Effect of xenon on the excited States of phototropic receptor flavin in corn seedlings

The chemically inert, water-soluble heavy atom gas, xenon, at millimolar concentrations specifically quenches the triplet excited state of flavin in solution without quenching the flavin singlet excited state. The preferential quenching of the flavin triplet over the singlet excited state by Xe has been established by showing that the flavin triplet-sensitized photooxidation of NADH is inhibited while the fluorescence intensity and lifetime of flavin are not affected by Xe.     

Effect of flavin structure and redox state on catalysis by and flavin-pterin energy transfer in Escherichia coli DNA photolyase

5-DeazaFAD bound to a hydrophobic site in apophotolyase and formed a stable reconstituted enzyme, similar to that observed with FAD. Although stoichiometric incorporation was observed, the flavin ring modification in 1-deazaFAD interfered with normal binding, decreased protein stability, and prevented formation of a stable flavin radical, unlike that observed with FAD. The results suggest that an important hydrogen bond is formed between the protein and N (1) in FAD, but not N (5), and that there is sufficient space at the normal flavin binding site near N (5) to accommodate an additional hydrogen but not near N (1). Catalytic activity was observed with enzyme containing 5-deazaFADH2 (42% of native enzyme) or 1-deazaFADH2 (11% of native enzyme) as its only chromophore, but no activity was observed with the corresponding oxidized flavins, similar to that observed with FAD and consistent with a mechanism where dimer cleavage is initiated by electron donation from excited reduced flavin to substrate. The protein environment in photolyase selectively enhanced photochemical reactivity in the fully reduced state, as evidenced by comparison with results obtained in model studies with the corresponding free flavins. Phosphorescence was observed with free or photolyase-bound 5-deazaFADH2, providing the first example of a flavin that exhibits phosphorescence in the fully reduced state. Formation of an enzyme-substrate complex resulted in a nearly identical extent of quenching of 5-deazaFADH2 phosphorescence (85.1%) and fluorescence (87.5%). The data are consistent with a mechanism involving exclusive reaction of substrate with the excited singlet state of 5-deazaFADH2, analogous to that proposed for FADH2 in native enzyme. Direct evidence for singlet-singlet energy transfer from enzyme-bound 5-deazaFADH2 to 5,10-CH(+)-H4folate was provided by the fact that pterin fluorescence was observed upon excitation of 5-deazaFADH2, accompanied by a decrease in 5-deazaFADH2 fluorescence. On the other hand, the fluorescence of enzyme-bound pterin was quenched by 5-deazaFADox, consistent with energy transfer from pterin to 5-deazaFADox. In each case, the spectral properties of the chromophores were consistent with the observed direction of energy transfer and indicated that transfer in the opposite direction was energetically unlikely. Unlike 5-deazaFAD, energy transfer from pterin to FAD is energetically feasible with FADH2 or FADox. The results indicate that the direction of flavin-pterin energy transfer at the active site of photolyase can be manipulated by changes in the flavin ring or redox state which alter the energy level of the flavin singlet.     

Use of flavin photochemistry to probe intraprotein and interprotein electron transfer mechanisms

Photoexcitation of flavin analogs generates the lowest triplet state (via intersystem crossing from the first excited singlet state) in the nanosecond time domain and with high quantum efficiency. The triplet, being a strong oxidant, can abstract a hydrogen atom (or an electron) from a reduced donor in a diffusion-controlled reaction. If the donor is a redox protein, the oxidation process can be used to initiate an electron transfer sequence involving either intramolecular or intermolecular reactions. If the donor is an organic compound such as EDTA, the neutral flavin semiquinone will be produced by H atom abstraction; this is a strong reductant and can subsequently transfer a hydrogen atom (or an electron) to an oxidized redox protein, thereby again initiating a sequence of intramolecular or intermolecular processes. If flavin photoexcitation is accomplished using a pulsed laser light source, the initiation of these protein electron transfer reactions can be made to occur in the nanosecond to microsecond time domain, and the sequence of events can be followed by time-resolved spectrophotometry to obtain rate constants and thus mechanistic information. The present paper describes this technology, and selected examples of its use in the investigation of redox protein mechanisms are given.     

