15N-NMR spectroscopy. 34. Stereospecificity of the polymerization of D,L-leucine N-carboxyanhydride and γ-OMe-D,L-glutamic acid N-carboxyanhydride

D ,L -Leucine-N-carboxyanhydride (D ,L -Leu-NCA) and γ-methyl-D ,L -glutamic acid N-carboxyanhydride (γ-OMe-D ,L -Glu-NCA) were synthesized with ca. 2.5% ¹⁵N enrichment. Their polymerizations were conducted under a variety of conditions using benzylamine, triethylamine potassium tert-butanolate, and pyridine as initiators. The 40.55-MHz ¹⁵N-nmr spectra of the resulting stereocopolypeptides measured in trifluoroacetic acid display at least four signals, representing the isotatic, syndiotactic, and two heterotactic triads. From the signal intensities it was concluded that these NCAs behave nearly identically. With benzylamine initiation the formation of isotactic blocks is slightly favored, and they are still more predominant when strong bases are used as initiators. Initiation by pyridine favors the formation of syndiotactic sequences. However, in all cases the average lengths of the stereoblocks never exceeded 4 monomer units. The low stereospecificity of most polymerizations of D ,L -NCAs is confirmed by the high degree of solubility of the resulting poly(D ,L -amino acids) in aprotic solvents. Penultimate effects are weak or absent, so that most polymerizations follow Bernoullian type statistics. Deviations from these statistics were found for polymerizations in pyridine.     

Stereochemistry of the incorporation of valine methyl groups into methylene groups in cephalosporin C.

'Chiral methyl valines', i.e. samples of valine labelled stereospecifically in the methyl groups with 2H and 3H, were incorporated into cephalosporin C by a suspension of washed cells of Cephalosporium acremonium. Analysis by 3H n.m.r. of the cephalosporin C produced showed that the conversion of the 3-pro-S-methyl group of valine into the acetoxymethyl side-chain was a highly stereospecific process. By contrast, conversion of the 3-pro-R-methyl group into the endocyclic methylene group of the dihydrothiazine ring was shown to proceed by a non-stereospecific process.     

Stereochemistry of blennin A and blennin D from Lactarius blennius

From the ethanolic extract of Lactarius blennius, a new sesquiterpene blennin D (2-hydroxyblennin A) was isolated and its structure elucidated together with the stereochemistry of the already known blennin A. Treatment of blennin A mesylate with DBU afforded a compound with a new skeleton.     

Structural characterization of a lipase-catalyzed copolymerization of epsilon-caprolactone and D,L-lactide

The copolymerization of epsilon-caprolactone (epsilon-CL) and d,l-lactide catalyzed by Candida antarctica lipase B was studied. Copolymerizations with different epsilon-CL-to-lactide ratios were carried out, and the product was monitored and characterized by MALDI-TOF MS, GPC, and (1)H NMR. The polymerization of epsilon-CL, which is normally promoted by C. antarctica lipase B, is initially slowed by the presence of lactide. During this stage, lactide is consumed more rapidly than epsilon-CL, and the incorporation occurs dimer-wise with regard to the lactic acid (LA) units. As the reaction proceeds, the relative amount of CL units in the copolymer increases. The nonrandom copolymer structure disappears with time, probably due to a lipase-catalyzed transesterification reaction. In the copolymerizations with a low content of lactide, macrocycles of poly(epsilon-caprolactone) and copolymers having up to two LA units in the ring were detected.     

Polymerization of L- and DL-phenylalanine N-carboxyanhydride by poly(N-methyl-L-alanine)

Polymerizations of L - and DL -phenylalanine N-carboxyanhydride in nitrobenzene by poly (N-methyl-L -alanine) of varying degrees of polymerization (n = 1–30) were investigated. Poly(N-methyl-L -alanine) was prepared by the polymerization of N-methyl-L -alanine NCA with N-methyl-L -alanine diethylamide and the degree of polymerization was controlled by the molar ratio [NCA]/[Catalyst] + 1. This polymer was shown to be an asymmetrically selective catalyst which polymerized L -phenylalanine NCA at a faster rate than DL -phenylalanine NCA. With increasing degree of polymerization the stability of the secondary structure of poly(N-methyl-L -alanine) increased. This was confirmed by circular dichroism spectra. However, the degree of asymmetric selection did not increase as the stability of the secondary structure of poly(N-methyl-L -alanine) increased. These findings indicate that the interaction of a growing polypeptide in an ordered structure with NCA molecules prior to the reaction does not lead to an asymmetric selection, and that the mechanism of the asymmetric selection by poly(N-methyl-L -alanine) should be different from those proposed so far.     

