Polysarcosine-containing copolymers: Synthesis, characterization, self-assembly, and applications

13C‐NMR sequence analysis. 11. Anionic copolymerization of lactams

Sarcosine N-carboxyanhydride (Sar-NCA) was polymerized with 1,12-diaminododecane (DAD) or 1,13-diamino-4,7,10-trioxatridecane (DATT) as initiators, and telechelic blocks having two secondary amino endgroups were obtained. Their structure was proven by 1 H NMR and MALDI-TOF mass spectrometry. The SEC measurements indicated narrow molecular weight distribution with polydispersity indices in the range of 1.1-1.2. These difunctional polysarcosines were reacted with three different coupling agents: 4,4'-diisocyanatodiphenylmethane (MDI), N-hydroxysuccinimide sebacate, and sebacoyl bisimidazolide. Although the 1 H NMR spectra proved high conversions, only a moderate chain extension was observed. The MALDI-TOF mass spectra indicated a high extent of cyclization, which limited the chain growth.

Chain Extension and Cyclization of Telechelic Polysarcosines

Five N-protected tetrapeptide esters of the structure Gly-Pro-X-X*-O-methyl were synthesized in such a way that one of the two variable amino acid residues (X) was isotopically enriched in 15N (denoted by*). The variable amino acids are glycine, alanine, leucine, valine, and phenylalanine. For the natural abundance 15N-nmr spectra of these tetrapeptide derivatives in methylene chloride only the signals of the Gly-Pro trans isomer were found. In a 2:1 mixture of acetone and dimethylsulfoxide, signals for both the cis and trans isomers were observed. Three of the five tetrapeptide derivatives show cis/trans splitting of all four nitrogen signals. The 15N-nmr spectra of Z-Pro-Pro-OH and of (D,L-proline)n were measured in a 2:1 mixture of acetone and dimethylsulfoxide as well as in water. The effects of solvents and neighboring residues and the influence of the cis/trans isomerism on the nmr spectra are discussed. The determination of the cis/trans equilibria and the assignment of the 15N-nmr signals of all oligopeptides were achieved by selective isotopic enrichment and by means of 13C-nmr spectra.

15N‐nmr spectroscopy. 20. Cis/trans isomerism and neighboring residue effects of proline‐containing peptides

The 15N NMR spectra of various oligopeptide derivatives of the ZXYYOMe structure, where X and Y are variable amino acids and Z is the benzyloxycarbonyl group, were measured in several protic and aprotic solvents. The shift difference of the 15N of the YY and XY bond (neighbouring residue effect) is discussed with respect to the nature of X and Y with respect to the solvent. Oligopeptides of the ZXYYOH and nH3XYYOMe structures were compared with the Z-peptide esters to investigate the spectroscopic influence of the protecting groups. The methyl ester hydrochlorides of the 25 most common amino acids were measured in water and DMSO to elucidate the solvent dependence of the substituent effects. Moreover, the methyl ester hydrochlorides were compared with Z-amino acids and N-acetyl-amino acid methyl esters in DMSO to establish whether the substituent effects depend on the nature of the amino acid derivatives. In this connection the assignments of the serine, threonine and glycine signals are discussed with respect to silk proteins. Furthermore, the assignments of the signals of copolypeptides by comparison with oligo- and homo-polypeptides are discussed. Finally, it was demonstrated that intramolecular H bonds cause downfied shifts of 7–10 ppm of the acceptor amide groups.

15N NMR spectroscopy. 30—structure/shift relationships of oligopeptides and copolypeptides, including gramicidin S

The natural abundance 15N nmr spectra of linear polysarcosine (DP = 35) has been recorded in Me2SO and H2O solution. Because of cis/trans isomerization at the peptide bond, a broad signal with several splittings was observed. These splittings appear to reflect the influence of three peptide bonds on a single N atom. The 15N signals from the sequence polypeptides (β-Ala-Sar-Gly)n and (β-Ala-Sar-D,L-Ala)n also show a cis/trans splitting, as well as chemical shifts which are dependent on the peptide sequence. The tertiary nitrogen of the sarcosyl residue has a T1 relaxation time which is longer than the T1 for secondary nitrogens of the other amino acids. The nuclear Overhauser effect is also discussed.

15N nmr spectroscopy. I. Polysarcosine and related sequence polypeptides.

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15N nmr spectroscopy. I. Polysarcosine and related sequence polypeptides.