Photophysical Properties of Structurally and Electronically Modified Flavin Derivatives Determined by Spectroscopy and Theoretical Calculations

Abstract

Four different riboflavin (RF) derivatives, two electronically modified compounds (1- and 5-deazariboflavin, 1DRF and 5DRF) and two sterically modified compounds (7,8-didemethyl- and 8-isopropylriboflavin, DMRF and iprRF), were subjected to a combination of time-resolved measurements (absorption and fluorescence) and high-level quantum chemical investigations. Both alkyl-modified flavins showed similar fluorescence properties as the parent compound, yet 5DRF had a larger quantum yield of fluorescence (PhiF = 0.52) than RF (PhiF = 0.27). Interestingly, 1DRF did not show fluorescence at all under these steady state conditions. The triplet quantum yield was different for the modified flavins such that no triplet formation was found for 1DRF, whereas the other compounds all formed triplet states (PhiTR for 5DRF of 0.64 and 0.50 and 0.23 for iprRF and DMRF, respectively). The triplet states of the two alkyl-modified flavins decayed with similar time constants as the parent compound, whereas a shorter lifetime was measured for 5DRF (tauTR = 15 micros, compared to tauTR = 29 micros for RF). In the calculations, the flavin derivatives were modeled as lumiflavins, that is, without the ribityl chain. We conclude that for aqueous solutions of DMRF, iprRF, and 5DRF intersystem crossing (ISC) takes place from the S1 1(pipi*) to the T2 3(pipi*) state by a vibronic spin-orbit coupling mechanism, a process common to most flavins, whereas ISC is slow in excited 1DRF due to the absence of a close-by triplet state.

Electronic and Protein Structural Dynamics of a Photosensory Histidine Kinase

“…These bands are attributed to a S 0 -S 1 transition in the isoalloxazine moiety (15) and are qualitatively identical to that measured for phot1-LOV2 (Figure 1A). The near-UV flavin absorption band of LovK, can be attributed to S 0 -S 2 / S 0 -S n transitions (16, 17), and is blue-shifted relative to phot1-LOV2. Both LovK and phot1-LOV2 have two peaks in this wavelength range with similar gaps: 371/355 nm and 376/360 nm, respectively (Figure 1A).…”

Spin–orbit coupling and intersystem crossing in molecules

“…In principle, efficient prescreening of the FCFs can extend the applicability of this approach,124,125 but the fundamental problem persists: With increasing number of normal modes and/or increasing adiabatic electronic energy difference, the VDOS grows to an extent that the computation of rate constants for nonradiative transitions, even in the Condon approximation, becomes computationally very demanding. The time‐independent ansatz using a finite search interval has so far been the main approach to compute ISC rates in our laboratory 16,17,110,111,126–133. With a slight modification it may also be employed for computing rates for the transition from a bound initial state to the vibrational continuum of a dissociative state.…”

Computing UV/vis spectra from the adiabatic and vertical Franck-Condon schemes with the use of Cartesian and internal coordinates

“…We note in this context that a common strategy in the calculation of FC spectra is to focus on the vibrational modes that affect the spectral shape most strongly. 45,46 Our approach may be regarded as a different kind of mode selection, in which we include all modes except those that are technically problematic and are not expected to influence the spectrum too much. An alternative would be the independent mode displaced harmonic oscillator model (IMDHO 12,47 ) that uses the ground-state vibrational modes in a VFC-type treatment and thus disregards the differences in the shapes of the ground-state and excited-state PES, which might also cause artifacts in the computed spectra (see the section on FC Spectra).…”