Stereochemistry of the kynureninase reaction

The steric course of the replacement of the anthranilyl group of kynurenine by hydrogen was determined by conversion of (2S,3R)- and (2S,3S)-[3-3H]kynurenine into alanine with kynureninase in D2O, followed by chirality analysis of the alanine methyl group. To minimize enolization, the labeled substrates were generated in situ from the corresponding stereospecifically tritiated tryptophan species. The result, replacement in a retention mode, together with the finding that tritium from the alpha-position of the substrate is recycled and appears both at C alpha and C beta of the product suggests a single base mechanism and an active site geometry of the pyridoxal phosphate-substrate complex in which H alpha and the beta-substituent are syn oriented.     

Synthesis of some aminoacyl-AH-sepharoses via the N-carboxyanhydride method (author's transl)]

(Aminoacyl)n=1-10-AH-Sepharoses can easily be prepared by the N-carboxy anhydride method from amino acid N-carboxy anhydrides and AH-Sepharose 4B at pH 10.2, 0degreesC in aqueous borate buffer. Syntheses of cystinyl-, cysteinyl-, isoleucyl-, leucyl- and phenylalanyl-AH-Sepharoses are described. An improved preparation for cystine bis(N-carboxy anhydride) is presented.     

Stereochemistry of collagen

This review article, based on a lecture delivered in Madras in 1985, is an account of the author's experience in the working out of the molecular structure and conformation of the collagen triple-helix over the years 1952-78. It starts with the first proposal of the correct triple-helix in 1954, but with three residues per turn, which was later refined in 1955 into a coiled-coil structure with approximately 3.3 residues per turn. The structure readily fitted proline and hydroxyproline residues and required glycine as every third residue in each of the three chains. The controversy regarding the number of hydrogen bonds per tripeptide could not be resolved by X-ray diffraction or energy minimization, but physicochemical data, obtained in other laboratories during 1961-65, strongly pointed to two hydrogen bonds, as suggested by the author. However, it was felt that the structure with one straight NH...O bond was better. A reconciliation of the two was obtained in Chicago in 1968, by showing that the second hydrogen bond is via a water molecule, which makes it weaker, as found in the physicochemical studies mentioned above. This water molecule was also shown, in 1973, to take part in further cross-linking hydrogen bonds with the OH group of hydroxyproline, which occurred always in the location previous to glycine, and is at the right distance from the water. Thus, almost all features of the primary structure, X-ray pattern, optical and hydrodynamic data, and the role of hydroxyproline in stabilising the triple helical structure, have been satisfactorily accounted for. These also lead to a confirmation of Pauling's theory that vitamin C improves immunity to diseases, as explained in the last section.     

Polymerization of optical isomers of phenylalanine N-carboxyanhydride by various secondary amines

In the polymerization of phenylalanine NCA initiated by some secondary amines, the two enatimorphs of phenylalanine NCA were polymerized with the same rate, which was almost twice as high, as that found for the racemic mixture. This stereoselectivity was observed only when the polymerization was initiated by secondary amines which are sterically crowded and reluctant to undergo a nucleophilic addition to NCA. Poly(DL -phenylalanine) produced in the stereoselective polymerization had a higher molecular weight than that produced in nonstereoselective polymerization. These findings point to the possibility that the stereoselectivity arises only in those polymerizations which are propagated by the activated monomers and not in the propagation involving the terminal amine of the growing polymer. A possible mechanism for the stereoselective polymerization is proposed and examined.     

Stereochemistry of spathulin

The stereochemistry of the pseudoguaianolide spathulin has been determined by NMR spectrometry at 270 mHz.     

Regularly alternating L,D-peptides. III. Hexacyclic peptides from valine or phenylalanine

In the present paper we describe the single-crystal x-ray analyses of two cyclic hexapeptides containing an equal number of alternating L,D-residues as putative analogues of the metal binding compounds, enniatin and beauvericine. Both the molecules of c(L-Val-D-Val)3 and C(L-Phe-D-Phe)3 retain in the solid state the center of symmetry and crystallize with six and eight trifluoroacetic acid molecules, respectively. The peptides are strongly hydrogen bonded to the solvent molecules. We estimate, on the basis of the molecular geometry and spatial arrangement of the peptide carbonyl groups and in comparison with other metal binding cyclic peptides, the ability of these molecules to interact with metal ions as 1:1 complexes.