Nanosecond dynamics of a mimicked membrane-water interface observed by time-resolved stokes shift of LAURDAN

We studied the dipolar relaxation of the surfactant-water interface in reverse micelles of AOT-water in isooctane in the nanosecond and subnanosecond time ranges by incorporating the amphipathic solvatochromic fluorescent probes LAURDAN and TOE. A negative component was observed in the fluorescence decays in the red edge of the emission spectrum-the signature of an excited state reaction-with LAURDAN but not for TOE. The deconvolution of the transient reconstructed spectra of LAURDAN based on a model constructed by adding together three log-normal Gaussian equations made it possible to separate the specific dynamic solvent response from the intramolecular excited state reactions of the probe. The deconvoluted spectrum of lowest energy displayed the largest Stokes shift. This spectral shift was described by unimodal kinetics on the nanosecond timescale, whereas the relaxation kinetics of water-soluble probes have been reported to be biphasic (on the subnanosecond and nanosecond timescales) due to the heterogeneous distribution of these probes in the water pool. Most of this spectral shift probably resulted from water relaxation as it was highly sensitive to the water to surfactant molar ratio (w(0)) (60-65 nm at w(0) = 20-30). A small part of this spectral shift (9 nm at w(0) = 0) probably resulted from dipolar interaction with the AOT polar headgroup. The measured relaxation time values were in the range of the rotational motion of the AOT polar headgroup region as assessed by LAURDAN and TOE fluorescence anisotropy decays.     

Limited rotational motion of amphiphilic flavins in dipalmitoylphosphatidylcholine vesicles

The rotational motion of amphiphilic flavins in dipalmitoyl phospholipid bilayers was investigated with fluorescence anisotropy decay measurements. At temperatures between 10 and 50°C the rotation proved to be anisotropic, which indicated composite motion of both the aliphatic side-chain and the isoalloxazine moiety of the octadecyllumiflavin derivatives. Above the phase transition temperature (crystalline→liquid-crystalline state) the depolarization is complete within the average flavin fluorescence lifetime, implicating unrestricted motion and resulting in a non-ordered microenvironment. In the gel or crystalline state the flavin motion can best be characterized as a limited rotation or librational motion. The fluorescence decay of the flavins is heterogeneous at temperatures between 10 and 50°C, which is explained by assuming nanosecond relaxation of the polar phosphatidyl head-groups around the excited flavin. The lack of a significant cholesterol effect suggests that the isoalloxazine is located at the interphase of the bilayer and not in the hydrocarbon region. The microstructure is fluid-like, not in agreement with a preferred static localization of the flavins in the bilayer.     

Singlet oxygen generation by UVA light exposure of endogenous photosensitizers

UVA light (320-400 nm) has been shown to produce deleterious biological effects in tissue due to the generation of singlet oxygen by substances like flavins or urocanic acid. Riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), beta-nicotinamide adenine dinucleotide (NAD), and beta-nicotinamide adenine dinucleotide phosphate (NADP), urocanic acid, or cholesterol in solution were excited at 355 nm. Singlet oxygen was directly detected by time-resolved measurement of its luminescence at 1270 nm. NAD, NADP, and cholesterol showed no luminescence signal possibly due to the very low absorption coefficient at 355 nm. Singlet oxygen luminescence of urocanic acid was clearly detected but the signal was too weak to quantify a quantum yield. The quantum yield of singlet oxygen was precisely determined for riboflavin (PhiDelta = 0.54 +/- 0.07), FMN (PhiDelta = 0.51 +/- 0.07), and FAD (PhiDelta = 0.07 +/- 0.02). In aerated solution, riboflavin and FMN generate more singlet oxygen than exogenous photosensitizers such as Photofrin, which are applied in photodynamic therapy to kill cancer cells. With decreasing oxygen concentration, the quantum yield of singlet oxygen generation decreased, which must be considered when assessing the role of singlet oxygen at low oxygen concentrations (inside tissue).     

