Combined solid‐phase/solution synthesis of large ribonuclease A C‐terminal peptides containing a non‐natural proline analog*

Abstract: Three large peptides corresponding to the 65–124 (60‐mer), 72–124 (53‐mer), and 77–124 (48‐mer) sequence of bovine pancreatic ribonuclease A (RNase A) were assembled from either two or three shorter protected peptide fragments by chemical coupling in solution. The fragments were synthesized manually by 9‐fluorenylmethyloxycarbonyl (Fmoc)‐based solid‐phase peptide chemistry in plastic syringes, and subsequently purified by normal‐phase high‐performance liquid chromatography on a silica gel column. The main aim of this work was to incorporate sterically hindered l ‐5,5‐dimethylproline (dmP) as a substitute for Pro⁹³ into the sequence of RNase A in order to constrain the –Tyr⁹²‐Pro⁹³– peptide group to a single cis‐conformation.