Synthesis and characterization of new polyamides derived from alanine and valine derivatives

Background Many efforts have been recently devoted to design, investigate and synthesize biocompatible, biodegradable polymers for applications in medicine for either the fabrication of biodegradable devices or as drug delivery systems. Many of them consist of condensation of polymers having incorporated peptide linkages susceptible to enzymatic cleavage. Polyamides (PAs) containing α-amino acid residues such as L-leucine, L-alanine and L-phenylalanine have been reported as biodegradable materials. Furthermore, polyamides (PAs) derived from C10 and C14 dicarboxylic acids and amide-diamines derived from 1,6-hexanediamine or 1,12-dodecanediamine and L-phenylalanine, L-valyl-L-phenylalanine or L-phenylalanyl-L-valine residues have been reported as biocompatible polymers. We have previously described the synthesis and thermal properties of a new type of polyamides-containing amino acids based on eight new symmetric meta-oriented protected diamines derived from coupling of amino acids namely; Fomc-glycine, Fmoc-alanine, Fomc-valine and Fomc-leucine with m-phenylene diamine or 2,6-diaminopyridine. Results revealed that incorporation of pyridine onto the polymeric backbone of all series decreases the thermal stability. Here we describe another family of polyamides based on benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid linked to benzidine and 4,4′-oxydianiline to study the effect of the dicarboxylic acid as well as the amino acids on the nature and thermal stability of the polymers. Results We report here the preparation of a new type of polyamides based on benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid linked to benzidine and 4,4′-oxydianiline to study the effect of the dicarboxylic acid as well as the amino acids on the nature and thermal stability of polymers. The thermal properties of the polymers were evaluated by different techniques. Results revealed that structure-thermal property correlation based on changing the dicarboxylic acid monomer or the diamine monomer demonstrated an interesting connection between a single change (changing the dicarboxylic acids in each series while the diamine is fixed) and thermal properties. The newly prepared polymers may possess biodegradability and thus may find some applications as novel biomaterials. Conclusions The thermal properties of the new type of polyamides based on benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid (alanine and valine) linked to benzidine and 4,4′-oxydianiline were evaluated by thermal gravimetric (TG), differential thermal gravimetric (DTG) and differential thermal analysis (DTA) techniques. Results revealed that the structure-thermal property correlation based on changing the dicarboxylic acid monomer or the diamine monomer demonstrated an interesting connection between a single change (changing the dicarboxylic acids in each series while the diamine is fixed) and thermal properties. In addition, pyridine-containing polymers exhibited semicrystalline characteristic with melting temperature, Tm. where none of the valine-containing polymers showed a melting and crystallization peak indicating that the polymers were amorphous. This is expected since L-valine side chain can inhibit close packing and eliminate crystallization. The newly prepared polymers may possess biodegradability and thus may find some applications as novel biomaterials.


Results:
We report here the preparation of a new type of polyamides based on benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid linked to benzidine and 4,4 0 -oxydianiline to study the effect of the dicarboxylic acid as well as the amino acids on the nature and thermal stability of polymers. The thermal properties of the polymers were evaluated by different techniques. Results revealed that structure-thermal property correlation based on changing the dicarboxylic acid monomer or the diamine monomer demonstrated an interesting connection between a single change (changing the dicarboxylic acids in each series while the diamine is fixed) and thermal properties. The newly prepared polymers may possess biodegradability and thus may find some applications as novel biomaterials.
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Conclusions:
The thermal properties of the new type of polyamides based on benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid (alanine and valine) linked to benzidine and 4,40-oxydianiline were evaluated by thermal gravimetric (TG), differential thermal gravimetric (DTG) and differential thermal analysis (DTA) techniques. Results revealed that the structure-thermal property correlation based on changing the dicarboxylic acid monomer or the diamine monomer demonstrated an interesting connection between a single change (changing the dicarboxylic acids in each series while the diamine is fixed) and thermal properties. In addition, pyridine-containing polymers exhibited semicrystalline characteristic with melting temperature, T m . where none of the valine-containing polymers showed a melting and crystallization peak indicating that the polymers were amorphous. This is expected since L-valine side chain can inhibit close packing and eliminate crystallization. The newly prepared polymers may possess biodegradability and thus may find some applications as novel biomaterials.

