Solvent effects on the absorption spectra of arginine-salicylaldimine

Arginine-salicylaldimine was synthesized, characterized and its absorption spectra were studied in various solvents. The molecule can exist either as keto or enol forms. These forms can be stabilized by the nature of the solvent. The results revealed that intramolecular hydrogen bonding is responsible for stabilizing the keto form in non-polar solvents whereas intermolecular hydrogen bonding favors the enol form. At the same time, an equilibrium can exist between the acidic and basic forms of the molecule which affects the solvation of the probe molecule. Three distinct species were found to exist in equilibria in aqueous solution and their pK _a values were determined (pK _a1 = 5.13, pK _a2 = 8.11, and pK _a3 = 10.88). The preferential interaction of arginine-salicylaldimine in mixed solvent system comprising of methanol and dichloromethane showed a sharp change in absorption energy at low concentrations of methanol, which indicated a strong preference for the probe molecule to undergo interaction with methanol. Time-dependent density functional theory (TD-DFT) calculations were used to assign the observed electronic transitions in the region 370-440 nm to a π-π* transition in the keto and enol forms, and distinguish between them in solution.