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The nuclear magnetic relaxation dispersion profiles of lyophilized globular proteins were measured in the frequency range of 10 kHz–30 MHz at temperatures from 156 to 302 K. The existent theory of proton relaxation in immobilized protein systems was critically tested and expended to include contributions of rapid motions of protein side-chain groups. The new theory takes into account the strong coupling between the side-chain protons and the protein backbone, when correlation function cannot be written as a product of the contributions. The measurements showed that while the relaxation rate constant of the protein backbone protons is a linear function of the absolute temperature the side-chain groups exhibit an exponential temperature dependence corresponding to an activated process. Measurements carried out on simple homopolypeptides, polyglycine and polyalanine, provide strong support of the proposed new theory. [ABSTRACT FROM AUTHOR]

The proton magnetic relaxation dispersion profiles are reported over the proton Larmor frequency range from 0.01 to 30 MHz for cross-linked gels and for the dry lyophilized bovine serum albumin covalently labeled at lysine with diethylenetriaminepentaacetic acid chelates of either Gd(III) or Mn(II) ions. The proton spin-lattice relaxation dispersion for the cross-linked paramagnetic protein gel is accurately represented as a sum of two major relaxation contributions. The diamagnetic term is a power law from the magnetic field dependence of the protein protons. The paramagnetic term is approximately described by the Solomon-Bloembergen-Morgan class of models. However, the paramagnetic relaxation mechanism in the dry lyophilized protein is fundamentally different and we develop a new quantitative description of the dispersion profile. In the dry case, no peak in the proton relaxation dispersion profile is detected from the field dependence of the electron spin relaxation times. The high-field paramagnetic relaxation dispersion is a power law in the Larmor frequency with an exponent of -0.8, which results from modulation of the electron-nuclear coupling by the intramolecular dynamics of the protein which primarily propagates along the primary structure of the protein. The low-field plateau is caused by the interruption of the electron-nuclear spin correlation by electron spin relaxation. This new quantitative description provides a simple approach to the measurement of electron spin-lattice relaxation times in paramagnetic protein systems at room temperature based on the magnetic field dependence of the proton spin-lattice relaxation rate constant. [ABSTRACT FROM AUTHOR]

The magnetic field dependence of the proton spin-lattice relaxation rate in polymeric materials and biological macromolecules may report important dynamical information. In the case where the system is dynamically heterogeneous as in plasticized polymeric systems or hydrated biopolymers, the spin-lattice relaxation of the liquid-proton population is generally coupled to the spin-relaxation behavior of the solid spins that often dominate the observable response of the liquid. In many of these systems the magnetic field dependence of the proton spin-lattice relaxation rate may be represented as a power law: 1/T[sub 1](ω)=Aω[sup -b] where a is a constant and b is usually found to be in the range of 0.5 to 0.8. We have shown that this power law may arise naturally from localized structural fluctuations along the backbone of chain molecules that modulate the proton dipole–dipole couplings, which form a network described by a fractal dimension that may be less than the Euclidean dimension. When the model for the solid spin-lattice relaxation rate constant is incorporated into the relaxation equations that couple the liquid and solid spin population responses, the composite model accounts quantitatively for proton spin-lattice relaxation rates measured in immobilized and hydrated protein systems and provides a fundamental basis for understanding the parametric dependence of proton spin-lattice relaxation rates in dynamically heterogeneous systems such as hydrated biopolymers, proteins, and biological tissues. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]