Use Deuterium Solid State NMR Simulations to Study Dynamics of Peptides Adsorbed onto Inorganic Surfaces
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This work is directed towards building an understanding of the dynamics of protein/peptide adsorbed on inorganic surfaces. Deuterium solid state NMR(SSNMR) is a very special tool for probing molecule dynamics in solid state. The probing of dynamic information is realized by simulation of the Deuterium line shape and relaxation data in the experiment. Two different SSNMR experiments are used: the quadrupole echo experiment and MAS experiment. The simulation of quadrupole echo experiment adopted the public simulation program MXET1, while the simulation of MAS experiment adopted the house made program KLDMAS. The theory and part of the KLDMAS code are described in detail in Chapter II. Then the codes were used to simulate the dynamics of peptide side chain on different inorganic surfaces. In this thesis, two peptides adsorbed on surfaces were studied, LKalpha14 and SN15. LKalpha14 is a 14 amino acid peptide with a periodic sequence of leucine and lysine residues consistent with an amphipathic alpha-helix. By using both quadrupole echo and MAS experiments, together with SFG method, it is found that the dynamics of the leucine side chains in this peptide is quantifiable, and is strongly upon the orientation of the polymer on the surface, which in turn depends on whether the LKalpha14 peptide adsorbs onto a polar or non-polar surface. Another peptide-surface system is SN-15 peptide binding on hydroxyapatite surface. The dynamics of phenylalanine side chains in SN-15 are studied using deuterium MAS line shape and relaxation. The spectra are interpreted using a model in which the phenylalanine side chain exchanges between rotameric states. Although rotameric exchange motions of phenylalanine have been observed for small peptides in solutions, this is the first time such exchange has been observed in solid state samples. Moreover, when the dynamics of two different phenylalanine side chains within the SN-15 peptide are compared, both rotameric populations and rotameric angles differ as functions of orientation and surface proximity. These results show that the side chain dynamics of surface-adsorbed peptides are very different with the same peptides in solid, but actually resemble to the dynamics of side chains in solution state.
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