Vibrational spectroscopy of an algal Phot-LOV1 domain probes the molecular changes associated with blue-light reception

Capturing the blue-light activated state of the Phot-LOV1 domain from Chlamydomonas reinhardtii using time-resolved serial synchrotron crystallography

Light-oxygen-voltage (LOV) domains are small photosensory flavoprotein modules that allow the conversion of external stimuli (sunlight) into intracellular signals responsible for various cell behaviors (e.g. phototropism and chloroplast relocation). This ability relies on the light-induced formation of a covalent thioether adduct between a flavin chromophore and a reactive cysteine from the protein environment, which triggers a cascade of structural changes that result in the activation of a serine/threonine (Ser/Thr) kinase. Recent developments in time-resolved crystallography may allow the activation cascade of the LOV domain to be observed in real time, which has been elusive. In this study, we report a robust protocol for the production and stable delivery of microcrystals of the LOV domain of phototropin Phot-1 from Chlamydomonas reinhardtii (CrPhotLOV1) with a high-viscosity injector for time-resolved serial synchrotron crystallography (TR-SSX). The detailed process covers all aspects, from sample optimization to data collection, which may serve as a guide for soluble protein preparation for TR-SSX. In addition, we show that the crystals obtained preserve the photoreactivity using infrared spectroscopy. Furthermore, the results of the TR-SSX experiment provide high-resolution insights into structural alterations of CrPhotLOV1 from Δt = 2.5 ms up to Δt = 95 ms post-photoactivation, including resolving the geometry of the thioether adduct and the C-terminal region implicated in the signal transduction process.

Coupling and regulation mechanisms of the flavin-dependent halogenase PyrH observed by infrared difference spectroscopy

Flavin-dependent halogenases are central enzymes in the production of halogenated secondary metabolites in various organisms and they constitute highly promising biocatalysts for regioselective halogenation. The mechanism of these monooxygenases includes formation of hypohalous acid from a reaction of fully reduced flavin with oxygen and halide. The hypohalous acid then diffuses via a tunnel to the substrate-binding site for halogenation of tryptophan and other substrates. Oxidized flavin needs to be reduced for regeneration of the enzyme, which can be performed in vitro by a photoreduction with blue light. Here, we employed this photoreduction to study characteristic structural changes associated with the transition from oxidized to fully reduced flavin in PyrH from Streptomyces rugosporus as a model for tryptophan-5-halogenases. The effect of the presence of bromide and chloride or the absence of any halides on the UV-vis spectrum of the enzyme demonstrated a halide-dependent structure of the flavin-binding pocket. Light-induced FTIR difference spectroscopy was applied and the signals assigned by selective isotope labeling of the protein moiety. The identified structural changes in α-helix and β-sheet elements were strongly dependent on the presence of bromide, chloride, the substrate tryptophan, and the product 5-chloro-tryptophan, respectively. We identified a clear allosteric coupling in solution at ambient conditions between cofactor-binding site and substrate-binding site that is active in both directions, despite their separation by a tunnel. We suggest that this coupling constitutes a fine-tuned mechanism for the promotion of the enzymatic reaction of flavin-dependent halogenases in dependence of halide and substrate availability.

Light-Oxygen-Voltage (LOV)-sensing Domains: Activation Mechanism and Optogenetic Stimulation

The light-oxygen-voltage (LOV) domains of phototropins emerged as essential constituents of light-sensitive proteins, helping initiate blue light-triggered responses. Moreover, these domains have been identified across all kingdoms of life. LOV domains utilize flavin nucleotides as co-factors and undergo structural rearrangements upon exposure to blue light, which activates an effector domain that executes the final output of the photoreaction. LOV domains are versatile photoreceptors that play critical roles in cellular signaling and environmental adaptation; additionally, they can noninvasively sense and control intracellular processes with high spatiotemporal precision, making them ideal candidates for use in optogenetics, where a light signal is linked to a cellular process through a photoreceptor. The ongoing development of LOV-based optogenetic tools, driven by advances in structural biology, spectroscopy, computational methods, and synthetic biology, has the potential to revolutionize the study of biological systems and enable the development of novel therapeutic strategies.

