Factors That Influence Acyl Proline Cleavage by Sodium in Liquid Ammonia

Abstract

The reductive cleavage of aminoacyl proline bonds by sodium in liquid ammonia was compared by a simple chromatographic technic for various protected and unprotected analogs of the C‐terminal tetrapeptide of the insulin B‐chain, ‐Thr‐Pro‐Lys‐Ala‐OH. The literature suggests that the Thr‐Pro bond in this peptide is especially sensitive to Na‐NN 3, compared to various other acyl proline bonds. The present study shows that this is true regardless of whether the sidechains, the α‐amino, and the α‐carboxyl groups are protected or free when they are introduced into liquid ammonia; there is no difference in the amount of reductive Thr‐Pro cleavage despite the different numbers of protons released into liquid ammonia by the functional groups of the residues surrounding the Thr‐Pro bond. The C‐terminal tetrapeptide and its blocked derivatives, prepared by solid phase synthesis, were reduced by Na‐NH 3 to the same extent as similarly synthesized B‐chains and natural B‐chain. Thr‐Pro cleavage could be minimized by sharply limiting the amount of sodium and the time of exposure of the peptide to it. The results are therefore useful in planning synthetic approaches to the B‐chain. The Ser‐Pro bond shares the lability of the Thr‐Pro bond to Na‐NH 3 reduction, whereas the Val‐Pro bond is only half as labile.