Alcohol induced structural and dynamic changes in β-lactoglobulin in aqueous solution: A neutron scattering study

Structural and dynamic properties of beta-lactoglobulin (beta-LG) were revealed as a function of alcohol concentration in ethanol- and trifluoroethanol(TFE)-water mixtures with circular dichroism (CD), small-angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS). The CD spectra showed that an increase in TFE concentration promotes the formation of the beta-sheet structure of beta-LG. The SANS-intensities were fitted using form factors for two attached spheres for the native and native-like states of the protein. At higher alcohol concentrations, where aggregation takes place, a form factor modelling diffusion limited colloidal aggregation (DLCA) was employed. The QENS-data were analyzed in terms of internal motions for all alcohol concentrations. While low concentrations of TFE (10% (v/v)) lead to an increase of the mean square amplitudes of vibrations <u(2)> and a retention of a native-like structure - but not to an increase of the characteristic radius of proton diffusion processes a. Addition of 20% (v/v) of TFE induces aggregation, going along with a further increase of <u(2)>. Further increase of TFE concentration to 30% (v/v) changes the nanoscale structure of the oligomeric nucleate, but induces no further significant changes in <u(2)>. The present study underlines the necessity of methods sensitive to the dynamics of a system to obtain a complete picture of a molecular process. (c) 2012 Elsevier B.V. All rights reserved.