Solvation dynamics in biological systems and organized assemblies

Abstract

Solvation dynamics of a polar probe in many biological systems and organized assemblies displays a surprising ultraslow component. The origin of this anomalous ultraslow component is investigated using experiments, theory and computer simulations. We first summarize some recent experimental results on solvation dynamics, e.g., temperature dependence, site dependence at active site of an enzyme and red edge excitation shift (REES). Then we propose two new theoretical models to explain the ultraslow component. The first one involves the motion of the 'buried water' molecules (both translation and rotation) accompanied by cooperative relaxation ('local melting') of several surfactant chains. An estimate of the time is obtained by using an effective Rouse chain model of chain dynamics, coupled with a mean first passage time calculation. The second explanation invokes self-diffusion of the (di)polar probe (created by optical excitation) itself from a less polar to a more polar region. This may also involve cooperative motion of the surfactant chains in the hydrophobic core, if the probe has a sizeable distribution inside the core prior to excitation. It may also involve escape of the probe to the bulk from the surface of the self assembly. The second mechanism should result in the narrowing of the full width of the emission spectrum with time, which has indeed been observed in recent experiments. It is argued that both the two mechanisms may give rise to an ultraslow time constant and may be applicable to different experimental situations. The effectiveness of solvation as a dynamical probe in such complex systems has been discussed. Finally, we give a brief overview of recent results on computer simulations on dynamics of water molecules around a protein and a micelle.