Background
Many efforts have been recently devoted to design, investigate and synthesize biocompatible, biodegradable polymers for applications in medicine for either the fabrication of biodegradable devices or as drug delivery systems [1][2][3][4][5]. Many of them consist of condensation polymers having incorporated peptide linkages susceptible to enzymatic cleavage. Polyamides (PAs) containing α-amino acid residues such as L-leucine, L-alanine and L-phenylalanine have been reported as biodegradable materials [6,7]. Jin et al. [8] prepared polyamides, and polyureas containing L-leucine and L-tyrosine residues in the chain. Polyester amides derived from α-amino acids and α-hydroxyacids, the polydepsipeptides, have also been investigated as biodegradable polymers [9,10]. Polyamides (PAs) derived from C 10 and C 14 dicarboxylic acids and amide-diamines derived from 1,6-hexanediamine or 1,12-dodecanediamine and L-phenylalanine, L-valyl-L-phenylalanine or L-phenylalanyl-L-valine residues have been reported as a biocompatible polymers [11][12][13]. Furthermore, an appropriate choice of the number and sequence of the α-amino acids, as well as a balance of hydrophilic and hydrophobic characteristics of the other constituents, makes these polymers susceptible to enzymatic cleavage of the peptide bonds by specific enzymes [6,[14][15][16][17][18][19].
We have previously [36] described the synthesis and thermal properties of a new type of polyamides-containing amino acids based on eight new symmetric meta-oriented protected diamines derived from coupling of four types of Fmoc-amino acids namely; Fomc-glycine, Fmoc-alanine, Fomc-valine and Fomc-leucine with m-phenylene diamine or 2,6-diaminopyridine. Results revealed that incorporation of pyridine onto the polymeric backbone of all series decreases the thermal stability [36].
Here we describe another family of polyamides based of benzene dicarboxylic acid, pyridine dicarboxylic acid, and α-amino acid linked to benzidine and 4,4 0 -oxydianiline to study the effect of the dicarboxylic acid as well as the amino acids on the nature and thermal stability of the polymer.
Melting points were determined with a Mel-Temp apparatus and are uncorrected. Infrared spectra (IR) were recorded on a FTIR-8400S Shimadzu-Japan or on a Perkin-Elmer 1600 series, Fourier transform instrument as KBr pellets. Absorption spectra were measured with a UV 500 UV-vis spectrometer at room temperature (rt) in DMSO with a polymer concentration of 1 mg/10 mL. Magnetic resonance spectra ( 1 H NMR and 13 C NMR spectra) were recorded on a JEOL 500 MHz spectrometer with chemical shift values reported in δ units (ppm) relative to an internal standard. Follow-up of the reactions and checks of the purity of the compounds was done by thin layer chromatography (TLC) on silica gel-protected aluminum sheets (Type 60 GF254, Merck) and the spots were detected by exposure to UV-lamp at λ 254 nm for a few seconds. Differential thermogravimetric (DTG) analyses were carried out in the temperature range from 20°C to 500°C in a steam of nitrogen atmosphere by Shimadzu DTG 60H thermal analyzer. The experimental conditions were: platinum crucible, nitrogen atmosphere with a 30 ml/min flow rate and a heating rate 10°C/min. Differential thermal analysis (TGA/DTA) analyses were carried out using SDT-Q600-V20.5-Build at the Institute of Graduate Studies and Research, Alexandria University and at the Microanalysis Center, Cairo University, Giza, Egypt. Elemental analyses were performed at the Microanalytical Unit, Cairo University and Center for mycology and biotechnology, Alazhar University, Cairo.

