Solvent effects on coupling yields during rapid solid-phase synthesis of CGRP(8-37) employing in situ neutralization

The success of solid-phase peptide synthesis is often dependent upon solvation of the resin and the growing resin-bound peptide chain. We investigated the relationship between solvent properties and solvation of the resin and peptide-resin in order to obtain satisfactory coupling yields for the rapid solid-phase peptide synthesis, using butyloxycarbonyl-(Boc)-amino acid derivatives, of human-α-calcitonin gene-related peptide(8-37) (CGRP(8-37)). Solvation of (p-methylbenzhydrylamine)copoly(styrene-1% divinylbenzene (DVB) (resin) and resin covalently bound to the fully protected amino acid sequence of CGRP(8-37) (peptide-resin) was correlated to solvent Hildebrand solubility (6) and hydrogen-bonding (δ_h) parameters. Contour solvation plots of δ_h vs. δ revealed maximum solvation regions of resin and peptide-resin. Maximum resin solvation occurred with N-methylpyrrolidinone (NMP), NMP:dimethylsulfoxide (DMSO) (8:2) and DMSO. Inefficient solvation of the peptide-resin occurred with these solvents and resulted in poor syntheses with average coupling yields of 78.1, 88.9 and 91.8%, respectively. Superior peptide-resin solvation was obtained using dimethylacetamide (DMA) and dimethylformamide (DMF), resulting in significantly higher average coupling yields of 98.0 and 99.5%, respectively. Thus, the region of maximum peptide-resin solvation shifts to solvents with higher δ_h values. DMF provided the most effective peptide-resin solvation and was the only solvent from which CGRP(8-37) was obtained as a single major product in the crude cleaved material.