Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies

Transcription

1 Supporting Information Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies Theodore E. Wiley 1, William R. Miller 1, Nicholas A. Miller 1, Roseanne J. Sension* 1 Piotr Lodowski 2, Maria Jaworska 2, Pawel M. Kozlowski,* 3,4 1 Department of Chemistry, University of Michigan, 930 N University Ave, Ann Arbor, MI Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, Katowice, Poland 3 Department of Chemistry, 2320 South Brook Street, University of Louisville, Louisville, Kentucky Visiting Professor at Department of Food Sciences, Medical University of Gdansk, Al. Gen. J. Hallera 107, Gdansk, Poland Table of Contents A. Steady State Photolysis Data pp. 2-3 B. Time-Resolved Excited State Absorption pp. 3-5 C. Calculation of Species Associated Difference Spectra (SADS) pp. 6-7 D. Truncated Structure Used in Calculations. p. 8 E. Potential Energy Curves and 2D Surface for HOCbl pp F. Experimental Methods pp G. Computational Methods p. 12 1

2 2 A. Steady State Photolysis. Under anaerobic conditions in the presence of an OH radical scavenger, sodium benzoate, photolysis to completion forming cob(ii)alamin is observed when 253 nm is used. No photolysis is observed when the sample is irradiated with 355 nm, 400 nm, or 532 nm laser excitation, or with a broadband incandescent visible source. Photolysis is observed with a 450W Xenon lamp and a pyrex filter, although the photolysis rate is ~2000 times slower than an adenosylcobalamin sample run concurrently (ϕ AdCbl 0.2). The threshold for photolysis of HOCbl falls between 290 nm and 350 nm. Figure S1. Photolysis of HOCbl using an unfiltered mercury pen lamp. Figure S2. The final spectrum is consistent with the cob(ii)alamin spectrum obtained following excitation of alkylcobalamins (red dashed line), indicating that the photolysis has gone to completion.

3 3 Figure S3. Difference spectra for the formation of cob(ii)alamin from HOCbl calculated from the photolysis data in Figure S1. B. Time-Resolved Excited State Absorption. Broadband femtosecond UV-Visible transient absorption spectroscopy was used to characterize the excited electronic states of HOCbl. Data was obtained using the second (404 nm) or third (269 nm) harmonic of a khz Ti:Sapphire laser femtosecond laser as the excitation source. Figure S4. Contour plot of the transient absorption data following excitation at 404 nm. The region around 404 nm is omitted because of pump scatter.

4 4 The transient spectra were analyzed by performing a global analysis of the data using the freely available program Glotaran (Snellenburg, J. J.; Laptenok, S.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M. 2012, 49(3), 1-22). In all cases the data was well modeled with two exponential decay components: τ 1 = 0.32 ± 0.08 ps and τ 2 = 5.50 ± 0.17 ps. The errors represent the scatter in the values obtained from analysis of eight independent data sets. There may be a faster component as well, but analysis at short time-delay is complicated by coherent signals from both solvent and solute. Figure S5. Fits to the transient absorption data following excitation at 269 nm at select time delays. Although there may be a small residual photoproduct yield following 269 nm excitation (<1.5%), overall the transient data is similar for both excitation wavelengths. Fits at select wavelength are shown in Figure S6 on the next page. Although the plots only extend to 30 ps, the data was obtained to hundreds of picoseconds to define any observable long time photoproduct.

5 Figure S6 Select kinetic traces and fits following excitation at 269 nm. 5

6 6 C. Calculation of Species Associated Difference Spectra (SADS) Assume a sequential model: hν 1 2 k k S S S S 0 n 1 0 The transient absorption signal is given by a sum of exponentials: I ( λ, t) = A ( λ ) e + A ( λ ) e + A ( λ ) k1t k2t Considering only the population excited by the optical pump pulse the population of the excited states are obtained by solving the coupled differential equations: n& ( t) = k n ( t) Sn 1 n& ( t) = k n ( t) k n ( t) S1 1 Sn 2 S1 n& ( t) = k n ( t) S0 2 S1 Sn Then the populations are given by: n ( t) = e Sn S0 k1t k k2t k1t ( ) 1 ns ( t) = e e 1 k1 k2 n k1t k2t k2e k1e ( t) = 1+ k1 k2 The transient absorption signal can be given in terms of the difference spectrum for each species. I ( λ, t ) = A ( λ ) n ( t ) + A ( λ ) n ( t ) + A ( λ ) n ( t ) Sn Sn S1 S1 S0 S0 Collecting terms for each exponential component the species associated difference spectra are given by: A ( λ ) = A ( λ ) + A ( λ ) + A ( λ) S n S ( k k ) A λ = A A 1 2 S ( ) 1 2( λ ) + 3( λ) k1 A ( λ ) = A ( λ) 0 3 Any permanent photoproduct is produced in sufficiently low yield that it does not modify the result significantly. This product could be produced directly upon excitation, from the S n state or from the S 1 state.

