H n.m.r. spectrum from the flexible N‐terminal segment of Streptomyces subtilisin inhibitor

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Abstract
The 1H n.m.r. spectrum of Streptomyces subtilisin inhibitor shows a limited number of unusually sharp signals at room temperature. Some of these signals are assigned uniquely to protons of the side chains of the N-terminal segment, Aspl-Ala2-Pro3-Ser4-Ala5-Leu6-Tyr7-based on experiments of spin decoupling, pH titration, and enzymatic cleavage of the protein. Quantitative examination of these signals indicates that the N-terminal end of this protein is heterogeneous in that the protein contains a considerable fraction whose sequence starts with Ala2 rather than with Asp1. The pKa values for the amino groups of Asp1 and Ala2 exposed at the N-terminus are determined to be 8.9 ± 0.4 and 9.0 ± 0.1, respectively. Furthermore, examination of the line-widths of the methyl proton resonances of Ala2 and Ala5 residues indicates that the N-terminal peptide segment is free and undergoes rapid segmental motions in the order of 10−9s.

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