| Summary: | 博士 === 國立臺灣師範大學 === 物理學系 === 100 === Spectral density function analysis is a direct description of protein dynamics and can be extracted from NMR relaxation measurements, such as spin-lattice relaxation (R1) dispersion. Toward that goal, we have built a field cycling device for measuring R1 field dispersion curves over the field range of 0-14.1T. The device permit the shuttling of solution protein samples in regular NMR tube to be shuttled up-and-down the 1 meter superconducting magnet bore stably and reproducibly in ~ 100 ms. Using this compact field cycling device, we have obtained the first set of 15N-R1 dispersion curve of a protein, ubiquitin from 0.9 T to 20 T. Surprisingly, the field-dependent 15N-R1 curves of many residues cannot be fit with conventional Lorentzian functions and new spectral density functions based on motions subjected to harmonic potential have to be derived. The backbone dynamic information derived accordingly showed that ubiquitin contains a rigid β-strands core and mobile termini. However, the most novel finding of this thesis work is our ability to determine the harmonic potential energy for each residue from analysis of the R1 dispersion curve. The results showed that potential energy can be correlated to conformational exchange and residues having large potential energy are those exhibiting slow conformational exchange. The discovery could be a further evidence of conformational selection for ubiquitin binding mechanism.
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