A study on interaction of Be++, Mg++ and Ca++ with phenylalanine: Binding energies, metal ion affinities and IR signature of complex stability

The interaction between divalent metal cations and amino acids plays an important role in many biological processes. In present report, we have examined the effect of metal cations (Be++, Mg++ and Ca++) interaction on structures, binding energies (BE), metal ion affinities (MIA) and infrared (IR) spectra of phenylalanine (Phe) molecule by density functional theory (DFT) calculations at B3LYP/6-311++G(d,p) level. Nine different ground state isomers of Phe molecule have been optimized at B3LYP/6-311++G (d,p) level of theory. The relative ground state energies of these nine isomers are lying between 0.0–1.9 kcal/mol with respect to the ground state energy of most stable Phe isomer. Seven most stable complexes of Phe molecule with Be++, Mg++ and Ca++ [Phe+M]++ (M = Be++, Mg++ and Ca++) were studied. The calculated values of metal ion affinity (MIA), BE and the Gibbs free energies of each [Phe+M] ++ complexes were found to be in the order of Be++ > Mg++ > Ca++. Among the seven [Phe+M]++ complexes, the most stable conformer has charge solvation structure where the metal cations coordinated through tridentate bonds with -N, -O atoms and benzene ring (N/O/Ring). The [Phe+Be]++ complex has maximum MIA value, 353.3 kcal/mol than that of [Phe+Mg]++ and [Phe + Ca]++ complexes. Thus, the complex [Phe+Be]++ is energetically more stable than that of [Phe+Mg]++ and [Phe + Ca]++. The IR spectra of each seven conformers of [Phe+M]++ complexes have been also calculated. The wavnumber position of (–CO) stretching mode was used to determine the charge/salt bridge structures of the [Phe+M]++ complex. The most stable [Phe+M]++ complex has been also verified through the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) analysis.