Effect of solvation on the structure conformation of human serum albumin in aqueous–alcohol mixed solvents

In the present paper, we analyze the effect of ethanol in the structure conformation of human serum albumin (HSA) by far- and near circular dichroism, UV–Vis spectroscopy and density and ultrasound velocity measurements. Both circular dichroism and absorbance data show changes in both tertiary and secondary structures at ethanol concentrations below 30% (v/v), which can be indicative of the presence of an equilibrium intermediate state. At higher ethanol concentrations in the mixed solvent (>30% (v/v)), a change from a rich--helix to a β-sheet and unordered secondary structure is observed. Moreover, it seems that certain protein aggregation starts to occur. From ethanol concentrations higher than 55% (v/v), a certain redissolution of these aggregates takes place, which seems to be accompanied by an increase in -helix content. Trends found by volume and compressibility determinations seem to be in accordance. Both volume and compressibility increase with alcohol concentration up to 50% (v/v) but in two well-defined steps indicating a different nature of these two changes, the first of them with a variation close to that shown by a molten globule intermediate state. Moreover, at ethanol concentrations higher than 55% (v/v), a decrease in both quantities occurs confirming the breakdown of protein aggregates.