Solid‐phase peptide synthesis by ion‐paired α‐chymotrypsin in nonaqueous media

Solid‐phase synthesis of dipeptides in low‐water media was achieved using AOT ion‐paired α‐chymotrypsin solubilized in organic solvents. Multiple solvents and systematic variation of water activity, a w , were used to examine the rate of coupling between N‐α‐benzyloxycarbonyl‐ L ‐phenylalanine methyl ester (Z‐Phe‐OMe) and leucine as a function of the reaction medium for both solid‐phase and solution‐phase reactions. In solution, the observed maximum reaction rate in a given solvent generally correlated with measures of hydrophobicity such as the log of the 1‐octanol/water partitioning coefficient (log P) and the Hildebrand solubility parameter. The maximum rate for solution‐phase synthesis (13 mmol/h g‐enzyme) was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a w of 0.30. For the synthesis of dipeptides from solid‐phase leucine residues, the highest synthetic rates (0.14–1.3 mmol/h g‐enzyme) were confined to solvent environments that fell inside abruptly defined regions of solvent parameter space (e.g., log P > 2.3 and normalized electron acceptance index <0.13). The maximum rate for solid‐phase synthesis was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a w of 0.14. In 90/10 and 70/30 (v/v) isooctane/tetrahydrofuran environments with a w set to 0.14, seven different N‐protected dipeptides were synthesized on commercially available Tentagel support with yields of 74–98% in 24 h. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 81: 809–817, 2003.