Chemical semisynthesis of aprotinin homologues and derivatives mutated in P' positions.

An extended concept for the replacement of amino acids in the P' region of aprotinin by chemical semisynthesis is presented. Either fragment condensation with dipeptides protected as tert-butyl ester or stepwise introduction of two single amino acid-tert-butyl esters into a partially esterified aprotinin derivative (with free Lys15-carboxyl group) lacking the amino acids Ala16 and Arg17 leads to aprotinin homologues and derivatives mutated in the P'1 and P'2 position. This method may complement the recently reported enzymatic synthesis by enabling access to aprotinin homologues and derivatives, which cannot be prepared enzymatically. The synthesis of [Ala17]BPTI and [seco-17/18]BPTI is described in detail.