Direct Observation of Coupling between Structural Fluctuation and Ultrafast Hydration Dynamics of Fluorescent Probes in Anionic Micelles

The coupling of structural fluctuation and the dynamics of associated water molecules of biological macromolecules is vital for various biological activities. Although a number of molecular dynamics (MD) studies on proteins/DNA predicted the importance of such coupling, experimental evidence of variation of hydration dynamics with controlled structural fluctuation even in model macromolecule is sparse and raised controversies in the contemporary literature. Here, we have investigated dynamics of hydration at the surfaces of two similar anionic micelles sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) as model macromolecules using coumarin 500 (C500) as spectroscopic probe with femtosecond to picosecond time resolution up to 20 ns time window. The constituting surfactants SDS and SDBS are structurally similar except one benzene moiety in the SDBS may offer additional rigidity to the SDBS micelles through a-stacking and added bulkiness. The structural integrity of the micelles in the aqueous medium is confirmed in dynamic light scattering (DLS) studies. A variety of studies including polarization gated fluorescence spectroscopy and quasielastic neutron scattering (QENS) have been used to confirm differential structural fluctuation of SDS and SDBS micelles. We have also employed femtosecond-resolved Forster resonance energy transfer (FRET) in order to study binding of a cationic organic ligand ethidium bromide (EtBr) salt at the micellar surfaces. The distance distribution of the donor (C500) acceptor (EtBr) in the micellar media reveals the manifestation of the structural flexibility of the micelles. Our studies on dynamical coupling of the structural flexibility with surface hydration in the nanoscopic micellar media may find the relevance in the "master-slave" type water dynamics in biologically relevant macromolecules.