13C NMR‐sequenzanalyse, 9. 1HNMR‐ und 13C NMR‐spektroskopische Untersuchungen zur cis/trans‐Isomerie bei niedermolekularen und polymeren Derivaten des Sarkosins

Abstract

AbstractThe cis/trans‐isomerism of N‐formylsarcosine (1), its trimethylsilyl ester (3) and of N‐acetyl‐, N‐propionyl‐ as well as N‐pivaloyl sarcosine methyl ester (2a–c) was investigated by 1H and 13C NMR spectroscopy in various solvents. It was found that bulky N‐acyl groups favour the trans form, so that in the case of the pivaloyl group no cis form is detectable. The nuclear Overhauser effect of the formyl proton in 1 and 3 was determined in various solvents and the signals of cis and trans form could be assigned thereby unambiguously. The position of cis and trans signals relative to each other was found to be dependent on solvent and molecular structure. 1H and 13C NMR signals of polysarcosine (10) show in dimethyl sulfoxide more than four splittings, so that the influence of three peptide bonds on one signal is detectable. In water, formic acid and trifluoroacetic acid 1H NMR signals of 10 show only simple cis/trans splittings which demonstrate that trans bonds are preferred in these solvents. Sequence polypeptides like (‐β‐Ala‐Sar‐Gly‐)n (5a), (‐β‐Ala‐Sar‐D,L‐Ala‐)n (5b), or (‐β‐Ala‐Sar‐β‐Ala‐)n (5c) can be characterized by 13C NMR but not by 1H NMR spectroscopy. Only 13C NMR signals of the β‐Ala residue attached to the sarcosyl nitrogen show cis/trans splitting, whereas amino acids attached to the sarcosyl CO group do not. The free energy of activation (ΔG) for the rotation round the CO‐Sar bond was determined for some monomeric sarcosine derivatives as well as for 10 and its sequence polypeptides. Nearly identical values were found (ΔG = 75,3–76,2 kJ/mol (18,0–18,2 kcal/mol)) for all products.