Network analysis of chromophore binding site in LOV domain

Photoreceptor proteins are versatile toolbox for developing biosensors for optogenetic applications. These molecular tools get activated upon illumination of blue light, which in turn offers a non-invasive method for gaining high spatiotemporal resolution and precise control of cellular signal transduction. The Light-Oxygen-Voltage (LOV) domain family of proteins is a well-recognized system for constructing optogenetic devices. Translation of these proteins into efficient cellular sensors is possible by tuning their photochemistry lifetime. However, the bottleneck is the need for more understanding of the relationship between the protein environment and photocycle kinetics. Significantly, the effect of the local environment also modulates the electronic structure of chromophore, which perturbs the electrostatic and hydrophobic interaction within the binding site. This work highlights the critical factors hidden in the protein networks, linking with their experimental photocycle kinetics. It presents an opportunity to quantitatively examine the alternation in chromophore's equilibrium geometry and identify details which have substantial implications in designing synthetic LOV constructs with desirable photocycle efficiency.

Real-Time Tracking of Proton Transfer from the Reactive Cysteine to the Flavin Chromophore of a Photosensing Light Oxygen Voltage Protein

LOV (light oxygen voltage) proteins are photosensors ubiquitous to all domains of life. A variant of the short LOV protein from Dinoroseobacter shibae (DsLOV) exhibits an exceptionally fast photocycle. We performed time-resolved molecular spectroscopy on DsLOV-M49S and characterized the formation of the thio-adduct state with a covalent bond between the reactive cysteine (C72) and C_4a of the FMN. By use of a tunable quantum cascade laser, the weak absorption change of the vibrational band of S-H stretching vibration of C57 was resolved with a time resolution of 10 ns. Deprotonation of C72 proceeded with a time constant of 12 μs which tallies the rise of the thio-adduct state. These results provide valuable information for the mechanistic interpretation of light-induced structural changes in LOV domains, which involves the choreographed sequence of proton transfers, changes in electron density distributions, spin alterations of the latter, and transient bond formation and breakage. Such molecular insight will help develop new optogenetic tools based on flavin photoreceptors.

QM calculations predict the energetics and infrared spectra of transient glutamine isomers in LOV photoreceptors

Photosensory receptors containing the flavin-binding light-oxygen-voltage (LOV) domain are modular proteins that fulfil a variety of biological functions ranging from gene expression to phototropism. The LOV photocycle is initiated by blue-light and involves a cascade of intermediate species, including an electronically excited triplet state, that leads to covalent bond formation between the flavin mononucleotide (FMN) chromophore and a nearby cysteine residue. Subsequent conformational changes in the polypeptide chain arise due to the remodelling of the hydrogen bond network in the cofactor binding pocket, whereby a conserved glutamine residue plays a key role in coupling FMN photochemistry with LOV photobiology. Although the dark-to-light transition of LOV photosensors has been previously addressed by spectroscopy and computational approaches, the mechanistic basis of the underlying reactions is still not well understood. Here we present a detailed computational study of three distinct LOV domains: EL222 from Erythrobacter litoralis, AsLOV2 from the second LOV domain of Avena sativa phototropin 1, and RsLOV from Rhodobacter sphaeroides LOV protein. Extended protein-chromophore models containing all known crucial residues involved in the initial steps (femtosecond-to-microsecond) of the photocycle were employed. Energies and rotational barriers were calculated for possible rotamers and tautomers of the critical glutamine side chain, which allowed us to postulate the most energetically favoured glutamine orientation for each LOV domain along the assumed reaction path. In turn, for each evolving species, infrared difference spectra were constructed and compared to experimental EL222 and AsLOV2 transient infrared spectra, the former from original work presented here and the latter from the literature. The good agreement between theory and experiment permitted the assignment of the majority of observed bands, notably the ∼1635 cm-1 transient of the adduct state to the carbonyl of the glutamine side chain after rotation. Moreover, both the energetic and spectroscopic approaches converge in suggesting a facile glutamine flip at the adduct intermediate for EL222 and more so for AsLOV2, while for RsLOV the glutamine keeps its initial configuration. Additionally, the computed infrared shifts of the glutamine and interacting residues could guide experimental research addressing early events of signal transduction in LOV proteins.

