The remarkable sensitivity to acid‐catalyzed peptolysis of peptide chains (endopeptolysis) having a succession of three <i>N</i>‐alkylated amino acid residues

Abstract

An investigation on the resistance against acids of over 30 peptides containing a repetition of N ‐alkylated amino acids has revealed the extremely facile cleavage of the peptide bond (endopeptolysis; ranking from a few minutes to 1 hour in Tfa) between the second and third residue in a sequence of three l ‐imino acid residues (called “triad”)‐ This is pronouncedly so if Pro is the head member of the triad. Triads become resistant against hydrolysis, however, when both the first and second positions are occupied by Pro, and even more so if the second member is d ‐MePhe. On the other hand, as an exception, diads of iminoacids preceded by Gly also are sensitive to hydrolysis. Two possible mechanisms are proposed, among which the one that is characterized by an intramolecular attack of the N ‐nucleophilic site of the first residue on the peptide bond between the second and third unit and thus forming a diketopiperazine‐onium intermediate, is our preference. This sensitivity towards acidolysis and acid‐catalyzed hydrolysis should not be overlooked, not only during the planning of peptide syntheses (having especially implications during acidic deprotection protocols), but it will compromise any strategy that makes use of strong acid conditions, e.g. in chiral determinations of N ‐alkylated amino acids in peptide hydrolysates, because acidic peptide breakdown is accompanied by appreciable stereo‐mutation. Finally, the weakened resistance of iminoacid peptide chains is obviously related to trans‐cis isomerization about the first peptide bond within the triads and suggests that endopeptidases may well act initially through a trans‐cis isomerase stage.