Ab Initio Study of Preferential Interactions Between Aromatic Side Chains

摘要

The study of the trytophan-histidine adducts, derived from the crystal structures available in the Brookhaven Protein Data Band (PDB), using as model systems indole and (#delta#) 5-methylimidazole [G. Alagona, C. Ghio, S. Monti, J. Phys, Chem. A, 102, 6152 (1998)], has been extended for three of them (1esaB, 1s01, and 1lla, named after the PDB file to which they belong) to also include #epsilon# and protonated 5-methylimidazole at the Hartree-Fock (HF) and the second-order Moller-Plesset (MP2) levels, employing the 6-31G~* basis set with the d exponents reduced to 0.25, thus named 6-31G~*(0.25), and the HF/6-31G~* internal geometries of the isolated partners. For these arrangements having a shallow or even repulsive HF interaction energy, the counterpoise correction to the basis set superposition error was introduced both at the HF and MP2 levels, using the 6-31G~*(0.25) basis set, to test the reliability of the results obtained along the whole approaching path. The position (either #delta# or #epsilon#) of the proton on 5-methylimidazole does not affect much the results when the aromatic rings are almost parallel or in a T-shaped imidazole-across arrangement: the MP2 energy gap between the two adducts (favoring 1s01 #epsilon# and 1lla #epsilon#) is about 1.5 and 0.9 kcal/mol, respectively, and becomes 1.2 and 0.7 kcal/mol when counterpoise (CP) corrected. On the contrary there is a sensitive stabilization of the adduct when the imidazole H points toward the indole ring #pi# density (lesaB#epsilon