Amide hydrogen exchange rates of model peptides in dimethyl sulphoxide solutions

Abstract

Proton Transfers from and to the nitrogen atom (s) α (or α and β) of a doubly protected dipeptide. Gly-Gly or PG 2. (or tripeptide : Gly-Gly-Gly or PG 3) in anhydrous DMSO was followed by DMNR under conditions of both acid (sulphuric) and base (tetramethylguanidine) catalyzed exchange. Two kinetic processes are observed, one catalyzed by the hydrogen ion and the other by the conjugate base (P-) of the peptide (P). The results at 25°C are summarized by the equation. rate of peptide proton exchange = k H[H +] + k B[P]/[H +], with k H = 6.3 : 17.1 : 8.85 M-ls -1 and k B = 1.45 ; 5.62 ; 9.55 × 10 -13s -1 for PG 2(α) and PG 3 (α and β), respectively. This allows us to predict maximum half-reaction times for NH-NH (or NH-ND) exchanges of ca. 10 4 mn at pH 7 to 8 in DMSO. pK values of N-protonation (pK NH 2 +) are estimated from log k H and pK values of N-deprotonation (pK NH) are measured by three independent methods. pK NH = 19.4 : 19.7 : 19.1 (± 0.2) for PG 2 and PG 3 (α and β). Compared to N-methylacetamide. pKNH 2 + and log k H of peptides are decreased by ca. two units, while pK NH and log k B are decreased and increased by ca. four and two units. respectively.