7 Figure S7. Species associated difference spectra determined for the S n state (0.32 ± 0.08 ps lifetime) and S 1 state (5.50 ± 0.17 ps lifetime) following excitation at 404 nm and 269 nm. 7

8 8 D. Truncated Structure Used in Calculations. Figure S8. Structural model of HOCbl employed in present work and denoted as Im-[Co III (corrin)]-oh +.

9 9 E. Potential Energy Curves and 2D Surface for HOCbl. Figure S9. Potential energy curves of the ground and lowest singlet excited states of the Im-[Co III (corrin)]-oh + model complex along the Co-C bond stretch computed at the ground state optimized geometry.

10 Figure S10. Potential energy surface for the S 1 electronic state of the Im-[Co III (corrin)]-oh + model complex in the optimized geometry of the excited state plotted as a function of axial bond lengths. 10

11 11 F. Experimental Methods The hydroxocobalamin, adenosylcobalamin, sodium benzoate, TEMPO, and sodium carbonate salts were obtained from Sigma Aldrich and used without further purification. The ph was buffered at ca using a 12 mm carbonate buffer solution to insure that the dominant form was HOCbl (pk a ~8). The samples were held in 1 mm path length quartz static cuvettes or flow cells (NSG precision cells) for all experiments. The HOCbl concentrations of the sample were ~ 0.3 mm, in order to ensure an optical density (OD) of 1.0 at the pump wavelength (269 nm). Photolysis: All photolysis experiments were conducted under anaerobic conditions to prevent the reaction of cob(ii)alamin with dissolved oxygen. Samples were bubbled under dry nitrogen in sealed vials for ~1 hour prior to experiments. After bubbling, samples were transferred to nitrogen purged 1 mm quartz cuvettes via syringe. Sodium benzoate was added to the HOCbl solutions in a 5:1 molar ratio to serve as an OH radical scavenger. Adenosylcobalamin (AdoCbl) references were prepared in deionized water. TEMPO was added to the AdoCbl solution in a 5:1 molar ratio to serve as an adenosyl radical scavenger. Steady state photolysis experiments were carried out using the 253 line of an Oriel Hg pen lamp, the 355 nm (3 rd harmonic) of an Nd:YAG laser (EKSMA), a 532 nm CW laser, or filtered output of a 450W Xenon lamp. A 13 mm pyrex filter was used to cut off the Xenon lamp spectrum at ~ 290 nm. AdoCbl was photolyzed concurrently with HOCbl in all experiments to serve as an intensity reference. Transient absorption: A home built Ti:Sapph oscillator was used to produce an 88 MHz pulse train of ~808 nm pulses with a bandwidth of ~30 nm. A pulse picker (Quantum Technologies) operating at 1 khz selected a subset of the pulse train to seed a multi-pass amplifier, producing ~500 mw ~60 fs pulses. The output is split between the pump and probe arms of the transient absorption system. The probe beam is a white light continuum generated by focusing an 808 nm or 404 nm beam into a 3 mm thick calcium fluoride crystal. Generating the continuum with 808 nm pulses produces wavelengths from nm, while the continuum produced using 404 goes deeper into the UV ( nm). The calcium fluoride is slowly translated in a plane normal to the incoming beam to prevent optical damage. Time delays between the pump and probe are achieved by delaying the probe with respect to the pump using a mechanical stage (Newport, ILS150PP). A 350 mm spherical mirror focuses the probe into the sample to a spot size of ~ 75 um. The probe is collected by fiber optic cable, which transmits it to a 1350 pixel CCD detector (AvaFast, Avantes), which operates at 500 Hz for shot-to-detection. The pump beam is either the 2 nd or 3 rd harmonic of the laser, 404 or 269 nm respectively. An optical chopper (Thorlabs, Inc.) operating at 500 Hz modulates the pump beam. A λ/2 plate is used to maintain the polarization of the pump at magic angle (54.7 ) with respect to the probe polarization. The pump beam is focused into the sample by a 250 mm fused silica lens to a spot

12 12 size of ~ 150 um. The pump pulse energy is maintained at ~250 nj by a variable neutral density filter. G. Computational Methods All of the calculations reported in this work were carried out using DFT and TD-DFT with the nonhybrid (GGA) Becke Perdew (BP86) exchange-correlation functional and the TZVPP for Co, C, N, and TZVP for H basis sets, as implemented in the Turbomole suite of programs for electronic structure calculations. This level of theory is appropriate for predicting both geometries and energies for cobalt-corrinoids. In calculations, the Resolution of the Identity approach for computing the electronic coulomb interaction (RI-J) was applied with the corresponding auxiliary basis sets for RI-DFT. To account for environmental effects on geometries and electronic properties, ground and excited states of all investigated structures were computed with the use of Conductor-like Screening Model (COSMO) and water as solvent.