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J Phys Chem B. 2013 Jan 10;117(1):104-10. doi: 10.1021/jp308863h. Epub 2012 Dec 27.

Lysozyme hydration in concentrated aqueous solutions. Effect of an equilibrium cluster phase.

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  • 1Department of Physics, La Sapienza University of Rome and CNR-INFM-SOFT, Piazzale A. Moro 5, I-00185 Rome, Italy.


Water close to proteins plays a key role in determining their structural and functional properties. Despite being a subject of considerable interest, the characterization of hydration water, as far as its total amount is concerned, is still controversial and its influence on protein structure and folding is not yet fully understood. In this work, we have investigated the dielectric properties of lysozyme aqueous solutions over the frequency range where the orientational polarization relaxation of the aqueous phase occurs (from 500 MHz to 50 GHz). Measurements extend over a wide concentration range, up to 300 mg/mL, corresponding to a volume fraction of the order of 0.4. The analysis of the dielectric spectra, based on the decrease of the dielectric increment of the γ-dispersion as a function of protein concentration, allows us to estimate the total amount of hydration water (both bound water and loosely bound water) present in the system investigated. We observe a decrease of the hydration number as a function of the protein concentration. This behavior is well accounted for by considering the formation of small equilibrium clusters with aggregation number of some units, as recently reported by Stradner et al. (1) on the basis of small-angle X-ray and neutron scattering measurements.

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