Stepwise two-color laser photolysis studies of alpha-cleavage in highly excited triplet states of alpha-acyl-4-phenylphenols.

The photochemical properties of alpha-cleavage of C-O bond in highly excited triplet states (T(n) with n>2) of p-biphenyl acetate and p-biphenyl benzoate (Me-OBP and Ph-OBP) in solution were investigated in comparison with those in the lowest excited singlet and triplet states by using single laser and sequential two-color two-laser photolysis techniques. Upon 266 nm laser photolysis of Me-OBP and Ph-OBP, occurrence of C-O bond cleavage in the excited singlet state was recognized from the observation of the formation of p-phenylphenoxy radical (PPR) in the transient absorption. The quantum yields (Phi(rad)) of the PPR formation were determined to be 0.29 and 0.24 for Me-OBP and Ph-OBP, respectively. Triplet sensitization using acetone (Ac) provided efficient formation of the lowest triplet states (T(1)) of Me-OBP and Ph-OBP, and the molar absorption coefficients of the triplet-triplet absorption were determined. No photochemical reactions were found in the T(1) state. Upon 355 nm laser flash photolysis of the T(1) states of Me-OBP and Ph-OBP, formation of PPR accompanied with decomposition of the triplet state was confirmed in the transient absorption. This observation indicated that alpha-cleavage proceeds in the highly excited triplet state. The quantum yields (Phi(dec)) of the decomposition in the dissociative highly excited triplet state (T(R)) were determined to be 0.25 and 0.15 for Me-OBP and Ph-OBP, respectively. The reaction mechanism for alpha-bond cleavage in the T(R) state was discussed.     

Flavin-photosensitized reactions of retinal and stilbene

1. Retinol and stilbene are both isomerized when they are illuminated anaerobically in the presence of flavins. 2. Triplet quenchers (e.g. oxygen, potassium iodide and paramagnetic ions) inhibit the reaction more efficiently than they quench flavin fluorescence. At 77 degrees C in a diethyl ether-isopentane-ethanol (5:5:2) glass retinol quenches flavin phosphorescence, but not its fluorescence. 3. For the stilbene reaction cis/trans photostationary-state mixtures are obtained with different flavins and these are linearly related to the phosphorescence transition energies of the flavins used. 4. The reaction involves the triplet state of flavin and a scheme for the reaction is suggested. 5. The dependence of the rate of reaction on substrate concentration is explicable in terms of this scheme. 6. The photobleaching of rhodopsin sensitized by flavin is also demonstrated.     

Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2'-aminoacetophenone and its derivatives in solution.

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X-C6H4-COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2(AAP), NHCH3(MAAP), N(CH3)2(DMAAP), NHCOCH3(AAAP), NHCOCF3(TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n,pi*-pi,pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic)= 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S1(pi,pi*) and S2(n,pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.     

The pH dependence of the reactions of flavin triplet states with amino acids: A laser flash photolysis study

The pH dependence of the rate constants of reaction of several amino acids with the triplet states of flavin mononucleotide in aqueous solution has been determined. In addition, the relative contributions of hydrogen atom transfer, electron transfer and physical deactivation to the overall process of triplet quenching by amino acids have been estimated.Analogous experiments to those with amino acids were carried out with EDTA as the substrate. The results indicate that the flavin triplet state abstracts an electron from EDTA but does not form an excited state flavin-EDTA complex as suggested in a previous study.     

Photophysical properties of methyl beta-carboline-3-carboxylate mediated by hydrogen-bonded complexes--a comparative study in different solvents

The hydrogen bonding interactions of methyl beta-carboline-3-carboxylate (BCCM) in both ground and first singlet excited electronic states have been studied in solvents with different properties in the presence of acetic acid, a hydrogen-bonding donor/acceptor. The methyl ester substituent reduces the pyridinic nitrogen basicity of this beta-carboline derivative. This fact has let us study the hydrogen bonding interactions in a higher range of acetic acid concentrations than for other beta-carboline derivatives previously studied. Steady and non-steady photophysical studies have been carried out in two non-polar solvents, benzene and p-dioxane; and in two polar solvents, acetonitrile and dichloromethane. Six different fluorescence emissions have been isolated corresponding to the uncomplexed BCCM, the protonated species and four different complexes between BCCM and acetic acid whose structures we have tried to elucidate.     