Results and discussion
Chemical preparation of the polyamides containing amino acids Preparation of the Fmoc-protected diamines 5-8 The preparation of the new symmetric diamines 9-12, Scheme 1, for the stepwise polymerization was our first target. Fmoc-alanine (Fmoc-Ala-OH) 1 and Fmoc-valine (Fmoc-Val-OH) 2 were used in this investigation. Reactions of two equivalent amounts of the aforementioned amino acids with benzidine 3, or 4,4 0 -oxydianiline 4 were performed using two equivalents of HATU [37] as coupling reagent in presence of four equivalent of diisopropylethyl amine (DIEA) as base in dimethylformamide (DMF) to furnish the corresponding bis-Fmoc-protected diamines (5)(6)(7)(8) in good yield and purity. The structures of the prepared protected diamines 5-8 were fully characterized by IR, 1 H-NMR, 13 C-NMR and elemental analyses. IR spectra of the protected diamines exhibited characteristic absorption bands in the range 3452-3288 cm −1 corresponding to the N-H bond. In addition the bands corresponding to the amide CONH group is observed in the range 1692-1668 cm −1 . 1 H-NMR spectra of compounds 5-8 in DMSO-d 6 showed signals corresponding to four NH protons. The signals at the range δ 6.82-7.80 ppm correspond to two NH protons, and the other two NH protons are observed at the range δ 9.83-10.47 ppm. The 24 aromatic protons of compounds 5 and 6 are observed at the range δ 7.30-7.86 ppm, while those of compounds 7 and 8 are observed at the range 6.90-7.84 ppm. The 13 C-NMR spectra of compounds 5-8 in DMSO-d 6 show two signals corresponding to the four carbonyl groups at the range 152.99-154.00 ppm and at the range 174.29-175.60.

Preparation of the diamines-containing amino acids 9-12
Treatment of the protected diamines 5-8 with (3:7 Et 2 NH/CH 3 CN v/v) easily furnished the required diamines 9-12, respectively, in high yield, Scheme 1. Noteworthy, the byproduct dibenzofulvene was easily removed from the crude materials by washing with n-hexane. The crude products obtained were used as such without further purification.

Preparation of the polyamides 16-25 by low temperature solution polycondensation
The aromatic acid chlorides, namely isophthaloyl dichloride 13, pyridine-2,6-dicarbonyl dichloride 14, and pyridine-3,5dicarbonyl dichloride 15 used in this investigation were prepared by the reaction of their corresponding dicarboxylic acids, isophthalic acid, pyridine-2,6-dicarboxylic acid, and pyridine-3,5-dicarboxylic acid respectively, with thionyl chloride in the presence of few drops of DMF. Direct polycondensation reaction of an equimolar mixture of the acid chloride 13 with the diamines 9-12 in DMAc solution at 0-5°C furnished the corresponding polyamides containing amino acids 16-19, respectively in high yields, Scheme 2. In a similar manner, reactions of the acid chlorides 14 and 15 with the diamines-containing amino acids 9-11 furnished the corresponding polyamides 20-22 and 23-25, Scheme 2. The polymer structures were confirmed by elemental analysis, IR and UV spectroscopy.

Physical properties of the prepared polyamides containing amino acids Solubility
The prepared polymers 16-25 showed similar solubility behavior in different organic solvents. Moderate to complete dissolution was observed in a variety of aprotic solvents such as NMP, DMSO, DMAc, boiling alcoholic solvents such as methanol, ethanol while insoluble in halogenated solvents such as CHCl 3 , CCl 4 , CH 2 Cl 2 , ClCH 2 CH 2 Cl or in ethers such as Et 2 O, THF, 1,4-dioxane or 1,2-dimethoxyethane (DME).

FTIR Spectroscopy
The FTIR spectra of the polymers exhibited characteristic absorbance at the range of υ 3477-3230 cm −1 and 1692-1639 cm −1 , corresponding to the N-H and C = O stretching of the amide group, respectively. Bands around υ 2900 cm −1 were assigned to the alkyl H-C stretching, while bands appeared around υ 3050, 1598 and υ 1524 cm −1 assigned to the aromatic C-H and C=C aromatic, respectively.

Optical properties
The optical properties of polymers 16-25 were investigated by UV-vis spectroscopy in DMSO with a polymer concentration of 1 mg/10 mL. The spectra were recorded from 600 nm to 200 nm and the maximum absorbances (λmax) of the prepared polymers were recorded. Ala-containing polymers 18, 20, 23 and 25 exhibited bathochromic or red shifted peaks maxima at λ 362 nm, 321 nm, 322 nm and 367 nm, respectively may be attributed to the n→π transition while peaks at lower wavelengths appeared, respectively at 276 nm, 268 nm, 276 nm and 269 nm and could be attributed to the π→π transitions. 18 and 23 showed additional peak maxima at 268 nm, due to π→π transitions. Ala-containing polymers 16 and 22 exhibited peaks at wavelengths, appeared respectively at 304 nm and 276 nm which could be attributed to the π→π transitions. They also showed additional peak maxima at 272 nm and 268 nm due to π→π transitions.