Systematic ab initio studies of the conformers and conformational distribution of gas-phase tyrosine -: art. no. 134313

A full structural assignment of the conformers of gaseous tyrosine is presented. A total of 1296 unique trial structures were generated by allowing for all combinations of internal single-bond rotamers and optimized at the B3LYP/6-311G(*) level of theory and then subjected to further optimization at the B3LYP/6-311++G** level. A total of 76 conformers are found and their dipole moments, rotational constants, and harmonic frequencies are determined. Accurate relative energies are given at the MP2/6-311G(2df,p)//B3LYP/6-311++G** level of theory. Characteristic H-bonding types are classified and listed for all the conformers. The four most stable conformers display an intramolecular H bond, COOH center dot center dot center dot NH2, and an additional H-bonding interaction between the amino group and pi electron of the aromatic ring. The results further confirm that the global minimum conformations of the aromatic amino acids have the same H-bonding type. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed and the temperatures with which the theoretical results match that of experiments are indicated. (C) 2005 American Institute of Physics.