A new approach in determining local residue flexibility from base-amino acid contact frequencies is applied to the twelve million lattice chains modeling BIV Tat peptide binding to TAR RNA fragment. Many of the resulting key features in flexibility correspond to RMSD calculations derived from a set of five NMR derived structures (X. Ye, R. A. Kumar, and D. J. Patel, Protein Data Bank: Database of three-dimensional structures determined from NMR (1996)) and binding studies of mutants (L. Chen and A. D. Frankel, Proc. Natl. Acad. Sci. USA 92, 5077-5081 (1995)). The lattice and RMSD calculations facilitate the identification of peptide hinge regions that can best utilize the introduction of Gly or other flexible residues. This approach for identifying potential sites amenable to substitution of more flexible residues to enhance peptide binding to RNA targets could be a useful design tool.