Polymerization of N-carboxy–amino acid anhydrides in the solid state. II. Relation between polymerizability and molecular arrangement in L-leucine NCA and L-alanine NCA crystals

The polymerizability of N-carboxy–amino acid anhydrides (NCAs) of L -leucine and L -alanine was examined in the solid state and in solution. L -leucine NCA shows much higher reactivity in the solid state (when immersed in hexane) than in solution (in acetonitrile), but the opposite is true for L -alanine NCA. However, the two NCAs give similar values of apparent activation energy in each polymerization system. Rather high-molecular-weight polypeptides were obtained in the polymerization of L -leucine NCA in the solid state compared with those obtained in solution, while the molecular weight of polymers obtained from L -alanine NCA was higher in solution than in the solid state. IR spectra showed that α helices form mainly in the polymerization of both L -leucine NCA and L -alanine NCA in the solid state; a small amount of the β structure forms in the latter polymerization. X-ray diffraction and electron microscopy revealed that L -leucine NCA polymerizes predominantly along the c axis in the crystal, while the polymer chains grow in random directions in the crystal of L -alanine NCA. The difference can be explained by the molecular arrangement in the crystal. There are two requirements for high reactivity in the solid state: the five-membered rings of the monomer must form a layer structure and the polymer must occupy nearly the same space as the reacting monomer.     

Polymerization of N-carboxy-amino acid anhydrides in the solid state. I. Polymerizability of the various α-amino acid NCAs in the solid state

The solid-state polymerization of various α-amino acid NCAs was investigated and the results were compared with those obtained by heterogeneous polymerization in acetonitrile. Essential differences were found in the polymerizability of the NCAs in these two systems. In the solid state, L-leucine NCA was the most reactive among the NCAs examined, and its reactivity was even higher than in the precipitation polymerization of acetonitrile solutions. On the other hand, glycine NCA was the most inert among the NCAs examined in the solid state. The difference between the reactivities of glycine NCA and L-alanine NCA was interpreted in terms of their crystal structures. Several kinetic features of the solid-state polymerization were studied on γ-benzyl-L -glutamate NCA.     

Rapid and Mild Synthesis of Amino Acid N‐Carboxy Anhydrides: Basic‐to‐Acidic Flash Switching in a Microflow Reactor

Polymerization of N‐carboxy anhydrides (NCAs) is the primary process used to prepare polypeptides. The synthesis of various pure NCAs is key to the efficient synthesis of polypeptides. The only practical method that can be used to synthesize NCAs requires harsh acidic conditions that make acid‐labile substrates unusable and results in an undesired ring opening of NCAs. Basic‐to‐acidic flash switching and subsequent flash dilution technology in a microflow reactor was used to demonstrate the synthesis of NCAs. It is both rapid (0.1 s) and mild (20 °C) and includes substrates containing acid‐labile functional groups. The basic‐to‐acidic flash switching enabled both an acceleration of the desired NCA formation and avoided the undesired ring opening of NCAs. The flash dilution precluded the undesired decomposition of acid‐labile functional groups. The developed process allowed the synthesis of various NCAs which cannot be readily synthesized using conventional batch methods.     

Large-scale synthesis of α-amino acid-N-carboxyanhydrides

Hetero- and homopolymers prepared from α-amino acid-N-carboxyanhydrides (NCAs) monomers are widely useful products. The preparation of pure NCA monomers has been extensively studied in the past. Purification methods including repeated crystallizations, extraction, and flash column chromatography have been devised. However, these methods are not easily amendable to large-scale NCA preparations. This article describes the synthesis of numerous highly purified NCAs on a >100?g scale using a simple filtration step through diatomaceous earth (celite). The resulting NCAs provided polyethylene glycol (PEG)–amino acid triblock polymers devoid of low-molecular-weight by-products that were routinely observed when unfiltered batches of NCAs were used. Also disclosed is the preparation of NCAs at ambient temperature. Traditionally, NCA reactions using a phosgene source are heated. This study shows these reactions can be driven by the slight exotherm that forms upon reagent mixing. This eliminates the need for an external heating source, simplifying large-scale reactions.     

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.     

