Multichain copoly(α-amino acid). I. Multichain copoly(α-amino acid) prepared by reaction of copoly(L-lysine γ-methyl-L-glutamate) with N-carboxy anhydride of Nϵ-carbobenzyloxy-L-lysine

Polymerization of the N-carboxy anhydride of N^?-carbobenzyloxy-L -lysine in the presence of multifunctional polymeric initiator, copoly(L -lysine γ-methyl-L -glutamate) was studied in N,N-dimethylformamide containing 3% (v/v) of dimethyl sulfoxide. Multichain copoly(α-amino acid), i.e., multi-N^?-poly(N^?-carbobenzyloxy-L -lysine)copoly(L -lysine γ-methyl-L -glutamate), was obtained with linear poly(N^?-carbobenzyloxy-L -lysine) as by-product that could be removed by reprecipitation as was evidenced by gel-permeation chromatography. The degree of polymerization of the branch polymer chains estimated by the osmometric molecular weight determination and amino acid analysis was between 20 and 60, which decreased with increasing lysine content of the polymeric initiator. The stability of α-helical conformation of the multichain copoly(α-amino acid) was studied in the chloroform–dichloroacetic acid system at 25°C by the ORD technique. The α-helical conformation of poly(N^?-carbobenzyloxy-L -lysine) branches was less stable than those of linear poly(N^?-carbobenzyloxy-L -lysine) and the core molecular chains of the multichain copoly(α-amino acid).     

Multichain copoly(α-amino acid). II. Preparation and properties of multi-Nϵ-poly(γ-benzyl-L-glutamyl)copoly(L-lysine γ-methyl-L-glutamate)

A number of multi-N^?-poly(γ-benzyl-L -glutamyl)copoly(L -lysine γ-methyl-L -glutamate)s with branches having various degrees of polymerization and with various intervals of the grafting sites in the core molecule were prepared in N,N-dimethylformamide containing dimethyl sulfoxide by the reaction of N-carboxy anhydride of γ-benzyl L -glutamate with random copoly(L -lysine γ-methyl-L -glutamate)s of different composition with various anhydride-initiator ratios. The relationship between the intrinsic viscosity measured in a coil solvent, dichloroacetic acid (DCA), and the number-average molecular weight determined by osmometry was found to be expressed by the Mark–Houwink–Sakurada equation for the multichain copoly(α-amino acid)s which were made from the same polymeric initiator. The observed α values of the multichain copoly(α-amino acid)s in the equation were lower than that of linear poly(γ-benzyl-L -glutamate). The solvent induced helix–coil transition of the multichain copolymer was investigated in the chloroform?DCA system by the ORD technique. Two kinds of transition regions were clearly distinguished: The α-helices of the core molecules underwent the transition at lower DCA concentration and those of the branch chains at higher DCA concentration. The reduced viscosity of the multichain copoly-(α-amino acid) increased slightly between the two transition regions, in contrast to the large decrease in the reduced viscosity of linear poly(γ-benzyl-L -glutamate) during the helix–coil transition.     

Copolymerization of N-carboxy Nϵ-carbobenzoxy L-lysine anhydride with N-carboxy β-benzyl L-aspartate anhydride in acetonitrile

Copolymerization of N-carboxy N^?-carbobenzoxy L -lysine anydride with N-carboxy β-benzyl L -aspartate anhydride was initiated with n-butylamine in acetonitrile. The copolymerization proceeded almost homogeneously except for the initial stage, when the proportion of N-carboxy anhydride (NCA) in the polymerization mixture varied from 25 to 75 mol %. This was due to the fact that the copolypeptides formed were soluble or highly swollen in the solvent, in contrast to the homopolymerization of NCAs such as N^?-carbobenzoxy L -lysine NCA and β-benzyl L -aspartate NCA in acetonitrile, which proceeds heterogeneously. The compositions of the copolymers obtained were, within experimental error, the same as their monomer feed compositions. The initial rates of copolymerization were almost the same as the rate of homopolymerization of β-benzyl L -aspartate NCA, which propagates with a nonhelical polypeptide, but were slower than the rate of homopolymerization of N^?-carbobenzoxy L -lysine NCA, which propagates with a helical polypeptide.     

