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Proton nmr studies of protected alpha amino iso butyric acid containing peptides chemical shift nonequivalence of benzyloxycarbonyl methylene protons



Proton nmr studies of protected alpha amino iso butyric acid containing peptides chemical shift nonequivalence of benzyloxycarbonyl methylene protons



International Journal of Peptide & Protein Research 18(8): 208-213



The benzylic methylene protons in a large number of benzyloxycarbonyl .alpha.-aminoisobutyric acid (Z-Aib) containing peptides, show chemical shift nonequivalence. The magnitude of the geminal nonequivalence is correlated with the involvement of the urethane carbonyl group, in an intramolecular H-bond. Studies of the model compounds Z-Aib-Aib-Ala-NHMe, and Z-Aib-Aib-Aib-Pro-OMe clearly establish the presence of intramolecular H-bonds, involving the urethane CO group. In both compounds marked anisochrony of the benzylic methylene protons is demonstrated. In Z-Aib-Aib-Pro-OMe, where a 4 .fwdarw. 1 H-bonded .beta.-turn is not possible, the benzylic-CH2-protons appear as a singlet in CDCl3 and have a very small chemical shift difference in (CD3)2SO. The observation of such nonequivalence is of value in establishing whether the amino terminal Aib-Pro .beta.-turn is retained in large peptide fragments of alamethicin.

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Accession: 006215925

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