Effect of thienyl groups on the photoisomerization and rotamerism of symmetric and asymmetric diaryl-ethenes and diaryl-butadienes.

Five symmetric (bis-substituted) and asymmetric (mono-substituted) analogues of E-stilbene and EE-1,4-diphenylbutadiene, where one or both the side aryls are 2'-thienyl or 3'-thienyl groups, have been studied by stationary and pulsed fluorimetric techniques, laser flash photolysis, conventional photochemical methods and theoretical calculations. The results obtained for these compounds and the comparison with those previously reported for three other compounds of the same series, allowed the effects of the position of the heteroatom and of the extension of the olefin chain on the excited state relaxation properties to be understood. The presence of one or two thienyl groups and their positional isomerism affect the spectral behaviour, the relaxation properties (radiative/reactive competition), the photoisomerization mechanism (singlet/triplet) and the ground state rotamerism. For the dienes containing the 3'-thienyl substituent(s), two rotamers were evidenced whose radiative and photochemical properties were obtained by selective excitation.     

Resonance Raman studies of ETE dehydrogenase (an iron sulfur flavoprotein)

ETF Dehydrogenase is an iron sulfur flavoprotein responsible for the transfer of electrons between electron transfer flavoprotein (ETF) and CoQ of the electron transport chain. We have determined the resonance Raman spectrum of this enzyme observing in the process at least seven of thirteen flavin bands in the 1100cm-1-1600 cm-1 region of the Raman spectrum. The positions of three of these bands, II, IX, and X (see Figure I and Table I for band numbering system) in ETF dehydrogenase is very similar to their positions in aqueous solution of flavins in which water is hydrogen bonded to N-1, N-5, C=0(2), C=0(4), and N-H(3) of flavin. Conversely the positions of the flavin Raman bands are considerably shifted from those of flavin in nonhydrogen bonding solvent. The positions of bands II, IX, and X are nearly identical to those in the flavoprotein glutathione reductase; x-ray structural investigations on this enzyme indicate that there is extensive hydrogen bonding between FAD and protein in this molecule. A previous study in our laboratory has demonstrated that metal complexation at N-5 and C=0(4) with either Ru or Ag produces large shifts in the positions of Raman bands II, VI, IX, and X. None of these shifts are observed in ETF dehydrogenase indicating that there is no direct inner sphere coordination of Fe to flavin. In addition to the Raman bands of flavin observed in our spectrum, we also observe one band that is in the Fe-S stretching region observed for a variety of Fe-S proteins. This band is located at 331 cm-1. The frequency of the band corresponds to the 335 cm-1 band associated with the strongest Fe-S stretching mode in the 4Fe-4S protein ferrodoxin from C. pasterianum. The observed frequency is quite different from that of the 3Fe-3S proteins such as ferrodoxin(II) from D. gigas. Finally, ETF dehydrogenase shows no loss of activity or visual evidence of photodegradation in the laser beam as most other FeS proteins do.     

Interactions of flavins with melanin. Studies on equilibrium binding of riboflavin to dopa-melanin and some spectroscopic characteristics of flavin-melanin complex

