The importance of each side chain of a cross-linked interfacial peptide inhibitor of HIV-1 protease was evaluated using an alanine scanning approach. Whereas the parent inhibitor has an IC50 value of 350 nM, values for the mutations reported here range from 280 - 9200 nM. The relative importance or each residue was thus assigned and correlated to the solvent accessible surface area (SASA) exposed upon mutation. (C) 1999 Elsevier Science Ltd. All rights reserved.