Effect of alkali ions on optical properties of flavins: vibronic spectra of cryogenic M⁺lumiflavin complexes (M = Li–Cs)

Abstract

Cryogenic ion spectroscopy of metal–lumiflavin (M+LF) complexes at the level of vibrational resolution illustrates the large impact of alkali ions on the optical properties of this prototypical flavin molecule.

Vibronic optical spectroscopy of cryogenic flavin ions: the O2+ and N1 tautomers of protonated lumiflavin

“…The previous VISPD study on the S 1 ' S 0 transitions of the related O4+ protomers of M + LF (M = Li-Cs) using the same computational approach yields differences of less than 650 cm À1 between measured and computed adiabatic S 1 origins. 33 Additionally, deviations in the same order of magnitude have been reported for the S 1 transitions of H + LC(N5) (809 cm À1 ) and M + LC(O4+) (up to 1154 cm À1 ). 31,32 Clearly, the S 1 origins predicted for the other H + LF protomers fit less well.…”

“…In particular, the computed S 1 origins of the N5 and O4AE tautomers exhibit large redshifts of B4000-7000 cm À1 , similar to those observed for O4+ of M + LF. 33 The OH+AE tautomers also have substantial computed S 1 redshifts of B1500 cm À1 . The only other protomer with a predicted nearby S 1 origin is O2À with DS 1 = À107 cm À1 .…”

“…20 The small difference between the experimental and computed S 1 origins of B1000 cm À1 (B0.1 eV) confirms that the chosen DFT level describes the electronic structure of this type of flavin well. [31][32][33] In general, the protonation-induced DS 1 shifts depend strongly on the site of protonation and span a large range from À6826 to +578 cm À1 . Interestingly, for the N1 protomer the order of the excited S 1 and S 2 states changes, when going from vertical to adiabatic energies.…”

“…Subsequent studies characterize the effects of metalation of LC and LF with alkali metal cations (M = Li-Cs) on their optical properties, illustrating that the M + Á Á ÁLF/LC interaction strength and thus the S 1 excitation energy shifts depend strongly on the metalation site. 32,33 Herein, we extend these VISPD studies to H + LF. The major questions to be addressed are the observed protonation sites, the effects of protonation on the optical properties of LF, and the differences between H + LF and H + LC/M + LF studied previously by the same spectroscopic and computational approach.…”

“…† Multidimensional Franck-Condon simulations at T = 0 K are carried out to generate vibronic excitation spectra. 37 In previous studies on related flavins (H + LC, M + LC/LF with M = Li-Cs), [31][32][33] the employed PBE0/cc-pVDZ level was shown to be a computationally efficient and reliable DFT approach for calculating vibronic spectra. Orbitals contributing to the observed transitions are determined using the natural transition orbital (NTO) approach.…”