Natural melanins are photoprotective pigments that in mammals are principally found in the skin, hair, and eyes. Although the molecular mechanism of photoprotection of pigmented cells has not yet been established, several hypotheses have been proposed with melanin acting as a light filter, free radical scavenger, and quencher of electronically excited states of reactive intermediates. It can be expected that the detoxicating efficiency of melanin should be enhanced if the melanin and potentially cytotoxic species are brought close together. Such a situation may occur for a number of photosensitizing dyes that have the ability to bind to melanin. The interaction of melanin with flavins has been studied under strictly controlled experimental conditions. The equilibrium dialysis method has been employed to determine dissociation constants and the number of binding sites in melanin at pH 5-9. The data reveal that synthetic DOPA-melanin has two different classes of binding sites with dissociation constants of 10(-6) and 10(-5) M, respectively. The overall binding capacity of melanin, at pH 7, is 250 nmol RF/mg melanin. The amount of bound-to-melanin RF increases with pH. The absorption spectra of melanin complexes with RF and lumiflavin indicate that hydrophobic interaction may be involved in the binding of these flavins by melanin. No changes in flavin fluorescence have been detected after binding of flavin to melanin. It appears that, contrary to cationic photosensitizing dyes, the singlet excited state of flavin molecules is not quenched by melanin.     

The single chlorophyll a molecule in the cytochrome b6f complex: unusual optical properties protect the complex against singlet oxygen

The cytochrome b(6)f complex of oxygenic photosynthesis mediates electron transfer between the reaction centers of photosystems I and II and facilitates coupled proton translocation across the membrane. High-resolution x-ray crystallographic structures (Kurisu et al., 2003; Stroebel et al., 2003) of the cytochrome b(6)f complex unambiguously show that a Chl a molecule is an intrinsic component of the cytochrome b(6)f complex. Although the functional role of this Chl a is presently unclear (Kuhlbrandt, 2003), an excited Chl a molecule is known to produce toxic singlet oxygen as the result of energy transfer from the excited triplet state of the Chl a to oxygen molecules. To prevent singlet oxygen formation in light-harvesting complexes, a carotenoid is typically positioned within approximately 4 A of the Chl a molecule, effectively quenching the triplet excited state of the Chl a. However, in the cytochrome b(6)f complex, the beta-carotene is too far (> or =14 Angstroms) from the Chl a for effective quenching of the Chl a triplet excited state. In this study, we propose that in this complex, the protection is at least partly realized through special arrangement of the local protein structure, which shortens the singlet excited state lifetime of the Chl a by a factor of 20-25 and thus significantly reduces the formation of the Chl a triplet state. Based on optical ultrafast absorption difference experiments and structure-based calculations, it is proposed that the Chl a singlet excited state lifetime is shortened due to electron exchange transfer with the nearby tyrosine residue. To our knowledge, this kind of protection mechanism against singlet oxygen has not yet been reported for any other chlorophyll-containing protein complex. It is also reported that the Chl a molecule in the cytochrome b(6)f complex does not change orientation in its excited state.     

A Q63E Rhodobacter sphaeroides AppA BLUF domain mutant is locked in a pseudo-light-excited signaling state

The AppA BLUF photoreceptor from Rhodobacter sphaeroides contains a conserved key residue, Gln63, that is thought to undergo a shift in hydrogen-bonding interactions when a bound flavin is light excited. In this study we have characterized two substitution mutants of Gln63 (Q63E, Q63L) in the context of two constructs of the BLUF domain that have differing lengths, AppA1-126 and AppA17-133. Q63L mutations in both constructs exhibit a blue-shifted flavin absorption spectrum as well as a loss of the photocycle. Altered fluorescence emission and fluorescence quenching of the Q63L mutant indicate significant perturbations of hydrogen bonding to the flavin and surrounding amino acids which is confirmed by (1)H-(15)N HSQC NMR spectroscopy. The Q63E substitution mutant is constitutively locked in a lit signaling state as evidenced by a permanent 3 nm red shift of the flavin absorption, quenching of flavin fluorescence emission, analysis of (1)H-(15)N HSQC spectra, and the inability of full-length AppA Q63E to bind to the PpsR repressor. The significance of these findings on the mechanism of light-induced output signaling is discussed.     