Peripheral Methionine Residues Impact Flavin Photoreduction and Protonation in an Engineered LOV Domain Light Sensor

Proton-coupled electron transfer reactions play critical roles in many aspects of sensory phototransduction. In the case of flavoprotein light sensors, reductive quenching of flavin excited states initiates chemical and conformational changes that ultimately transmit light signals to downstream targets. These reactions generally require neighboring aromatic residues and proton-donating side chains for rapid and coordinated electron and proton transfer to flavin. Although photoreduction of flavoproteins can produce either the anionic (ASQ) or neutral semiquinone (NSQ), the factors that favor one over the other are not well understood. Here we employ a biologically active variant of the light-oxygen-voltage (LOV) domain protein VVD devoid of the adduct-forming Cys residue (VVD-III) to probe the mechanism of flavin photoreduction and protonation. A series of isosteric and conservative residue replacements studied by rate measurements, fluorescence quantum yields, FTIR difference spectroscopy, and molecular dynamics simulations indicate that tyrosine residues facilitate charge recombination reactions that limit sustained flavin reduction, whereas methionine residues facilitate radical propagation and quenching and also gate solvent access for flavin protonation. Replacement of a single surface Met residue with Leu favors formation of the ASQ over the NSQ and desensitizes photoreduction to oxidants. In contrast, increasing site hydrophilicity by Gln substitution promotes rapid NSQ formation and weakens the influence of the redox environment. Overall, the photoreactivity of VVD-III can be understood in terms of redundant electron donors, internal hole quenching, and coupled proton transfer reactions that all depend upon protein conformation, dynamics, and solvent penetration.

In-cell infrared difference spectroscopy of LOV photoreceptors reveals structural responses to light altered in living cells

Proteins are usually studied in well-defined buffer conditions, which differ substantially from those within a host cell. In some cases, the intracellular environment has an impact on the mechanism, which might be missed by in vitro experiments. IR difference spectroscopy previously has been applied to study the light-induced response of photoreceptors and photoenzymes in vitro Here, we established the in-cell IR difference (ICIRD) spectroscopy in the transmission and attenuated total reflection configuration to investigate the light-induced response of soluble proteins in living bacterial cells. ICIRD spectroscopy on the light, oxygen, or voltage (LOV) domains of the blue light receptors aureochrome and phototropin revealed a suppression of the response of specific secondary structure elements, indicating that the intracellular environment affects LOV photoreceptor mechanisms in general. Moreover, in-cell fluorescence spectroscopy disclosed that the intracellular environment slows down the recovery of the light-induced flavin adduct. Segment-resolved ICIRD spectroscopy on basic-region leucine zipper (bZIP)-LOV of aureochrome 1a from the diatom Phaeodactylum tricornutum indicated a signal progression from the LOV sensor to the bZIP effector independent of unfolding of the connecting A'α-helix, an observation that stood in contrast to in vitro results. This deviation was recapitulated in vitro by emulating the intracellular environment through the addition of the crowding agent BSA, but not by sucrose polymers. We conclude that ICIRD spectroscopy is a noninvasive, label-free approach for assessing conformational changes in receptors in living cells at ambient conditions. As demonstrated, these near-native responses may deviate from the mechanisms established under in vitro conditions.

Directly monitoring the active sites of charge transfer in heterocycles in situ and in real time

Coherent degenerate infrared-infrared-visible sum frequency generation vibrational spectroscopy provides a powerful label-free sensitive probe for charge transfer active sites in heterocyclic molecules in situ and in real time.

Characterization of the Blue-Light-Activated Adenylyl Cyclase mPAC by Flash Photolysis and FTIR Spectroscopy

The recently discovered photo-activated adenylyl cyclase (mPAC from Microcoleus chthonoplastes) is the first PAC that owes a light-, oxygen- and voltage-sensitive (LOV) domain for blue-light sensing. The photoreaction of the mPAC receptor was studied by time-resolved UV/vis and light-induced Fourier transform infrared (FTIR) absorption difference spectroscopy. The photocycle comprises of the typical triplet state LOV₇₁₅ and the thio-adduct state LOV₃₉₀ . While the adduct state decays with a time constant of 8 s, the lifetime of the triplet state is with 656 ns significantly shorter than in all other reported LOV domains. The light-induced FTIR difference spectrum shows the typical bands of the LOV₃₉₀ and LOV₄₅₀ intermediates. The negative S-H stretching vibration at 2573 cm^-1 is asymmetric suggesting two rotamer configurations of the protonated side chain of C194. A positive band at 3632 cm^-1 is observed, which is assigned to an internal water molecule. In contrast to other LOV domains, mPAC exhibits a second positive feature at 3674 cm^-1 which is due to the O-H stretch of a second intrinsic water molecule and the side chain of Y476. We conclude that the latter might be involved in the dimerization of the cyclase domain which is crucial for ATP binding.

