Multicanonical Monte Carlo calculation of the free-energy map of the base-amino acid interaction

The specific interactions between base pairs and amino acids were studied by the multicanonical Monte Carlo method. We sampled numerous interaction configurations and side-chain conformations of the amino acid by the multicanonical algorithm, and calculated the free energies of the interactions between an amino acid at given Cα positions and a fixed base pair. The contour maps of free energy derived from this calculation represent the preferred Cα position of the amino acid around the base, and these maps of various combinations of bases and amino acids can be used to quantify the specificity of intrinsic base–amino acid interactions. Similarly, enthalpy and entropy maps will provide further details of the specific interactions. We have also calculated the free-energy map of the orientations of the Cα—Cβ bond vector, which indicates the preferential orientation of the amino acid against the base. We compared the results obtained by the multicanonical method with those of the exhaustive sampling and canonical Monte Carlo methods. The free-energy map of the base–amino acid interaction obtained by the multicanonical simulation method was nearly identical to the accurate result derived from the exhaustive sampling method. This indicates that a single multicanonical Monte Carlo simulation can produce an accurate free-energy map. Multicanonical Monte Carlo sampling produced free-energy maps that were more accurate than those produced by canonical Monte Carlo sampling. Thus, the multicanonical Monte Carlo method can serve as a powerful tool for estimating the free-energy landscape of base–amino acid interactions and for elucidating the mechanism by which amino Correspondence to: A. Sarai; e-mail: [email protected] ∗Present address: Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305-8568, Japan †Present address: Department of Chemistry, University of Pennsylvania, 231S 34 St. Philadelphia, PA 19104 Contract/grant sponsor: Science and Technology Agency of