Identification of N,N‐Dimethylproline as the N‐Terminal Blocking Group of Crithidia oncopelti Cytochrome c557

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Abstract
The N-terminal tryptic peptide of C. oncopelti cytochrome c557 X-Pro-Me3Lys-Ala-Arg, in which X represents an unknown N-terminal blocking group was characterized by electrophoresis at pH 2 and by 1H and 13C NMR. 1H-NMR spectra of the tryptic peptide suggested that the blocking group X was N,N-dimethylproline, although the electrophoretic mobility of the peptide suggested a larger MW. The peptides X-Pro-Me3Lys and X-Pro were generated by treatment of the tryptic peptide with thermolysin and carboxypeptidase, and the free-blocking group X was prepared by acid hydrolysis. Comparison of the 1H-NMR spectra of these peptides with spectra of synthetic N,N-dimethylproline and N,N-dimethylprolylproline demonstrated that the blocking group was N,N-dimethylproline. The 13C-NMR spectrum of the tryptic peptide was consistent with this conclusion although unambiguous assignments to all resonances could not be obtained because of the small amount of material available. The origin of the dimethylproline blocking group is discussed.