The role of enhanced aromatic π-electron donating aptitude of the tyrosyl sidechain with respect to that of phenylalanyl in intramolecular interactions

An exhaustive ab initio and DFT search for energetically stable conformers from the topologically possible set was undertaken on the N-acetyl-phenylalanyl-N-methylamide and N-acetyl-tyrosyl-N-methylamide systems. The geometries of all 81 phenylalanyl and 162 tyrosyl possible. rotamers, described under the rules outlined by Multi-Dimensional Conformational Analysis (MDCA), were attempted at each of the RHF/3-21G, RHF/6-31G(d) and B3LYP/6-31G(d) levels of theory. A total of 32 and 66 stable conformational minima were found for the phenylalanyl and tyrosyl amino acid diamides, respectively, at the B3LYP/6-31G(d) level. From the tyrosyl set, 33 unique conformers emerge when the orientation of the A(i)(3) dihedral angle (p-OH orientation) is disregarded. A total of 31 conformers were common to both sets and showed nearly identical geometries. The comparison of the optimized DFT geometries of the two systems showed near by perfect linear fits with R-2 values of 0.9997, 0.9994, 0.9997, and 0.9996 for the phi(i), psi(j), A(i)(1) and A(i)(2) dihedral angles, respectively. Relative energies of the matching 31 conformers also fitted to a linear plot with an R-2 value of 0.9985. The geometric centroid of the aromatic ring in the sidechain of both systems was found to be within 4.1 Angstrom of the H and 0 atoms of the peptide groups, in 21 and 2 of the conformers, respectively. None of the non-matching conformers showed any such interaction distance less than or equal to4.1 Angstrom.