Synthesis of sequential polypeptides containing L-isoleucine for assignment of the far-i.r. band characteristic of isoleucyl in a peptide α-helix
Sequential co-polypeptides containing L-isoleucine and L-methionine or L-alanine (Met-Ile-Met)n, (Ala-Ile-Ala)n, (Ala-Ala-Ile-Ala)n, and (Ala-Ile-He-Ala)N have been synthesized by polycondensation of peptide N-hydroxysuccinimide (ONSu) esters with corresponding sequences of amino acids. The tripeptide ONSu esters were prepared by the conventional stepwise method for peptide synthesis, while the tetrapeptide ONSu esters were synthesized by a so-called ‘back-up procedure’ from the tripeptide ONSu esters. The sequential co-polypeptides with the α-helical conformation showed a common far-i.r. band near 453 cm–1, the strength of which changed in accordance with a change in the proportion of L-isoleucine in the polypeptides. Thus, the band at near 453 cm–1 was assigned to that characteristic of L-isoleucine in a peptide with an α-helical conformation.