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=Aminoisobutyric Acid NCA: Interpretation According to the Hypothesis of a Multiple Mechanism

The basic salt-initiated polymerization of α-aminoisobutyric acid NCA in acetonitrile was studied using various alkaline alcoholates and in the presence or absence of various protic (very weak acid) additives. The cation effect observed was the one expected from either the N-carboxy-α-amino acid anhydride (NCA) anion mechanism (activated monomer mechanism) or the alcoholate anion mechanism (Blout's mechanism). The anion effect appeared to be abnormal for the former mechanism, but did not agree nor disagree substantially with the latter. Furthermore, such additives as methanol (conjugate acid of the initiator), 3-methylhydantoin, 2-oxazolidone, and N-acetylglycine NCA (prototype of the chain growing through the NCA anion mechanism) considerably enhanced the rate of initiation. A still higher rate of initiation could be obtained by the combined use of two additives. IR and DTA analyses of the polymerization products showed the formation of 5,5-dimethylhydan-toin-3-isobutyric acid in the sample using the alcohol-free initiator, hence the NCA anion mechanism is operative. This acid was absent in the low DP polymer obtained in the presence of added methanol, and this agrees with the alcoholate anion mechanism without, however, proving it. Thus, while only part of the results could be explained by one or the other of the previous interpretations, all the experimental facts were accounted for, without noticeable contradiction, by the hypothesis of a multiple mechanism which contains both interpretations among its elements.     

Homochiral oligopeptides via a lattice-controlled polymerisation in racemic crystals of valine N-carboxyanhydride suspended in aqueous solutions

As part of our program on the biochirogenesis of homochiral peptides, we report the formation of racemic parallel (p) beta sheets composed of alternating R and S chains of up to 14-15 repeat units of the same handedness through the polymerisation of (R,S)-valine N-carboxyanhydride (NCA) crystals suspended in aqueous solutions of a primary amine as the initiator. The occurrence of such a lattice-controlled reaction accompanied by a reduction in volume implies the operation of a mechanism that differs from that of the common solid-state polymerisation in vinyl systems. The topotacticity of the reaction is explained through the operation of a multistep nonlinear process comprising lattice control coupled with an asymmetric induction in the formation of homochiral short peptides followed by their self-assembly into racemic p beta sheets, which operate as efficient templates in the ensuing process of enantioselective chain elongation at the polymer/crystal interface. The composition of the diastereoisomeric libraries of oligopeptides was determined by MALDI-TOF and MALDI-TOF-TOF MS analyses of the products obtained from monomers enantioselectively labelled with deuterium. The structure of the p beta sheets could be determined by initiating the polymerisation reaction with water-soluble esters of enantiopure alpha-amino acids or short peptides. The same reaction performed with the monomer crystals suspended in hexane yielded a complex mixture of diastereoisomeric oligopeptides, thus highlighting the indispensable role played by water in controlling the stereoselectivity of the reaction. By contrast, polymerisation of (R,S)-leucine NCA crystals, with a different packing arrangement that presumably does not endorse the formation of periodic peptide templates, yielded, both in aqueous and hexane suspensions, libraries of peptides dominated by heterochiral diastereoisomers.     

Terpolymerization kinetics of amino acid N‐carboxy anhydrides

Based on their versatility with respect to amino acid type and sequence, polypeptides have become attractive for a number of biological applications such as drug delivery, biomineralization, and drugs. N‐carboxy anhydride (NCA) polymerization is a convenient way to rapidly prepare high‐molecular weight polypeptides with good control over molecular weight and polydispersity. However, the kinetics of the incorporation of NCA monomers into copolypeptides during random copolymerization are poorly understood. Here, kinetic data is presented that allows insight into the NCA polymerization of a terpolymer composed of three commercially relevant amino acids, namely, glutamic acid, lysine, and tyrosine. Furthermore, kinetic data and copolymerization parameters from the copolymerization of binary mixtures of these three amino acid NCAs is used to make predictions of the terpolymer composition. This study provides access to the information necessary to prepare functional copolypeptides with better‐defined sequence architecture that will be essential for the future development of polypeptide‐based materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1228–1236     

Analysis of aromatic amines in water samples by liquid-liquid-liquid microextraction with hollow fibers and high-performance liquid chromatography

