Fast-atom bombardment tandem mass spectrometry studies of organo-alkali-metal ions of small peptides. Competitive interaction of sodium with basic amino acid substituents.
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The fragment ions obtained from the [M + Na]+Ion of model peptides (formed by fast-atom bombardment desorption ionization) are rationalized by binding of the Na+ion to a specific site of the peptide. The dissociation reactions of the [M + Na]+ions of peptides containing tltratable R groups are consistent with the site of attachment of Na+being dependent upon the relative alkali-metal ion affinity of the basic side chain. Also, the proton and alkall-metal ion affinity of histidine is shown to be higher than expected based on the pKaof the imidazole N. The anomalously high proton and alkali-metal ion affinity is rationalized by formation of an ionic complex involving the imidazole and amide nitrogens. 1988, American Chemical Society. All rights reserved.