Polymerization of α-amino acid N-carboxy anhydride. IX. Copolymerization of α-amino acid N-carboxy anhydride in heterogeneous system

Copolymerization of NCA's was undertaken in a heterogeneous system in acetonitrile, which is not a solvent of the polypeptides. The reactivity ratio was calculated by using the Lewis-Mayo equation. Further, the conversion rate in the copolymerization and the configuration of the copolymer produced were compared with those of the copolymerization in the homogeneous system in nitrobenzene, in which the copolypeptides are swollen. The rate of copolymerization in acetonitrile was between the rates of polymerization of the individual monomers. It has been reported that the configuration of the copolymer obtained in dimethylformamide, in which the copolypeptides are swollen, is of the block type. On the other hand, many polypeptides obtained in acetonitrile, which is not a solvent of the copolypeptides, had a random configuration near to an alternating configuration.     

Conformational transitions of poly(β-benzyl L-aspartate)

Conditions that affect the conformation of poly(β-benzyl L-aspartate) in films have been studied. A slow transition from the left-handed to the right-handed α-helical form is observed under certain conditions in half-dried film cast from chloroform solutions and after exposure of amorphous films of the left-handed β-helical form to chloroform vapor. This transition is caused by adsorption of moisture. Thermal transitions of these films were examined by differential scanning calorimetry and infrared spectroscopy. The right-handed α-helices change to left-handed ω-helices around 140°C, and sharp exothermic peak is observed in this region for some films.     

Circular dichroism of copoly(γ-stearyl-L-glutamate–γ-methyl-L-glutamate)

Copoly(γ-stearly-L -glutamate-γ-methyl-L -glutamate)s with various compositions were synthesized by the ester exchange reaction of poly(γ-methyl-L -glutamate). Circular dichroism studies were carried out for solution and solid film as a function of the degree of stearylation and temperature. The slight and gradual temperature dependence of molecular ellipticity was observed for solution of all the copolyglutamates studied here and for the solid film of the copolyglutamate with the degree of stearylation of 16%, indicative of no reversal in the helix sense. However the remarkable change in negative molecular ellipticity with temperature was detected for the solid film of the copolyglutamate with a low degree of stearylation, e.g., 52%, whereas the drastic change in molecular ellipticity from a negative to positive value appeared for that with a higher degree of stearylation. This is discussed in terms of the reversal in the helix sense from a right- to left-handed α helix with the increase of temperature occurring at the melting temperature of the ordered side chain region.     

Crystallinity and conformational changes in poly(β-benzyl L-aspartate)

The crystallinity of poly(β-benzyl L -aspartate) is highly variable, in a series of specimens prepared under various conditions: films cast from chloroform solutions at various evaporation rates, films cast comparatively slowly from chloroform–trifluoroacetic acid solutions, films prepared from dichloroacetic acid solution by treatment with ethyl alcohol, precipitates formed from trifluoroacetic acid solution by addition of ether. Film cast slowly from chloroform is in the highly crystalline ω form. In contrast, the conformation of the benzene rings in the ω helix obtained from the α helix by heating is distorted to some extent in comparison with the structure of the highly crystalline ω form. Crystallization and conformational changes from the α to the ω form, and from the ω to the β form upon heating, are correlated with the dispositions of the side chains, the packing of the benzene rings, and the motion of the side chains. The main chain of the α helix is distorted into the ω form when its side chains are in a favorable conformation. The α helix is stable in the disordered conformation, and it is distorted to some extent at high temperature.     

Polymerization of α-amino acid N-carboxy anhydrides initiated by histamine in N,N-dimethylformamide

The polymerization of α-amino acid N-carboxy anhydrides (NCAs) initiated by 4-aminoethylimidazole (histamine) was studied in order to synthesize poly(amino acids) containing an imidazole nucleus at the end of polymer chain. On the basis of the kinetical measurements, it was found that the rate of polymerization is proportional to the first order in both NCA and initiator concentrations and that the initiation reaction is predominantly caused by the primary amine with the highest basicity in a histamine molecule. Binding of the histamine fragment to the end of polymer chain was confirmed by elementary analysis, nuclear magnetic resonance spectroscopy, and measuring the number-average molecular weight of the resulting polymers. It was thus possible to prepare poly(amino acids) with a pendant histamine. In addition, the lowering of the number-average degree of polymerization of the polymers prepared was observed under the condition that the initial molar ratio of NCA to histamine was larger. It was caused by the reinitiation of polymerization by the imidazole nucleus at the chain end.     

