Total synthesis of bovine pancreatic ribonuclease A. Part 4. Synthesis of the protected tetraoctacontapeptide ester (positions 41–124)

Abstract

Commencing with the protected hexapentacontapeptide corresponding to the sequence Z(OMe)-(69–124)-OBzl of bovine pancreatic RNase A, chain elongation was carried out to the tetraoctacontapeptide, Z(OMe)-(RNase 41–124)-OBzl, by seven successive azide condensations of the peptide fragments Z(OMe)-Cys(MBzl)-Lys(Z)-Asn-Gly-NHNH-Troc (16), Z(OMe)-Asn-Val-Ala-NHNH₂(17), Z(OMe)-Val-Cys(MBzl)-Ser-Gln-Lys(Z)-NHNH₂(18), Z(OMe)-Asp(OBzl)-Val-Gln-Ala-NHNH-Troc (19), Z(OMe)-Glu(OBzl)-Ser-Leu-Ala-NHNH-Troc (20), Z(OMe)-Phe-Val-His-NHNH₂(21), and Z(OMe)-Lys(Z)-Pro-Val-Asn-Thr-NHNH₂(22). The Troc group was removed from fragments (16), (19), and (20) by treatment with Zn prior to each condensation. T.l.c. assessment of the homogeneity of the products was not possible, due to lack of a suitable solvent system. The ratios of newly incorporated amino-acids in the acid hydrolysate to phenylalanine, however, gave an important clue to purity. The purity of Z(OMe)-(RNase 41–124)-OBzl was confirmed by gel-filtration on Sephacryl S-200.