XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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A sequential QM/MM study of the absorption spectrum of vitamin B12 cyanocobalamin in explicit water environment.
Paula Jaramillo, Kaline Coutinho and Sylvio Canuto
Instituto de Física, Universidade de São Paulo, São Paulo, Brazil
The cobalt-corrin ring is the basic unit of the vitamin B12 coenzymes which catalyses radical rearrangements and methyl transfer in biological systems [1]. Their enzymatic activities are based on the properties of a unique Co-C bond. This organometallic compound has interesting physical and biological properties. Because of its great biological importance [1,2] several theoretical studies have been performed, but mostly restricted to the central ring. In this work, the absorption spectrum of the full vitamin cyanocobalamin B12 in water is calculated with the sequential QM/MM method. The experimental spectrum in water [2] shows two bands: the intense gamma band near 350 -380 nm and the combined alpha-betha bands between 460-560 nm. Different procedures using continuum, discrete and explicit solvent models were used to include the solvation effect on the calculated absorption spectrum. The discrete and explicit models used Monte Carlo simulation [3] to generate the liquid structure. INDO/CIS (ZINDO) and TDDFT calculations were performed to obtain the excitation energies. Two different functionals, B3LYP and BP86, were used for the spectrum calculations with continuum and discrete models. The explicit model was able to correctly describe the experimental betha and gamma bands, but the low lying alpha band is not obtained with the B3LYP functional. The BP86 calculations give a qualitative and quantitative description with differences between 4 and 10 nm in comparison with the experimental bands. The best result was obtained with the discrete model where the full vitamin B12 is calculated embedded in the electrostatic field of 2000 water. This work gives a good theoretical description and consequently strongly emphasizes the importance of considering the full molecular structure and the role of the solvent in the absorption spectrum of the vitamin B12.
References
[1] K. L. Brown. Chem. Rev. 105, 205 (2005)
[2] T. A. Stich, A. J. Brooks, N. R. Buan, T. C. Brunold. J. Am. Chem. Soc. 125, 5897 (1996)
[3] K. Coutinho, S. Canuto. J. Chem. Phys. 113, 9132 (2000).


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