Stability of Flavin Semiquinones in the Gas Phase: The Electron Affinity, Proton Affinity, and Hydrogen Atom Affinity of Lumiflavin

Abstract

Examination of electron transfer and proton transfer reactions of lumiflavin and proton transfer reactions of the lumiflavin radical anion by Fourier transform ion cyclotron resonance mass spectrometry is described. From the equilibrium constant determined for electron transfer between 1,4-naphthoquinone and lumiflavin the electron affinity of lumiflavin is deduced to be 1.86 ± 0.1 eV. Measurements of the rate constants and efficiencies for proton transfer reactions indicate that the proton affinity of the lumiflavin radical anion is between that of difluoroacetate (331.0 kcal/mol) and p-formyl-phenoxide (333.0 kcal/mol). Combining the electron affinity of lumiflavin with the proton affinity of the lumiflavin radical anion gives a lumiflavin hydrogen atom affinity of 59.7 ± 2.2 kcal/mol. The ΔG298 deduced from these results for adding an H atom to gas phase lumiflavin, 52.1 ± 2.2 kcal/mol, is in good agreement with ΔG298 for adding an H atom to aqueous lumiflavin from electrochemical measurements in the literature, 51.0 kcal/mol, and that from M06-L density functional calculations in the literature, 51.2 kcal/mol, suggesting little, if any, solvent effect on the H atom addition. The proton affinity of lumiflavin deduced from the equilibrium constant for the proton transfer reaction between lumiflavin and 2-picoline is 227.3 ± 2.0 kcal mol(-1). Density functional theory calculations on isomers of protonated lumiflavin provide a basis for assigning the most probable site of protonation as position 1 on the isoalloxazine ring and for estimating the ionization potentials of lumiflavin neutral radicals.

IRMPD spectroscopy of metalated flavins: structure and bonding of Mq+–lumichrome complexes (Mq+ = Li+–Cs+, Ag+, Mg2+)

“…Basic properties of flavins, their ground-state structure, their stability, and their interaction with metal ions and solvent must be characterized to generate a consistent molecular-level description of their activity and function. 16 However, due to their difficult preparation in the gas phase, experiments on isolated flavins have not been performed until recently, [17][18][19] and studies of their metal and solvent adducts are completely lacking. The few available studies include photo-and collision-induced fragmentation of protonated flavin mononucleotide (FMN), 19 a fluorescence spectrum of lumichrome (LC) in superfluid He droplets, 20 and the determination of the proton affinity of lumiflavin (LF) by mass spectrometry.…”

“…The few available studies include photo-and collision-induced fragmentation of protonated flavin mononucleotide (FMN), 19 a fluorescence spectrum of lumichrome (LC) in superfluid He droplets, 20 and the determination of the proton affinity of lumiflavin (LF) by mass spectrometry. 17 Moreover, the preferred protonation site of a variety of fundamental protonated flavins, including LC, LF, riboflavin (RF, vitamin B 2 ), and FMN, have recently been determined by infrared multiphoton dissociation (IRMPD) spectroscopy and quantum chemical calculations. 18 It was shown that the protonation site strongly depends on the flavin substituent and that the strongly IR active CO stretch modes are sensitive indicators of the various protonation sites.…”

Photodegradation of Riboflavin under Alkaline Conditions: What can Gas-Phase Photolysis Tell Us About What Happens in Solution?

“…80 fold in alkaline media (correlated with the increased reactivity of the triplet state) until pH 10, when the anion is formed [4]. Unsurprisingly, the extensive solution-phase photochemical behavior of riboflavin has accrued much experimental attention over the years [1,5]; however, studies of its intrinsic (i.e., gas-phase) excited-state properties, and those of other flavins, have been sparse until very recently [6][7][8][9][10][11][12][13]. To date, the only gas-phase study of a system involving riboflavin is that of Dopfer and co-workers on alkali metal coordination [9].…”

“…For anionic flavin systems, Matthews et al have compared the intrinsic electronic behavior of deprotonated LC and its related chromophore alloxazine (AL), finding novel near-threshold transient anion resonance states, which were assigned to dipole-bound excited states [6]. Recently, Bull et al have utilized tandem ion mobility spectrometry and action spectroscopy to bring to light the influence that deprotonation sites have on the electronic absorption properties and photochemistry of the FAD anion [8], following on from earlier experimental work assigning probable sites of (de)protonation of flavins in the gas phase [9][10][11][12][13][14]. Non-statistical (i.e., occurring during the excited-state lifetime) LC formation of the FAD mono-anion was found to proceed through a photo-induced intramolecular proton-coupled electron transfer of riboflavin [9].…”

Uma nova estratégia para o cálculo de afinidades eletrônicas

“…A título de exemplo, AEs desempenham papel na química do fulereno [1], células solares [2,3], semicondutores [4][5][6], sensores ópticos [7,8], química de interfaces [9], etc. Pesquisas também enfatizam o papel da AE no entendimento de mecanismos de reação [10], estabilidade e papel biológico de compostos químicos [11].…”