N-Carboxyanhydrides of amino acids (NCAs) are very reactive monomers able to polymerize into oligopeptides. They are assumed to be prebiotic precursors of the first polypeptides. Few reports have been published on the study of NCA polymerization in aqueous solution. In this work, a kinetic study focused on the hydrolysis of NCA and its coupling with amino acids and homopeptides (up to tripeptide) was carried out, taking L-valine derivatives as model compounds. For that purpose, capillary electrophoresis appeared to be an effective and reliable technique for the measurement of the kinetic constants. The electrophoretic separation conditions, the procedure for stopping NCA reactivity, as well as the conditions of reaction are discussed in detail. We report the variation of the kinetic constant of the coupling reaction of the NCA of valine with an oligovaline as a function of its degree of polymerization. Finally, a temperature study also allowed us to estimate the activation energies associated with the NCA of valine hydrolysis and its coupling reaction with valine.     

Molecularly Imprinted Polymers with Mandelic Acid as Template

In this report, molecularly imprinted polymers (MIPs) with racemic and L-mandelic acid as the templates were synthesized. Several structural analogues were chosen to study the selectivity of the MIPs prepared using 4-vinylpyridine (4-VP) as the monomer (MIP_4-VP). At the same time, the chromatographic behaviors of the MIP using acrylamide (AA) as the monomer (MIP_AA) in two different mobile phases, acetonitrile and hexane/ethyl acetate (1:1, v/v), were investigated and the separation of the enantiomers was tried. The results indicated that MIP_4-VP had great retention with a template comparable to other analogues. However, no difference in the k′ values of the enantiomers was observed, as the interaction between the polymer and the enantiomers was mainly dependent on the acidity, which showed no difference. The MIP_AA showed a more obvious imprinting effect for the template in hexane/ethyl acetate (1:1, v/v) than in acetonitrile. But the separation of the enantiomers was not achieved even in the less polar solvent, though there was an obvious difference between the retention time of L-MA and D-MA on the L-MA imprinted polymer.     

Synthesis and Crystal Structure of Amino Acid Modified NDI Lanthanide Coordination Complex

The synthesis and crystal structure of γ‐aminobutyric acid naphthalene diimides derivative with the La^III coordination complex, [La(L)(DMF)Cl]_n, was reported, which is a twofold interpenetrating metal‐organic framework architecture. The coordination polymer was characterized by elemental analysis, infrared spectroscopy, thermal gravimetric analysis, and single‐crystal X‐ray diffraction. The optical properties of the crystallized complex were investigated both in solution and the solid state.     

The peptide formation mediated by cyanate revisited. N-carboxyanhydrides as accessible intermediates in the decomposition of N-carbamoylamino acids

Similar to many ureas, N-carbamoylamino acids were shown to be hydrolyzed in aqueous solution through elimination mechanisms at close to neutral pH, the nucleophilic attack of water being a minor process. Two competing elimination mechanisms can take place involving either cyanate or isocyanate transient intermediates. Peptide formation was observed and attributed to the latter pathway through the intermediacy of amino acid N-carboxyanhydride (NCA). Eventually, cyanate and its precursors (including urea) unexpectedly behave as amino acid activating agents because of their ability in amino acid carbamoylation. Owing to its ability to generate a background prebiotic production of NCAs on the primitive Earth, this reaction is suggested to have contributed to the origin of life process.     

Chiral Amplification of Oligopeptides in the Polymerization of α‐Amino Acid N‐Carboxyanhydrides in Water

This article is concerned with the chiral amplification of oligopeptides formed in the polymerization of chiral, nonracemic mixtures of the N‐carboxyanhydride (NCA) of Leu and Glu in aqueous solution. Labeling (deuteration) of one enantiomer and reversed‐phase and normal‐phase high‐performance liquid chromatography mass spectrometry (RP‐ and NP‐HPLC/MS, respectively) were used to determine the product distribution, both with respect to oligopeptide chain length and stereoisomer distribution. Starting the polymerization with an enantiomeric excess (ee) of 20% of the L ‐enantiomer (L ‐amino acid/D ‐amino acid 6 : 4) leads to an ee of 73% at the level of the homochiral enantiomeric (Leu)₅, and of 71% at the level of the homochiral enantiomeric (Glu)₇. For the Leu system and in the presence of a solid support (quartz), the ee reached values of up to 100%. We argue that such amplification processes could be relevant for the chemical evolution towards single‐handedness.     