Modulation of the flavin-protein interactions in NADH peroxidase and mercuric ion reductase: a resonance Raman study

NADH peroxidase (Npx) and mercuric ion reductase (MerA) are flavoproteins belonging to the pyridine nucleotide:disulfide oxidoreductases (PNDO) and catalyzing the reduction of toxic substrates, i.e., hydrogen peroxide and mercuric ion, respectively. To determine the role of the flavin adenine dinucleotide (FAD) in the detoxification mechanism, the resonance Raman (RR) spectra of these enzymes under various redox and ligation states have been investigated using blue and/or near-UV excitation(s). These data were compared to those previously obtained for glutathione reductase (GR), another enzyme of the PNDO family, but catalyzing the reduction of oxidized glutathione. Spectral differences have been detected for the marker bands of the isoalloxazine ring of Npx, MerA, and GR. They provide evidence for different catalytic mechanisms in these flavoproteins. The RR modes of the oxidized and two-electron reduced (EH₂) forms of Npx are related to very tight flavin-protein interactions maintaining a nearly planar conformation of the isoalloxazine tricycle, a low level of H-bonding at the N₁/N₅ and O₂/O₄ sites, and a strong H-bond at N₃H. They also indicate minimal changes in FAD structure and environment upon either NAD(H) binding or reduction of the sulfinic redox center. All these spectroscopic data support an enzyme functioning centered on the Cys-SO^-/Cys-S^- redox moiety and a neighbouring His residue. On the contrary, the RR data on various functional forms of MerA are indicative of a modulation of both ring II distortion and H-bonding states of the N₅ site and ring III. The Cd(II) binding to the EH₂-NADP(H) complexes, biomimetic intermediates in the reaction of Hg(II) reduction, provokes important spectral changes. They are interpreted in terms of flattening of the isoalloxazine ring and large decreases in H-bonding at the N₅ site and ring III. The large flexibility of the FAD structure and environment in MerA is in agreement with proposed mechanisms involving C_4a(flavin) adducts.

Hydrogen Bonding Environment of the N3-H Group of Flavin Mononucleotide in the Light Oxygen Voltage Domains of Phototropins

The light oxygen voltage (LOV) domain is a flavin-binding blue-light receptor domain, originally found in a plant photoreceptor phototropin (phot). Recently, LOV domains have been used in optogenetics as the photosensory domain of fusion proteins. Therefore, it is important to understand how LOV domains exhibit light-induced structural changes for the kinase domain regulation, which enables the design of LOV-containing optogenetics tools with higher photoactivation efficiency. In this study, the hydrogen bonding environment of the N3-H group of flavin mononucleotide (FMN) of the LOV2 domain from Adiantum neochrome (neo) 1 was investigated by low-temperature Fourier transform infrared spectroscopy. Using specifically ¹⁵N-labeled FMN, [1,3-¹⁵N₂]FMN, the N3-H stretch was identified at 2831 cm^-1 for the unphotolyzed state at 150 K, indicating that the N3-H group forms a fairly strong hydrogen bond. The N3-H stretch showed temperature dependence, with a shift to lower frequencies at ≤200 K and to higher frequencies at ≥250 K from the unphotolyzed to the intermediate states. Similar trends were observed in the LOV2 domains from Arabidopsis phot1 and phot2. By contrast, the N3-H stretch of the Q1029L mutant of neo1-LOV2 and neo1-LOV1 was not temperature dependent in the intermediate state. These results seemed correlated with our previous finding that the LOV2 domains show the structural changes in the β-sheet region and/or the adjacent Jα helix of LOV2 domain, but that such structural changes do not take place in the Q1029L mutant or neo1-LOV1 domain. The environment around the N3-H group was also investigated.

