Conformational changes of l-phenylalanine - Near infrared-induced mechanism of dimerization: B3LYP studies

Structures of l-phenylalanine (phe) conformers were subjected to geometry optimization. Conformers of phe were optimized at the B3LYP and MP2 levels using the Gaussian 03 package. The relative energies, free energies and dipole moments of these conformers were subsequently calculated. The aggregation process of phe molecule has been also studied with B3LYP calculations using 6-31G* basis set. We suggest the possible mechanism of phenylalanine dimer formation. The interaction between two identical molecules of the amino acid involves hydrogen bonding CO⋯HOOC, forming cyclic dimers. Before dimer formation conformational changes (trans/cis isomerization) should occur. The trans form is energetically more stable than the cis one, but through an entropy-effect of association, the cis form can be dominating as it results in two hydrogen bonds. The conformational barrier height of cis/trans isomerization was calculated. The results are a continuation of our experimental studies on phenylalanine amino acid.