Interfacial water at protein surfaces: wide-line NMR and DSC characterization of hydration in ubiquitin solutions.
Wide-line 1H-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 x g(-1) x K(-1), and the magnitudes are higher than that of pure/bulk water (4.2 J x g(-1) x 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 x g(-1) x 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.
Tompa
Kálmán
K
Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest, Hungary.
Bánki
Péter
P
Bokor
Mónika
M
Kamasa
Pawel
P
Lasanda
György
G
Tompa
Péter
P
D002152:Calorimetry, Differential Scanning; D005612:Freeze Drying; D005615:Freezing; D006358:Hot Temperature; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D012996:Solutions; D013499:Surface Properties; D013816:Thermodynamics; D025801:Ubiquitin / Q000737:chemistry* / Q000378:metabolism*; D014867:Water / Q000737:chemistry* / Q000378:metabolism*
D012996:Solutions; D025801:Ubiquitin; D014867:Water