assignation of carbon atom 11’C (66.0 ppm) linked to protons Ha’ (4.63 ppm) and Hb’ (4.48 ppm). Signal Hb’ exhibits correlation with 11”C (67.8 ppm), linked to Ha” and Hb”. Two cross signal are observed for Hc” (4.38 ppm), one with carbon atom at 12”C (60.4 ppm) and another with 21H linked to carbon atom 21C (62.3 ppm). The correlations of carbon atom 10”C (33.6 ppm) with Hd”(1.91 ppm) and He”(1.78 ppm), and carbon atom 9”C (25.6 ppm) with Hf”(1.51 ppm) and Hg”(1.42 ppm) confirmed the previous assignation.
4.4. Structural characterization of PTOBEE-ammonium
Table 6 shows the assignation of 1H and 13C-NMR chemical shifts (ppm) observed of polyester PTOBEE-ammonium chloride and calculated for the repeating unit and the aliphatic end group. In the 1H-NMR experiment, in CDCl3, peaks observed at 8.34, 7.36, 7.34, 8.18, and 8.16 ppm are assigned to 12H singlet, 8’H doublet, 8H doublet and 7’H doublet, and 7H doublet, respectively. Peak at 5.46 ppm is Hc‘ and 5.25 ppm is Hc. The double doublet at 4.60 ppm is interpreted as Ha’ because of its shape and the COSY correlation with Hc‘. An overlapped signal at 4.53 ppm is attributed to Hb’, by COSY and TOCSY cross signal with Ha‘. A weak multiplet at 4.15 ppm is assigned to Hc”, and this signal presented COSY correlation with Hb’, indicating the presence of Ha” and Hb”. The two double doublets at 3.76 and 3.74 ppm are identified as Ha and Hb and presented the expected COSY correlation with Hc. The overlapped signal at 1.90 ppm is Hd‘ correlated with Hc’. A very weak COSY cross signal is observed between Hc” and 1.88 ppm (Hd”). Triplet signal at 1.09 ppm with TOCSY correlation with Hc’ is interpreted as He’, while triplet at 1.03 ppm with TOCSY correlation with Hc was assigned to He. The weak triplet at 1.13 ppm corresponds to He”.
Signals of proton ammonium end group in DMSO-d6 are observed at (Table 5): 16H singlet (2.81 ppm), 15H multiplet (3.25 ppm), 14H multiplet (2.18 ppm), and 13H multiplet overlapped at (4.39 ppm) but presented COSY correlations with 2.18 ppm and TOCSY correlations with 3.25 ppm. The compound is positively charged, with the ammonium proton 17H observed at 10.3 ppm in DMSO-d6 and 13.2 ppm in CDCl3.
|Observed chemical shifts||Calc. chemical shifts||Observed chemical shifts||Calc. chemical shifts|
|Set of signal of system (’) and (“)||Set of signal of system without apostrophe ( )||Calculated chemical shift|
|Ha’, Hb’||4.60, 4.53||4.64 4.53||3’C||65.6||64.6||Ha, Hb||3.76, 3.74||3.96, 3.92||3C||45.23||45.6||4.80, 4.55||67.2|
|Ha”, Hb”||4.53||4.48*||3”C||67.8||*||4.53, 4.28||70.5|
|Hd”, He”||1.88, 1.13||1.90, 1.86*||2”C||*||1.48||26.8|
HSQC experiment exhibits several correlations of the complex proton signal at 4.53 ppm, carbon atoms. Double doublet Ha’(4.60 ppm) Hb’ (4.53) correlates with 3’C at (65.6 ppm). Another correlation is observed between proton Ha” and 3”C (67.8 ppm). Correlation between the overlapped signal of proton 13H (4.39 ppm), within the ammonium end group, and 13C (61.9 ppm) in CDCl3 is observed.
