Pressure dependence of the apparent specific volume of bovine serum albumin: Insight into the difference between isothermal and adiabatic compressibilities

There are some theoretical arguments related to interpreting the adiabatic compressibility (βs) of a protein determined from the sound velocity and the difference between βs and isothermal compressibility (βT). To address these problems experimentally, we constructed a high-pressure oscillating densitometer and used it to measure the apparent specific volume of bovine serum albumin as a function of pressure (0.1-78 MPa) and temperature (5-35 °C). The βT determined from plots of the apparent specific volume vs. pressure was slightly larger than βs at all temperatures examined, with the difference between the two compressibilities increasing as the temperature was decreased. Only at room temperature did the observed βT agree with those estimated from βs using the heat capacity and the thermal expansibility of the protein, suggesting that there are significant as-yet-unknown mechanisms that affect protein compressibility.