Deuteron field-cycling relaxation spectroscopy and translational water diffusion in protein hydration shells

Abstract

The deuterated hydration shells of bovine serum (BSA) albumin, and purple membrane sheets have been studied by the aid of deuteron field-cycling relaxation spectroscopy. The deuteron Larmor frequency range was 10(3) to 10(8) Hz. The temperature and the water content has been varied. The data distinguish translational diffusion on the protein surface from macromolecular tumbling or exchange with free water. A theory well describing all dependences has been developed on this basis. All parameters have successfully been tested concerning consistency with other sources of information. The concept is considered as a major relaxation scheme determining, apart from cross-relaxation effects, the water proton relaxation in tissue.

Molecular Dynamics in Biological Systems Observed by NMR Relaxation in a Rotating Frame

“…The physical origin of this law has been related to a relaxation mechanism, which correlate the value of power b 3 to some molecular dynamic parameters. In the case of immobilized proteins according to T 1 dispersion measurements, b 3 is evaluated to be near 0.75 and does not change with protein hydration [5]. In the case of proteins in aqueous solutions this parameter is greater, even exceeding 1 [6].…”

“…The second model applied here is the so-called power law model, often used in the interpretation of T 1ρ relaxation dispersion in biopolymers [5]. The final formula for evaluation of relaxation rates R 1ρ for uncorrelated homonuclear dipolar coupled two-spin systems has the form…”

“…One of these methods is nuclear magnetic relaxation dispersion [1][2][3][4][5][6][7], that is the measurement of nuclear magnetic relaxation rates R i = 1/T i (i = 1, 2, 1ρ) as a function of the magnetic field strength. In particular, there are some of interesting publications on molecular dynamics of proteins based on the spin-lattice relaxation rate, R 1 , dependence on B 0 [1][2][3][4][5] and the spin-lattice relaxation rate in the rotating frame, R 1ρ , dependence on B 1 [6][7][8][9].…”

“…Thus, the measurements of T 1ρ as a function of B 1 offers an excellent way of probing the motion of macromolecules characterized by frequencies in the mid-kHz region. In reported measurements the range of magnetic field B 1 strength was of 0.3 to 12 Gs [10 −4 T], which corresponds to correlation times τ typically observed for molecular motion of water adsorbed on macromolecule surface [5,6,9].…”

“…Although proton relaxation in free water has been successfully explained in terms of classic dipole-dipole interaction theory, when water molecules are bounded to biological macromolecules, the relaxation mechanism becomes more complicated and understanding of the bounded water relaxation is highly speculative. Therefore the obtained T 1ρ dispersion data are analyzed using two models: first one based on pure dipole-dipole interaction as a dominant mechanism in the relaxation of water protons and the second, simple power law, which is widely used in the interpretation of polymer relaxation [5].…”