Aqueous Ring‐Opening Polymerization‐Induced Self‐Assembly (ROPISA) of N‐Carboxyanhydrides

Reported here is the first aqueous ring‐opening polymerization (ROP) of N‐carboxyanhydrides (NCAs) using α‐amino‐poly(ethylene oxide) as a macroinitiator to protect the NCA monomers from hydrolysis through spontaneous in situ self‐assembly (ISA). This ROPISA process affords well‐defined amphiphilic diblock copolymers that simultaneously form original needle‐like nanoparticles.     

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.     

Noncoordinating anions in carbocationic polymerization

The initiation and catalysis of isobutylene polymerization from several new metallocene and nonmetallocene initiator-catalysts that contain the noncoordinating anions (NCA), B(C₆F₅)₄⁻ and RB(C₆F₆)₃⁻, is reported. Application of these initiator-catalysts is extended to styrenics and vinyl ethers. The NCA does not contribute to termination and can be used in low concentrations compared with conventional Lewis acids. These qualities provide for isobutylene polymerizations that yield low M_n oligomers or high M_n polymer, dependent upon the initiator and polymerization conditions. Mechanistic aspects of initiation, transfer and termination as well as the participation of adventitious water are considered for each class of initiator-catalyst. The influence of the NCA on the stereoregularity of cationic styrene polymerization is also considered. NCAs do not cause the stereospecific carbocationic polymerization of styrene. We suggest that under conditions not conducive to carbocationic polymerization, NCA/metallocenes mediate the coordination polymerization of styrene. © 1997 John Wiley & Sons, Inc.     

Single‐Step,Rapid, and Mild Synthesis of β‐Amino Acid N‐Carboxy Anhydrides Using Micro‐Flow Technology

β‐Amino acid N‐carboxy anhydrides (β‐NCAs) are rarely used in the synthesis of β‐peptides, which is due mainly to the poor availability of these potentially useful substrates. Herein, we describe the heretofore challenging synthesis of β‐NCAs via a single‐step, rapid, and mild formation using pH flash switching and flash dilution, which are aspects of micro‐flow technology. We synthesized 15 β‐NCAs in good to excellent yields that included acid‐labile β‐NCAs that cannot be readily synthesized using the conventional Leuchs approach. Scaled‐up synthesis using this process can be readily achieved via continuous operation.     

L-亮氨酸衍生物手性氨基醇的合成及其对布洛芬和扁桃酸对映异构体的手性识别

对甲基苯胺经甲基化、甲酰化得到5,N,N-三甲基-2-氨基苯甲醛;对L-亮氨酸经酯化、格氏反应得到二齿手性氨基醇.二齿手性氨基醇与上述醛经缩合、还原反应,得到三齿手性氨基醇.产物结构经IR,MS和1H NMR等进行了表征;通过改变主客体的浓度及手性羧酸的纯度,运用1H NMR分别考察了主体二齿手性氨基醇、三齿手性氨基醇对客体布洛芬和扁桃酸对映异构体的手性识别能力.结果表明:当主客体物质的量之比为1:1时,三齿手性氨基醇对布洛芬消旋体的a位甲基质子及扁桃酸消旋体的a位质子分别产生11.2和9.2 Hz的化学位移差值.     

Rethinking Cysteine Protective Groups: S‐Alkylsulfonyl‐l‐Cysteines for Chemoselective Disulfide Formation

The ability to reversibly cross‐link proteins and peptides grants the amino acid cysteine its unique role in nature as well as in peptide chemistry. We report a novel class of S‐alkylsulfonyl‐l ‐cysteines and N‐carboxy anhydrides (NCA) thereof for peptide synthesis. The S‐alkylsulfonyl group is stable against amines and thus enables its use under Fmoc chemistry conditions and the controlled polymerization of the corresponding NCAs yielding well‐defined homo‐ as well as block co‐polymers. Yet, thiols react immediately with the S‐alkylsulfonyl group forming asymmetric disulfides. Therefore, we introduce the first reactive cysteine derivative for efficient and chemoselective disulfide formation in synthetic polypeptides, thus bypassing additional protective group cleavage steps.