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Local Compressibilities of Proteins: Comparison of Optical Experiments and Simulations for Horse Heart Cytochrome-c

Physik-Department E14, Lehrstuhl für Physik Weihenstephan, Technische Universität München, Freising, Germany

Correspondence: Address reprint requests to Christina Scharnagl, Technische Universität München, Fakultät für Physik E14 und Lehrstuhl für Physik Weihenstephan, D-85350 Freising, Germany. Tel.: 49-8161-71-3557; E-mail: christina.scharnagl{at}physik.blm.tu-muenchen.de.

Spectroscopy with probe molecules yields local information on the environment of the probe. In this article we compare local compressibilities of cytochrome-c as obtained from molecular dynamics simulations with experimental results as obtained from spectroscopic measurements. The simulations show that the protein-core around the heme is much less compressible in a glycerol/water solvent than in pure water. The pocket is also much less compressible than the protein as a whole, although the compressibility of the water inside the rather incompressible protein-core is almost liquidlike. We show that the local compressibility values capture the collective correlations of local volume fluctuations with volume fluctuations in the surrounding protein-solvent system. The decoupling of the volume fluctuations of the core from the solvent shell explains the reduction of the heme-core-compressibility in glycerol/water solvent. This decoupling could be traced back to the suppression of the exchange between pocket-water and hydration-shell-water upon addition of glycerol as co-solvent.