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Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies Theodore E Wiley, William R Miller, Nicholas A Miller, Roseanne J. Sension, Piotr Lodowski, Maria Jaworska, and Pawel M Kozlowski J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02333 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 12, 2015
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The Journal of Physical Chemistry Letters
Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies Theodore E. Wiley1, William R. Miller1, Nicholas A. Miller1, Roseanne J. Sension*1 Piotr Lodowski2, Maria Jaworska2, Pawel M. Kozlowski,*3,4 1
Department of Chemistry, University of Michigan, 930 N University Ave, Ann Arbor, MI 48109-1055.
2
Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland
3
4
Department of Chemistry, 2320 South Brook Street, University of Louisville, Louisville, Kentucky 40292 and Visiting Professor at Department of Food Sciences, Medical University of Gdansk, Al. Gen. J. Hallera 107, 80-416 Gdansk, Poland
ABSTRACT: Hydroxocobalamin is a potential biocompatible source of photogenerated hydroxyl radicals localized in time and space. The photogeneration of hydroxyl radicals is studied using time-resolved spectroscopy and theoretical simulations. Radicals are only generated for wavelengths 350 nm. In contrast, photolysis to cob(II)alamin is readily observed following excitation at 253 nm. Photolysis is also observed with a xenon lamp using a pyrex filter to limit the UV, but the rate is ~2000 times slower than the comparable photolysis of adenosylcobalamin. The threshold for photolysis falls between 290 nm and 350 nm. (See Supporting Information Figures S1-S3).
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nm to 800 nm) and (270 nm to 600 nm) allowing characterization of transient species in the UV-visible region of the spectrum. A typical transient spectrum is plotted in Figure 2.
Figure 2. Contour plot of the transient absorption data following excitation at 269 nm. The data obtained following 404 nm excitation are similar (see Supporting Information Figure S4). The data were fit to a sum of exponentials using a global 6 analysis program. In all cases the data are well modeled using two exponential decay components: τ1 = 0.32 ± 0.08 ps and τ2 = 5.50 ± 0.17 ps. There may be a faster component as well, but analysis at short time-delays is complicated by coherent signals from both solvent and solute (see Supporting Information Figures S5-S6 for details of the fits). Typical fits at 540 nm are shown in Figure 3. The difference at early times results from a difference in pulse width and the presence of a strong coherent signal following 404 nm excitation. Following 404 nm excitation the transient absorption signal decays to baseline. The noise level of the measurement sets an upper limit of 100 ps with the steady state difference spectrum for photolysis to cob(II)alamin. These data set an upper limit of