The synthesis of β-heteroarylamino-α,β-dehydro-α-amino acid and β-heteroarylamino-α-amino acid derivatives

From heteroarylaminomethyleneoxazolones 4 , obtained from N-heteroarylformamidines 2 and 2-phenyl-5-oxo-4,5-dihydro-1,3-oxazole ( 3 ), the following β-heteroarylamino-α,β-dehydro-α-amino acid derivatives were prepared: methyl 8 and ethyl esters 9 , amides 10 and 11 , hydrazides 12 , and azides 15 . By catalytic hydrogenation the compounds 4 were converted into β-heteroarylamino substituted amides 18 and β-heteroarylamino-α-amino acids 20 .     

Polymerization of α-amino acid N-carboxy anhydrides. III. Mechanism of polymerization of L- and DL-alanine NCA in acetonitrile

The polymerization of L - and DL -alanine NCA initiated with n-butylamine was carried out in acetonitrile which is a nonsolvent for polypeptide. The initiation reaction was completed within 60 min.; there was about 10% of conversion of monomer. The number-average degree of polymerization of the polymer obtained increased with the reaction period, and it was found to agree with value of W/I, where W is the weight of the monomer consumed by the polymerization and I is the weight of the initiator used. The initiation reaction of the polymerization was concluded as an attack of n-butylamine on the C₅ carbonyl carbon of NCA. The initiation, was followed by a propagation reaction, in which there was attack by an amino endgroup of the polymer on the C₅ carbonyl carbon of NCA. The rate of polymerization was observed by measuring the CO₂ evolved, and the activation energy was estimated as follows: 6.66 kcal./mole above 30°C. and 1.83 kcal./mole below 30°C. for L -alanine NCA; 15.43 kcal./mole above 30°C., 2.77 kcal./mole below 30°C. for DL -alanine NCA. The activation entropy was about ?43 cal./mole-°K. above 30°C. and ?59 cal./mole-°K. below 30°C. for L -alanine NCA; it was about ?14 cal./mole-°K. above 30°C. and ?56 cal./mole-°K. below 30°C. for DL -alanine NCA. From the polymerization parameters, x-ray diffraction diagrams, infrared spectra, and solubility in water of the polymer, the poly-DL -alanine obtained here at a low temperature was assumed to have a block copolymer structure rather than being a random copolymer of D - and L -alanine.     

Effect of molecular weight on conformation of poly(β-benzyl L-aspartate) in films

In order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β-benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 10³. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left-handed α-helical form. All samples studied which have molecular weights above 2.3 × 10⁴ are similar in crystallinity and the left-handed α-helices in them crystallize to ω-helices during slow evaporation. In the low molecular weight region, however, the left-handed α-helices reverse to right-handed α-helices during slow evaporation, and the right-handed α-helices, in turn, reverse and crystallize to highly ordered ω-helices upon heat treatment, although there is some simultaneous conversion to the β-form. The transition temperatures of the quick-dried films for conversion from the left-handed α-helix to the ω-helix and from the ω-helix to the β-form increase linearly with increasing molecular weight up to about 2 × 10⁴, but no large molecular weight dependence is observed beyond that region.     

Enantioselective polymerization of γ-benzyl-DL-glutamate N-carboxy anhydride in the presence of poly(γ-benzyl-L-glutamate)

The polymerization of γ-benzyl-DL -glutamate NCA in the presence of poly(γ-benzyl-L -glutamate) was investigated. At the initial stage the D -enantiomer was preferentially polymerized (ca. 35% ee) by using triethylamine as an initiator. Enantioselectivity was independent of the molecular weight of preformed poly(γ-benzyl-DL -glutamate).     

