Ultrafast photoinduced deligation and ligation dynamics: DCM in micelle and micelle-enzyme complex

Abstract

We report studies on diffusion controlled deligation and ligation dynamics of a probe ligand 4-(dicyanomethylene)-2-methyl-6-( p-dimethylamino-styryl) 4H-pyran (DCM) with cationic cetyltrimethylammonium bromide (CTAB) micelles. In order to investigate the effect of spatial heterogeneity on the dynamics we study the DCM labeled micelle upon complexation with an enzyme α-chymotrypsin (CHT). The variation of fluorescence line-width ( Γ( t)) of DCM in the complex and also in the micelle indicates the diffusion dynamics of DCM through various environments of different polarities. The temporal behavior of Γ( t) reveals that at 50 mM CTAB concentration the excited DCM traverses 6.5 Å distance from the surface of a host micelle (deligation) before entering to a stern layer of another adjacent micelle (ligation). From neutron scattering experiment the distance 6.5 Å is found to be the thickness of a stern layer of CTAB micelle. No indication of ligation has been found at 2 mM CTAB concentration as the intermicellar distance is estimated to be very large (416 Å) compared to the previous case. The dynamical behavior of Γ( t) is also indicative of significantly slower diffusion of the ligand molecules (DCM) at the surface of the micelle in presence and absence of the enzyme compared to that in the bulk buffer. We have also studied the dynamics of solvation and local geometrical restriction on the probe DCM at the micellar surface with and without CHT. With picosecond time resolution, we found time constants of the solvation relaxation processes of the DCM labeled enzyme-micelle complex to be 230 ps (45%) and 870 ps (55%), which were comparable to those of the micelle without the enzyme. The time dependent anisotropy revealing local orientational motions of the probe in the complex was also found to be similar to that of DCM at the micellar surface in absence of CHT. These studies attempt to link the dynamical features for insight into the ligand mediated intercellular communication and the biological function of the enzyme α-chymotrypsin upon complexation with the CTAB micelle.