Absorption and luminescence spectroscopy of mass-selected Flavin Adenine Dinucleotide mono-anions

TL;DR

This study investigates the absorption and luminescence spectra of isolated FAD mono-anions, revealing significant charge-transfer character and large Stokes shift, which enhance understanding of flavin photophysics in the gas phase.

Contribution

It provides the first gas-phase absorption and luminescence spectra of FAD mono-anions, highlighting differences from solution-phase behavior and emphasizing vibronic effects.

Findings

The S0→S2 transition is strongly blue-shifted compared to aqueous solution.

Remaining absorption bands are close to solution-phase positions.

The gas-phase Stokes shift for S1 is 3000 cm$^{-1}$, larger than other molecular ions.

Abstract

We report the absorption profile of isolated Flavin Adenine Dinucleotide (FAD) mono-anions recorded using Photo-Induced Dissociation action spectroscopy. In this charge state, one of the phosphoric acid groups is deprotonated and the chromophore itself is in its neutral oxidized state. These measurements cover the first four optical transitions of FAD with excitation energies from 2.3 to 6.0~eV (210--550~nm). The $S_0\rightarrow S_2$ transition is strongly blue-shifted relative to aqueous solution, supporting the view that this transition has significant charge-transfer character. The remaining bands are close to their solution-phase positions. This confirms that the large discrepancy between quantum chemical calculations of vertical transition energies and solution-phase band maxima can not be explained by solvent effects. We also report the luminescence spectrum of FAD mono-anions \textit{in vacuo}. The gas-phase Stokes shift for $S_1$ is 3000~cm$^{-1}$, which is considerably larger than any previously reported for other molecular ions and consistent with a significant displacement of the ground and excited state potential energy surfaces. Consideration of vibronic structure is thus essential for simulating the absorption and luminescence spectra of flavins.