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-α-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 α-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.