We report the measurement of the adiabatic compressibility β for “molten globule” states of cytochrome b562 and cytochrome c. Precise density and sound velocity measurements allow determination of β in aqueous solution. Surprisingly, the molten globule apocytochrome b562 shows a β comparable to the native holocytochrome b562. We estimated the maximum difference in the hydration contribution to the compressibility of apocytochrome b562 relative to holocytochrome b562 to be only −2.6 × 10−11 Pa−1. Our results suggest that the intrinsic compressibility and the overall volume fluctuations in the molten globule state differ only slightly from those in the native protein. With cytochrome c we investigated the changes in β for the transitions from the native to the acid molten globule and expanded premolten globule states. The molten globule at pH 2.2 was found to be somewhat “softer” than the native protein at neutral pH. In contrast, a decrease in β of the premolten globule indicates an increased penetration of water into the macromolecule. In both of these intermediates the differences in adiabatic compressibility relative to the native state are small in comparison to the range of adiabatic compressibility observed for different native globular proteins.
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