Poly(β-amino acid)s. IV. Synthesis and conformational properties of poly(α-isobutyl-L-aspartate)

Poly(α-isobutyl-L -aspartate) was prepared by the polycondensation reaction of p-nitrophenyl ester of α-isobutyl-L -aspartate and the conformation of the poly(β-amino acid) was investigated by X-ray diffraction, polarized infrared, circular dichroism (CD), optical rotatory dispersion (ORD), and NMR spectroscopy. α-Isobutyl β-p-nitrophenyl-L -aspartate hydrochloride and hydrobromide were used as monomers and dimethylformamide, chloroform, and chlorobenzene, as solvents. A high-molecular-weight polymer with [η] 1.0 dl/g (dichloroacetic acid, 25°C) was formed in the polymerization of the hydrochloride in chloroform at 25°C. The X-ray diagram and polarized infrared spectrum of the stretched polymer film obtained from a chloroform solution suggested a cross-β-form as the most probable structure in the solid state. The CD spectra of the polymer in a 2,2,2-trifluoroethanol (TFE) solution and its film cast from the solution showed a peak at 205 nm and a trough at 190 nm which were assigned to a β-structure. The polymer was associated in chloroform. The NMR and ORD spectra in chloroform were similar to those in TFE, which suggests that the polymer also exists in the β-structure in chloroform. The addition of small amounts of dichloroacetic acid and sulfuric acid to chloroform and TFE solutions, respectively, destroyed the β-structure. A random copolymer of α-isobutyl-L -aspartate with β-alanine was also prepared by polycondensation reaction. The copolymer apparently did not form an ordered structure in the solid state or in solution.     

Supramolecular structure investigation of poly(γ-benzyl, α-l-glutamate)-benzyl alcohol system by static light scattering

The cholesteric mesophase of the PBLG-BA system in concentrated solutions and in the gel phase has been investigated by static laser light scattering. The observed ‘distortion’ of the patterns in terms of their dependence on the azimuthal angle was studied. Effect of form-optical rotation on the pattern is discussed. The manner of distortion in the H_v pattern determines the sense of cholesteric twisting. The observed scattering patterns are analogous to those obtained by previous investigators in other solvents. Quantitative measurements of the intensity as a function of scattering vector provide information on the sense and pitch of the cholesteric twisting, as a function of temperature and concentration. Optical microscopy studies reveal onset and build-up of the cholesteric structure. Optical rotation and differential scanning calorimetry (DSC) studies support the conclusions obtained from light-scattering experiments.     

Room-temperature polycondensation of β-amino acid derivatives. V. Synthesis of hydrophilic polyamide by the polycondensation of β-amino acid derivatives

N-(Hydroxyalkyl) β-alanine ester which was obtained from amino alcohol and acrylate yielded polyamide at room temperature in the presence of a basic catalyst. Alkali and alkali earth metal alkoxides had a strong catalytic effect on the room-temperature polycondensation of N-(hydroxyethyl)-β-alanine esters. The catalytic activity of metal alkoxides decreased in the order: Li > Na > K > Cs and Ca > Zn > Mg. Aluminum and titanium alkoxide had a weak catalytic effect, while boron (III), tin (IV), antimony (V), and tellurium (VI) alkoxides did not show any catalytic activity for the polycondensation. It was also found that solvent had an effect on the course of the polycondensation of N-(hydroxyethyl)-β-alanine esters, and the highest molecular weight polymer was formed only in methanol solution. The solid-phase polycondensation of the low molecular weight prepolymer resulted in a high molecular weight polymer with an inherent viscosity of 1.0 in the presence of a catalytic amount of phosphoric acid. The polymer obtained is hydrophilic and its moisture absorption is more than twice that of nylon 6.     

Bismuth(III) chloride-catalyzed one-pot Mannich reaction: three-component synthesis of β-amino carbonyl compounds

A simple and efficient method has been developed for the one-pot Mannich reaction of β-amino carbonyl compounds from aromatic aldehydes, aromatic ketones and aromatic amines in the presence of a catalytic amount of bismuth trichloride.     

The synthesis of β-heteroarylamino-α,β-dehydro-α-amino acid derivatives via thiazolones

2-Alkoxy-4-heteroarylaminomethylene-5(4H)-thiazolones 4 were converted with various nucleophiles into β-heteroarylamino-α,β-dehydro-α-amino acid derivatives 11, 14, 15, 16, 17, 18 , and 19 . Reduction of 4 with sodium borohydride in ethanol saturated with gaseous ammonia afforded the corresponding β-heteroaryl-amino substituted alanyl amides 20 . Thiazoledione derivative 7a was transformed with sodium methoxide in methanol into 1-(4,6-dimethylpyrimidinyl-2)-4-mercaptocarbonylimidazol-2(3H)-one ( 8a ).