4.5. Structural characterization of PTOBUME-amide
The structures of undec-10-enamide, 10-11-epoxy-undecanamide, and 10,11-dihydroxyundecanamide (
The structure of PTOBUME-amide,
|System (‘)||System without apostrophe ( )||Theoretical chemical shifts|
|Ha’, Hb’||4.95, 4.30||10’C||Ha, Hb||4.23, 4.18||10C||Ha,Hb||4.78, 4.53||10C||66.0|
|Hd’, He’||1.74||9’C||Hd, He||1.55||9C||Hd, He||1.67||9C||30.7|
|Hf’, Hg’||1.37||8’C||Hf, Hg||1.22||8C||Hf, Hg||1.25||8C||23.3|
|Experimental signals “end group”||Theoretical chemical shifts end group|
|Ha”, Hb”||10”C||Ha”, Hb”||3.86, 3.80||10”C||64.3|
|Hd”, He”||1.62||9”C||Hd”, He”||1.67||9”C||30.5|
|Hf”, Hg”||1.22||8”C||Hf”, Hg”||1.25||8”C||25.6|
|NH2 “||10.7||1”C||NH2 “||7.03||1”C||173.6|
4.6. Structural characterization of PTOBEE-amide
The structure of PTOBEE-amide,
|System (‘)||System without apostrophe ( )||Theoretical chemical shifts|
|Ha’, Hb’||4.63, 4.49||3’C||Ha,Hb||4.32, 4.22||3C||Ha,Hb||4.78, 4.53||3C||66.5|
|Hd’, He’||3.22, 3.10||2’C||Hd, He||1.05||2C||Hd, He||2.46||2C||38.9|
5. Thermal stability and differential scanning calorimetry (DSC)
The presence of choline group at the end of polymer chains causes in PTOBDME-choline a decrease in the thermal stability range compared to precursor PTOBDME. A 5% weight loss is observed for PTOBDME-choline at 230°C, while PTOBDME loses 5% weight at about 280°C. The thermal stability of PTOBEE-Choline is similar to that of polyester PTOBEE. PTOBEE-choline has 5% weight loss at 281°C, and PTOBEE at 280°C (see Figures 6 and 7). In the thermal stability curve of PTOBDME-choline, the first degradation step observed at 230°C is followed by two other weight loss step at 280 and 448°C. Two decomposition steps are observed at 280 and 466°C in PTOBEE-choline.
In the DSC experiment of PTOBDME-choline, performed at 10°C/min, Figure 6(b), a glass transition can be observed at 58.2°C, in the first heating run, and a weak endothermic peak at 99.5°C is interpreted as due to the first order transition from crystal phase to liquid crystal state. An exothermic peak at 171.2°C is also observed which is not explained, but the beginning of a second endothermic peak at 200°C can be attributed to fusion to the isotropic. In the cooling process, two exothermic peaks at 155°C and at 175 are observed, probably associated to crystal formation. In the second heating, a very broad endothermic peak at 100.2°C is observed again associated to the transition to liquid crystal mesophase.
In the DSC experiment of PTOBEE-choline at (10°C/min), Figure 6(c), a glass transition can be observed at 60°C, and an endothermic peak at 130.2°C is attributed to the transition crystal to liquid crystal. A decreasing of baseline from 183.7°C to the end of heating was also observed in the first heating run due to a nonconcluded endothermic process or to the beginning of degradation to the polymer. A broad exothermic peak observed the cooling around 145°C would correspond to a crystallization from the mesophase state. In the second heating, only two glass transitions can be observed at 65 and 85°C.
The presence of ammonium chloride group at the end in the polymer chains, in Figure 8-I, produces a decrease of the thermal stability range compared to precursor polyesters. At 278°C, PTOBEE-ammonium chloride loses 10% weight and PTOBDME-ammonium chloride at 260° C, while precursor PTOBDME and PTOBEE at 310°C. In the thermal stability curve of the ammonium-polymers, the first degradation step observed at 228 and 230°C, respectively, was not observed in PTOBEE and PTOBDME. The two next inflexion points at 310 and 311°C and 466 and 471°C were equivalents to the observed in the precursor polyesters, which would indicate the same type of decomposition to principal core of the chain.
In the DSC experiment of PTOBDME-ammonium chloride, at 10°C/min, Figure 8-II, a very broad exothermic peak centered at 96.8°C, is observed in the first heating, associated to low enthalpy value, which can be attributed to crystal to crystal transitions, involving molecular reordering between crystalline phases. An endothermic peak at 146.9°C is interpreted due to the transition to liquid crystal mesophase; finally, an exothermic peak at 186.8°C is observed. In the cooling run, very weak exothermic peaks at 154.4 and at 104.1°C were observed due crystallization process. In the second heating, a broad exothermic peak centered at 75.2°C, an endothermic peak at 149.1°C, and finally, an exothermic peak at 179.8°C were observed again.
