Study of electron transfer process in aqueous methanol system by using tryptophan based short peptide – Amino acid pairs

Abstract

The present study demonstrated the nature of synergistic solvation behaviour in water – methanol binary solvent mixture(s) and its role on photo-induced electron transfer (PET) process during the interactions of different tryptophan based peptide conjugates with selected non-aromatic amino acids and also with standard coumarins – amines pairs. The highest degree of solvation was obtained in water rich region near Xwater = 0.85 with ETN(30) value 1.05, which was observed more compared to both pure water (1.00) and methanol (0.76). An analytical solvent exchange model was used to estimate the extent of synergistic solvation and found ∼2 times stronger than the preferential solvation efficiency. Such unconventional associative solvation behaviour was confirmed by fluorescence study of the solvatochromic probe which suggested that the weak nature of intermolecular interaction is responsible for such behaviour. The selected pairs of different amino acids as donors, and established pre-synthesized new tryptophan based peptide conjugates as acceptors were used to check the PET event in the polar environment(s) to know the reorganization energy of the binary solvent mixture. An inverted region was observed and estimated the value of reorganization energy (λ) in Xwater = 0.85. The values of λ in water, methanol and Xwater = 0.85 solution were further used to determine its dependency on solvent parameters. Interestingly, our observations revealed that the solvent viscosity depicted linear dependence with reorganization energy (λ) where the value of λ increases with increasing solvent viscosity. Therefore, the photo-physical event i.e. PET, in present solvent system, is significantly dependent on the physical properties of polar solvents.