New stable folding of β-lactoglobulin induced by 2-propanol

beta-lactoglobulin A has been studied in 2-propanol-water mixtures by means of circular dichroism. fluorescence and small angle X-ray scattering. At a low ionic strength, 2-propanol induces an increase in alpha-helix structure followed by a further transformation which gives rise to a new feature, rich of beta-sheet fragments. The second step of the secondary structure transformation is time-dependent and depressed at high ionic strength. As a consequence, the tertiary structure is completely modified and a new stable protein folding may be hypothesized. Small angle X-ray scattering measurements reveal that 2-propanol induces a diffuse protein aggregation, but the complex equilibria among intra- and inter-molecular hydrophobic and electrostatic interactions may be modulated by balancing the ionic strength and/or the alcohol percentage. (C) 1998 Elsevier Science B.V.