Ion mobility action spectroscopy of flavin dianions reveals deprotomer-dependent photochemistry† †Electronic supplementary information (ESI) available: Further details of electronic structure calculations; ATDs of FAD dianions in N2 + ≈1% isopropyl alcohol buffer gas; photo-action ATDs for flavin mo

Photo-induced proton transfer, deprotomer-dependent photochemistry, and intramolecular charge transfer in flavin anions are investigated using action spectroscopy.


Figure S1
Labelling convention for deprotomers of FAD dianions and RB monoanions. The circles indicate deprotonation sites on the phosphate groups. For the N-3,PO 4 and OHx,PO 4 (x = 1-3) deprotomers, the remaining phosphate proton is shared between both PO 4 groups. Site OH4 is not relevant for FAD.   Table S2 Calculated relative energies, vertical transition wavelengths (df-CC2/6-31+G(d) level of theory, oscillator strengths in parentheses) and collision cross-sections for the lowest energy FAD dianion deprotomer structures -see illustrations of selected structures in Figure S2. Details for π-stacked conformations for the N-3,PO 4 and PO 4 ,PO 4 deprotomers are given in the lower section of the table. Efforts to locate a PO 4 ,PO 4 deprotomer in which one PO 4 group was closer to the N-3 hydrogen (i.e. for direct proton transfer) ultimately optimised to a PO 4 ,PO 4 structure in which the ribityl chain is directed away from the flavin unit.

Experimental collision cross-sections
Experimental collision cross-sections for the two FAD dianion ATD peaks in N 2 buffer gas ( Fig. 3(a) in the paper) were determined using the Mason-Schamp equation and the relevant instrument parameters: Here, K is the ion's mobility, z is number of elementary charges carried by the ion (2 for FAD dianions), e is the electronic charge, N is the number density of the buffer gas (pressure measured using a calibrated Baratron gauge was 6.7±0.1 Torr), m is the reduced mass of the colliding ion-neutral pair, k B is the Boltzmann constant, T is the absolute temperature, Ω is the collision cross-section, l is the length of the drift region (0.99 m including IF2), t d is the transit time through the drift region and V is the potential drop across the drift region.
In our instrument the measured arrival time of an ion packet, t, is given by: where t d , t oct and t quad are the ion transit times through the drift region (6.7 ±0.1 Torr), octupole ion guide (≈10 −4 Torr), and quadrupole mass filter (≈10 −6 Torr), respectively. Values of t oct and t quad were calculated from instrument parameters (dimensions and kinetic energy of the ions in the octupole ion guide and quadrupole mass filter) and are small (≈0.3 ms) compared with t d (≈12.2 ms).
For the two ATD peaks in Fig. 3(a) in the paper, values of t are 12.64 ms (isomer 1) and 12.47 ms (isomer 2). Corresponding values of t d are 12.34 and 12.17 ms, respectively. Substituting these data into the Mason-Schamp equation yields Ω values of 305±10 and 299±10 Å 2 , where the uncertainty is predominately associated with the buffer gas pressure. The uncertainty in relative collision cross-sections (assuming a ±0.02 ms uncertainty in t for each peak) is much less at ±0.5 Å 2 .
It is difficult to compare directly the experimental collision cross-sections with the calculated values given in Table  S2 due to the approximate nature of the MOBCAL approach and the lack of benchmarked parameters for interactions between N 2 and anions. Moreover, as discussed in the paper, there is probably rapid interconversion between FAD dianion conformations in the gas phase, meaning that the experimental cross-sections represent conformationallyaveraged values whereas the calculations assume static structures. For example, the difference in energy between the 'open' and 'π-stacked' conformations for the PO 4 ,PO 4 deprotomer is only 7 kJ/mol, however the corresponding calculated collision cross-sections differ by 12 Å 2 . The peak assigned to the PO 4 ,PO 4 deprotomer in our room-temperature ATDs is presumably associated with is a time-average of these and many other conformations. Arrival time distributions for FAD dianions using N 2 buffer gas seeded with ∼1% isopropyl alcohol: (a) FAD dianions under different ion funnel (IF1) conditions, and (b)/(c) isomers 1/2 gated with IG2. These ATDs show two well separated peaks with instrument limited widths (resolutions t/∆t∼110) consistent with the existence of two predominant dianion species in the gas phase. Note, the introduction of isopropyl alcohol dopant reduced the ion current making it difficult to collect action spectra. The identity of the photo-isomer is not known, although we note that the calculated collision cross-sections for the PO 4 and N-3 deprotomers are 227 and 209 Å 2 , respectively, suggesting that excitation causes conversion of the former to the latter. Note that an ATD without using IG2 was identical to the 'laser-off' ATD, which exhibits a single peak, consistent with the presence of only one isomer. Arrival time distribution for deprotonated RB monoanions (black curve) and laser induced difference signal or 'photo-action' (orange curve) at 500 nm. Only depletion consistent with electron detachment was observed. Note that an ATD without using IG2 was identical to the 'laser-off' ATD, which exhibits a single peak with instrumentally limited width, consistent with the presence of only one isomer. Comparison of action spectra for deprotonated RB monoanions recorded using the IMS instrument at the University of Melbourne (UM) and the SepI photodissociation instrument at Aarhus University. The action spectra of both main photo-fragments in the SepI experiments are identical, and both are very similar to the UM depletion spectrum. The addition of betaine induces a strong blue-shift.