Visible light-induced photooxidation of glucose sensitized by riboflavin

We conducted this study to evaluate the oxidation of glucose induced by visible light in the presence of sensitizers such as methylene blue and flavins (i.e., flavin mononucleotide and riboflavin). The concentration of the sensitizers was similar to that of flavin in parenteral nutrients. The photooxidation of glucose sensitized by flavin mononucleotide or riboflavin was greater than that which was observed in the presence of methylene blue, whereas the isotopic effect of deuterium oxide (D(2)O) was enhanced more substantially in the presence of methylene blue than in the presence of flavins. These results show that methylene blue exerts its action through singlet oxygen and that at a high substrate concentration (as was used in this work) flavin mononucleotide and riboflavin act preferentially as type I sensitizers. In the flavin photosensitized processes, the presence of hydrogen peroxide, superoxide anion, and hydroxyl radical was demonstrated. The photooxidation of glucose is favored by an increase in pH, and it also depends on the energy absorbed by the system. By using a specific reagent for glucose (i.e., o-toluidine), it was possible to quantify the photoconversion of glucose. The results obtained in this work should be considered in the management of glucose-containing parenteral nutrients that are exposed to visible light in the presence of a multivitamin complex containing flavin mononucleotide.     

Flavin-photosensitized oxidation of reduced c-type cytochromes. Reaction mechanism and comparison with photoreduction of oxidized cytochromes by flavin semiquinones

In order to compare the oxidation and reduction reactions of c-type cytochromes (cytochrome c552 from the green alga Monoraphidium braunii and horse heart cytochrome c) by different flavins (lumiflavin, riboflavin and FMN), laser flash photolysis studies have been carried out using either reduced or oxidized protein in the presence of triplet or semiquinone flavin, respectively. The reaction kinetics clearly demonstrate that cytochrome oxidation is mediated by the flavin triplet state. The rate constants for reduction are 20-100 times smaller than those for oxidation, indicating that the triplet state is a more effective reactant than is the semiquinone. This is attributed to its excited state nature and correspondingly high free energy content. The rate constants for both the reduction and oxidation of cytochrome c552 by riboflavin are significantly smaller than those obtained with lumiflavin, suggesting a steric interference of the ribityl side chain in the flavin-cytochrome interaction. The comparison between oxidation and reduction indicates that the former process is less affected by steric hindrance than the latter. Both reduction and oxidation of cytochrome c552 by FMN show an ionic strength dependence with the same sign, consistent with a negatively charged reaction site on the cytochrome. The magnitude of the electrostatic effect is slightly smaller for reduction than it is for oxidation. A pattern quite similar to that observed with cytochrome c552 was obtained when parallel experiments were carried out with horse cytochrome c, although differences were observed in the steric and electrostatic properties of the electron transfer site(s) in these two cytochromes. These results suggest that the same or closely adjacent sites on the proteins are involved in the oxidation and reduction reactions. The biochemical implications of this are discussed.     

Ultrafast excited-state deactivation of flavins bound to dodecin

Dodecins, a group of flavin-binding proteins with a dodecameric quaternary structure, are able to incorporate two flavins within each of their six identical binding pockets building an aromatic tetrade with two tryptophan residues. Dodecin from the archaeal Halobacterium salinarum is a riboflavin storage device. We demonstrate that unwanted side reactions induced by reactive riboflavin species and degradation of riboflavin are avoided by ultrafast depopulation of the reactive excited state of riboflavin. Intriguingly, in this process, the staggered riboflavin dimers do not interact in ground and photoexcited states. Rather, within the tetrade assembly, each riboflavin is kept under the control of the respective adjacent tryptophan, which suggests that the stacked arrangement is a matter of optimizing the flavin load. We further identify an electron transfer in combination with a proton transfer as a central element of the effective excited state depopulation mechanism. Structural and functional comparisons of the archaeal dodecin with bacterial homologs reveal diverging evolution. Bacterial dodecins bind the flavin FMN instead of riboflavin and exhibit a clearly different binding pocket design with inverse incorporations of flavin dimers. The different adoption of flavin changes photochemical properties, making bacterial dodecin a comparably less efficient quencher of flavins. This supports a functional role different for bacterial and archaeal dodecins.