Fluorescence Resonance Energy Transfer Small-AngleNeutron Scattering and Dynamic Light Scattering Study on Interactionsof Gemini Surfactants Having Different Spacer Groups with Proteinat Various Regions of Binding Isotherms

摘要

The binding interactions of three gemini surfactants having different spacer groups (12-4-12, 12-8-12, and 12-4(OH)-12) with a high concentration (150 μM) of bovine serum albumin (BSA) at various regions of binding isotherms have been studied by means of steady-state fluorescence and fluorescence anisotropy, time-correlated single-photon counting fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole, small-angle neutron scattering (SANS), and dynamic light scattering (DLS) measurements. The fluorescence resonance energy transfer phenomenon between the twisted intramolecular charge transfer fluorescent molecule, trans-2-[4-(dimethylamino)styryl]benzothiazole as an acceptor, and tryptophan 213 (Trp-213) of BSA as a donor has been successfully used to probe the binding interactions of gemini surfactants with protein at all regions of binding isotherms. The increasing order of energy transfer efficiency at a higher concentration range of surfactants is 12-8-12 > 12-4-12 > 12-4(OH)-12. Stronger binding of micelles of gemini surfactant molecules having a comparatively more hydrophobicspacer group with the hydrophobic segments of the protein resultsin closer approach of trans-2-[4-(dimethylamino)styryl]benzothiazolemolecules solubilized in micelles to Trp-213. The average excited-statelifetimes become shorter with a trend of increase in contributionfrom the fast component and decrease in contribution from the slowcomponent to the decay with increasing concentration of a surfactant.The nonradiative rate constant of trans-2-[4-(dimethylamino)styryl]benzothiazoleincreases with increasing concentration of a surfactant because theaverage microenvironment around it in protein–surfactant aggregatesis more polar as compared to that in native protein. SANS and DLSmeasurements were carried out for the study of the structural deformationsin the protein, on enhancement of the concentration of the geminisurfactants. The necklace and bead model has been used for the analysisof SANS data for the protein–surfactant complexes. At a higherconcentration range, 12-8-12 and 12-4-12 have a slightly smaller fractaldimension and a larger correlation length as compared to 12-4(OH)-12.DLS data show that the increasing order of hydrodynamic diameter forthe complexes of protein with three gemini surfactants in their highconcentration range is 12-4(OH)-12 < 12-4-12 < 12-8-12.