¹⁵N SOLID-STATE NMR DETECTION OF FLAVIN PERTURBATION BY H-BONDING IN MODELS AND ENZYME ACTIVE SITES

• Main Author: Cui, Dongtao
• Format: Others
• Published: UKnowledge 2010
• Subjects: Solid-state NMR; DFT; electronic structure; TPARF; flavoprotein; Biochemistry; Chemistry
• Online Access: http://uknowledge.uky.edu/gradschool_diss/48; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1052&context=gradschool_diss

Massey and Hemmerich proposed that the different reactivities displayed by different flavoenzymes could be achieved as a result of dominance of different flavin ring resonance structures in different binding sites. Thus, the FMN cofactor would engage in different reactions when it had different electronic structures. To test this proposal and understand how different protein sites could produce different flavin electronic structures, we are developing solid-state NMR as a means of characterizing the electronic state of the flavin ring, via the 15N chemical shift tensors of the ring N atoms. These provide information on the frontier orbitals. We propose that the 15N chemical shift tensors of flavins engaged in different hydrogen bonds will differ from one another. Tetraphenylacetyl riboflavin (TPARF) is soluble in benzene to over 250 mM, so, this flavin alone and in complexes with binding partners provides a system for studying the effects of formation of specific hydrogen bonds. For N5, the redoxactive N atom, one of the chemical shift principle values (CSPVs) changed 10 ppm upon formation of a hydrogen bonded complex, and the results could be replicated computationally. Thus our DFT-derived frontier orbitals are validated by spectroscopy and can be used to understand reactivity. Indeed, our calculations indicate that the electron density in the diazabutadiene system diminishes upon H-bond complex formation, consistent with the observed 100 mV increase in reduction midpoint potential. Thus, the current studies of TPARF and its complexes provide a useful baseline for further SSNMR studies aimed at understanding flavin reactivity in enzymes.