Reorientational dynamics of amino acids in aqueous solution probed by ultrafast optical Kerr effect spectroscopy

Reorientational relaxation of a solute in aqueous solution reflects the friction from its surroundings, yet observing solute-specific rotational motion at the microscopic level is difficult. Ultrafast optical Kerr effect (OKE) spectroscopy can follow molecular reorientational relaxation in liquid phase, but it is not obvious whether the slow picosecond component arises from the rotation of the solute itself or from water slowed by the increased viscosity when it is applied to aqueous solution systems. Here, we use OKE spectroscopy to investigate the reorientational dynamics of three amino acids (alanine, proline, and lysine monohydrochloride) in aqueous solution in terms of concentration, temperature, and chirality. Using poly(vinyl alcohol) (PVA) as a control, we found that increasing the macroscopic viscosity by approximately eightfold did not alter the reorientational relaxation time of water. This result indicates that OKE provides information on the local rotational friction associated with the solute and highlights its unique role as a technique complementary to rheometry, which measures macroscopic viscosity. This study provides new insight into the local rotational environment experienced by solutes in aqueous amino acid solutions.