The magnetic field dependence of the nuclear spin-lattice relaxation rate provides the Magnetic Relaxation Dispersion profile (MRD). The MRD contains most of the molecular dynamical information that is available from a nuclear magnetic resonance experiment, and offers a very powerful as well as high resolution tool for defining both intra and intermolecular motions. We have constructed an instrument that circumvents the standard difficulties of low sensitivity and resolution in MRD experiments by pneumatically moving a sample between a high field polarization and detection magnet and a satellite relaxation magnet. By varying the field in the relaxation magnet, we may map the MRD. We have largely solved the problems of sample containment, shuttle time, and relaxation rate windows. We propose to address molecular dynamics directly using this spectrometer to map spectral density functions, which are Fourier transforms of autocorrelation functions that characterize the time fluctuations of intermoment distances and orientations. The time scale that may be investigated ranges from tenths of milliseconds to tenths of picoseconds. We will measure localized translational diffusion constants for ions in condensation or double layer regions of membranes and linear polyelectrolytes, DNA in particular. We will measure localized translational diffusion constants for other solutes of interest at membrane surfaces. We will measure spectral density functions associated with small-molecule structural fluctuations and compare them with spectral density functions computed from molecular dynamics simulations in the time range of tens of ps. We will develop further the methods to measure internal dynamics in proteins using selected spin pairs that may be spectrally isolated by isotropic labeling methods combined with standard spectral editing sequences. Using this approach we expect to define important hinge motion frequencies in proteins as well as other low and high frequency dynamics such as crucial active site motions of amino acid side chains. Initial protein experiments will be focused on calmodulin and T4 lysozyme; however, the approaches are general. We expect that understanding these dynamics will impact our understanding of function significantly.
