Interfacial water at protein surfaces: wide-line nmr and dsc characterization of hydration in ubiquitin solutions

Tompa, Kálmán and Bánki, Péter and Bokor, Mónika Zsuzsanna and Kamasa, Pawel and Lasanda, György and Tompa, Péter (2009) Interfacial water at protein surfaces: wide-line nmr and dsc characterization of hydration in ubiquitin solutions. BIOPHYSICAL JOURNAL, 96 (7). pp. 2789-2798. ISSN 0006-3495

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Abstract

Wide-line H-1-NMR and differential scanning calorimetry measurements were done in aqueous solutions and on lyophilized samples of human ubiquitin between -70 degrees C and +45 degrees C. The measured properties (size, thermal evolution, and wide-line NMR spectra) of the protein-water interfacial region are substantially different in the double-distilled and buffered-water solutions of ubiquitin. The characteristic transition in water mobility is identified as the melting of the nonfreezing/hydrate water. The amount of water in the low-temperature mobile fraction is 0.4 g/g protein for the pure water solution. The amount of mobile water is higher and its temperature dependence more pronounced for the buffered solution. The specific heat of the nonfreezing/hydrate water was evaluated using combined differential scanning calorimetry, and NMR data. Considering the interfacial region as an independent phase, the values obtained are 5.0-5.8 J . g(-1) . K-1, and the magnitudes are higher than that of pure/bulk water (4.2 J . g(-1) . K-1). This unexpected discrepancy can only be resolved in principle by assuming that hydrate water is in tight H-bond coupling with the protein matrix. The specific heat for the system composed of the protein molecule and its hydration water is 2.3 J . g(-1) . K-1. It could be concluded that the protein ubiquitin and its hydrate layer behave as a highly interconnected single phase in a thermodynamic sense.

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