A DFT and TDDFT investigation of interactions between pyrene and amino acids with cyclic side chains

A computational investigation of photoinduced charge transfer complex formation between pyrene (Py) and amino acids with cyclic side chains (Phenylalanine: Phe, Tyrosine: Tyr, Tryptophan: Trp, Histidine, His) has been carried out in gas phase and in water. Geometry optimizations were performed by density functional theory (DFT) at ωB97XD/6-311++G(d,p) level. Time-dependent density functional theory (TDDFT) was used to calculate the electronic transitions of molecules at B3LYP/6-311++G(d,p) level using the above ground-state geometries. Polarizable Continuum Model (PCM) and SMD are used for calculations in water. Analyses and comparisons have been performed for total electronic energies, complexation energies, free energy differences, solvation energies, excitation wavelengths, and HOMO–LUMO energy gaps of complexes in gas phase and in solution. The intermolecular distances between Py and amino acids increased in water compared to the gas phase for all studied with the exception of Py–Tyr system. The optimized complexes display an increasing complex stability in the order Trp > Tyr > Phe > His. Analyses of first excited singlet states have indicated charge transfers transitions between Py and amino acids His and Trp through π–π stacking in gas phase at 345 nm and 393 nm, respectively. Py–Trp system has also charge transfer (CT) in water at 389 nm. Py–Trp systems have the most significant charge transfer between HOMO and LUMO (full CT, 70%). However, S0–S1 transition (393 nm) has weaker dipole moment and oscillator strength than the other studied systems. Due to its charge transfer character, Py–Trp systems seem to be appropriate models to investigate and design bioorganic photosensitive materials.