Low Frequency Dynamics of Confined Proteins
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Low Frequency Dynamics of Confined Proteins Jean-Pierre Korb1 and Robert G. Bryant2 1 Laboratoire de Physique de la Matière Condensée, UMR 7643 du CNRS, Ecole Polytechnique, 91128 Palaiseau, FRANCE 2 Chemistry Department, University of Virginia, Charlottesville, VA 22904-4319, USA ABSTRACT We present the frequency dependence of the proton spin-lattice relaxation rate 1/T1 in variously hydrated proteins. We present also the case of proteins confined in heavily hydrated gels where the rotation has been immobilized. The relaxation efficiency increases according to a power law at low frequencies. The temperature dependence of the protein protons T1 demonstrates that relaxation results from a direct spin-phonon process instead of a Raman process at temperatures above 273K. We propose a theory that accounts for experiments and depends on the dynamical distribution of states, the localization of the disturbances along and transverse to the peptide chains, and the spatial distribution of hydrogen in the structure. In hydrated and confined proteins, the motions of the backbone that dominate the relaxation process are transverse rather than along the peptide chain. We show that the protein structure adjusts to hydration from the lyophilized state to the fully hydrated state in small increment steps. INTRODUCTION There is considerable current effort to understand how structural fluctuations in proteins and other macromolecules provide access to functional conformations or provide energetic couplings that result in concerted and crucial changes in location or concentration as in muscle contraction or active transport [1-3]. The dynamical spectrum of a folded polymeric structure is complex and characterization requires examination over many decades in frequency or time. The nuclear magnetic spin-lattice relaxation dispersion provides a powerful approach to this class of problems because variations of the experiment may probe intra and intermolecular dynamics from the range of millisecond to picosecond [4]. We report here magnetic relaxation dispersion measurements on proteins that have been progressively hydrated and/or rotationally immobilized to suppress the rotational averaging of proton-proton dipole-dipole couplings. One price of immobilization is loss of high resolution usually associated with proton NMR spectroscopy; however, the magnetic relaxation dispersion measuremts provide a valuable characterization of the intramolecular protein dynamics at frequencies well below the rotational frequency of the protein in solution. EXPERIMENTAL DETAILS The 1H NMR spectrum of a rotationally immobilized protein, whether it is in a lyophilized powder or a heavily hydrated gel is broad and all features are lost in the linewidth, which is typically approximately 30 kHz [5]. The protein protons are strongly coupled by dipolar couplings between protons and the linewidth is homogeneous; irradiation in any portion of the line saturates the whole line, which is an important basis for magnetic transfer contrast imaging in medicine. Th
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