The DSC experiment of PTOBEE ammonium choride, at 10°C/min, Figure 8-III, shows in the first heating run a broad exothermic peak centered at 69.1°C, and a very strong endothermic peak at 146.2°C due to the fusion transition from crystalline phase to liquid crystal mesophase, and finally, a weak endothermic peak at 173.3°C, perhaps due to a partial fusion to isotropic. During the cooling, an exothermic peak appeared at 166°C would correspond to a crystallization from the mesophase state, and in the second heating, the broad exothermic peak observed in the first heating was observed to higher temperature centered at 114.8°C; the two endothermic peaks were again observed at 147.6 and 170.1°C.
The thermogravimetric curve and the DSC analysis of PTOBUME-amide are given in Figure 9. At 265°C, it loses 5% weight. At 340°C, a first decomposition step begins, followed by another three at 400, 450, and 510°C. In the first heating of the DSC, an endothermic peak is observed at 160°C interpreted as the transition to the mesophase state. In the cooling run, several week exothermic peaks could be associated to crystal formation processes.
6. Optical characterization
6.1. Optical activity of PTOBDME-ammonium and PTOBEE-ammonium
As in the polyester precursors PTOBEE-ammonium chloride and PTOBDME-ammonium chloride presented an unexpected optical activity and chiral morphology, although they were synthesized starting from equimolar quantities of TOBC and the racemic mixture of the corresponding glycol. The obtained chirality has been evaluated by optical rotatory dispersion, in Figure 10, the values of optical activity are given as [α]25°C, at different wavelengths. Table 9 shows the measured values.
|Polymers (0.2 g/100 ml in DMSO)||Hg (365 nm)||Hg (435 nm)||Hg (546 nm)||Hg (578 nm)||Na (589 nm)|
In the optical characterization of precursor cholesteric liquid crystal polyesters [1, 3], even an increase of chirality was observed for a second fraction of the polymer, obtained by precipitation, after days of reaction of the liquors mother with respect to the initial first fraction of the polymer. The optical activity of PTOBDME-choline, PTOBEE-choline, PTOBUME-amide and PTOBEE-amide, has not been studied at the end of the present article but will be reported in the future.
The synthetic methods of six new multifunctional cationic cholesteric liquid crystal polymers designed as PTOBDME-choline [(C34H36O8)n─C5H13N]; PTOBEE-choline [(C26H20O8)n─C5H13N]; PTOBDME-ammonium [(C34H36O8)n─C5H13N]; PTOBEE-ammonium [(C26H20O8)n─C5H13N]; PTOBUME-amide (C33H33O9N)n and PTOBEE-amide (C26H19O9N)n are given and their characterization by 1H, 13C-NMR, COSY, and HSQC is reported.
The NMR analysis let us to conclude that the enantiomeric polymer chains present stereo regular head-tail, isotactic structure, explained in terms of the higher reactivity of the primary hydroxyl group in the glycol, with respect to the secondary one, through the polycondensation reaction.
According to our previous experience, each enantiomer, with two independent sets of signals observed by 1H and 13C-NMR, differentiated with apostrophe (‘) and without it ( ), could be attributed to two diastereomeric conformers:
The presence of choline group or ammonium chloride groups at the end of polymer chains causes in precursor polyesters a decrease in their thermal stability range. PTOBDME-choline losses 5% weight at 230°C (PTOBDME at 280°C). The thermal stability of PTOBEE-choline is similar to that precursor PTOBEE, with 5% weight loss at 281°C.
At 260°C, PTOBDME-ammonium loses 10% weight and PTOBEE-ammonium at 278°C (precursor PTOBDME and PTOBEE at 310°C).
All the synthetized cationic liquid crystal polymers show in DSC an endothermic peak assigned to the first order transition from crystalline phase to liquid crystal mesophase: PTOBDME-choline at 99.5°C; PTOBEE-choline at 130.2°C; PTOBDME-ammonium at 146.9°C; and PTOBEE-ammonium at 146.2°C.
At 265°C, PTOBUME-amide loses 5% weight. At 340°C, it has a first decomposition step, followed by another three at 400, 450, and 510°C. In the DSC first heating, it shows the endothermic peak due to the mesophase transition at 160°C.
Optical ORD values are provided for the second fractions of PTOBDME-ammonium and PTOBEE-ammonium.
The author thanks Dr. Javier Sanguino Otero for his valuable help during the development of this Project. She also thanks the financial support obtained in the Project “Nuevos vectores no virales basados en polímero cristal-líquido colestérico (PCLC) y su uso para transfección génica”. PTR1995-0760-OP.