Date on Master's Thesis/Doctoral Dissertation

5-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Kozlowski, Pawel

Author's Keywords

Vitamin B12; Spectroscopy; DFT

Subject

Vitamin B12

Abstract

Enzymes incorporating B12 cofactors (cobalamins) are significant biological components as they are involved in the metabolism of every cell of the body. The absorbing riddle, yet unresolved, is the enormous acceleration of the reaction rate that is strictly related to the Co–C bond cleavage initiating every catalytic cycle. Consequently, the realistic explanation of the processes controlled by B12 derivatives demands a profound analysis of their structure coupled with the examination of factors influencing subsequent steps of the reaction as well as properties of the reagents and intermediates. Herein, the photochemical study of cobalamins was presented as it provides a sensitive probe for the investigation of the reactivity of the cobalt-carbon bond in terms of any structural or environmental modification. Theoretical investigations, guided by Density Functional Theory (DFT), Time-Dependent DFT (TD-DFT) and correlated ab initio methods were performed in order to explore the nature of electronically excited states of different vitamin B12 derivatives and the mechanism of events following the photon excitation in vitamin B12. Initially, the most appropriate density functional to study the photochemistry of these complex molecules was determined. Its accuracy was evaluated with respect to both its agreement with available experimental data and higher level ab initio outcomes. Next, the absorption (Abs), circular dichroism (CD) and magnetic CD were calculated for the first time. Based on the examination of certain spectral regions the electronic properties of B12 derivatives were examined. The correlation between the nature of specific electronic transitions and the structure was extensively discussed. In addition, the environmental effect on the character of excited states was investigated as the calculations were performed both in gas phase as well as in water solution modeled via the polarizable continuum model (PCM) or conductor-like screening model (COSMO). In addition, the study of cob(I)alamin, lacking axial ligands, is of significant value especially considering the fact that the complete model of cob(I)alamin was applied as opposed to a majority of previous theoretical efforts. Finally, the lack of photodissociation in CNCbl as opposed to other B12 analogs was investigated in detail. The electronically excited states of CNCbl were examined as a function of two axial ligands calculated at TD-DFT level of theory. By analyzing the shapes of potential energy surfaces (PESs) and the nature of its lowest excited states in different environments, the explanation of subsequent steps following photon excitation of that system was provided.

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