Unfolding of the C-Terminal Jα Helix in the LOV2 Photoreceptor Domain Observed by Time-Resolved Vibrational Spectroscopy

Light-triggered reactions of biological photoreceptors have gained immense attention for their role as molecular switches in their native organisms and for optogenetic application. The light, oxygen, and voltage 2 (LOV2) sensing domain of plant phototropin binds a C-terminal Jα helix that is docked on a β-sheet and unfolds upon light absorption by the flavin mononucleotide (FMN) chromophore. In this work, the signal transduction pathway of LOV2 from Avena sativa was investigated using time-resolved infrared spectroscopy from picoseconds to microseconds. In D2O buffer, FMN singlet-to-triplet conversion occurs in 2 ns and formation of the covalent cysteinyl-FMN adduct in 10 μs. We observe a two-step unfolding of the Jα helix: The first phase occurs concomitantly with Cys-FMN covalent adduct formation in 10 μs, along with hydrogen-bond rupture of the FMN C4═O with Gln-513, motion of the β-sheet, and an additional helical element. The second phase occurs in approximately 240 μs. The final spectrum at 500 μs is essentially identical to the steady-state light-minus-dark Fourier transform infrared spectrum, indicating that Jα helix unfolding is complete on that time scale.

Analysis of the Primary Photocycle Reactions Occurring in the Light, Oxygen, and Voltage Blue-Light Receptor by Multiconfigurational Quantum-Chemical Methods

The photocycle reactions occurring between the flavin mononucleotide cofactor and the reactive cysteine residue in the blue-light photoreceptor domain light, oxygen, and voltage (LOV) were modeled for a system consisting of lumiflavin and thiomethanol. The electronic structure and energies of the reactive species were estimated using the CASSCF and MCQDPT2 quantum-chemical methods. The reaction pathway for the S-C4a covalent adduct formation in the triplet state was determined. Concerted electron and proton transfer from the thiol to the flavin in the triplet electronic state results in a biradical complex that is, however, unstable because its structure corresponds to a triplet-singlet crossing. The covalent adduct dissociation in the ground electronic state is a reverse of the photoreaction proceeding via a single energy barrier for hydrogen transfer. Thus, both photo- and dark reactions were found to be single-step chemical transformations occurring without stable intermediates. The photoreaction yielding the S-C4a covalent adduct is an intrinsic property of the isoalloxazine-thiol complex in the specific geometry arranged by the protein in LOV. The S-C4a covalent adduct between lumiflavin and thiomethanol is rather stable implying that in LOV its dissociation is facilitated by the protein.

Microsecond Deprotonation of Aspartic Acid and Response of the α/β Subdomain Precede C-Terminal Signaling in the Blue Light Sensor Plant Cryptochrome

Plant cryptochromes are photosensory receptors that regulate various central aspects of plant growth and development. These receptors consist of a photolyase homology region (PHR) carrying the oxidized flavin adenine dinucleotide (FAD) cofactor, and a cryptochrome C-terminal extension (CCT), which is essential for signaling. Absorption of blue/UVA light leads to formation of the FAD neutral radical as the likely signaling state, and ultimately activates the CCT. Little is known about the signal transfer from the flavin to the CCT. Here, we investigated the photoreaction of the PHR by time-resolved step-scan FT-IR spectroscopy complemented by UV-vis spectroscopy. The first spectrum at 500 ns shows major contributions from the FAD anion radical, which is demonstrated to then be protonated by aspartic acid 396 to the neutral radical within 3.5 μs. The analysis revealed the existence of three intermediates characterized by changes in secondary structure. A marked loss of β-sheet structure is observed in the second intermediate evolving with a time constant of 500 μs. This change is accompanied by a conversion of a tyrosine residue, which is identified as the formation of a tyrosine radical in the UV-vis. The only β-sheet in the PHR is located within the α/β subdomain, ∼25 Å away from the flavin. This subdomain has been previously attributed a role as a putative antenna binding site, but is now suggested to have evolved to a component in the signaling of plant cryptochromes by mediating the interaction with the CCT.

Allosterically regulated unfolding of the A'α helix exposes the dimerization site of the blue-light-sensing aureochrome-LOV domain

Aureochromes have been shown to act as blue-light-regulated transcription factors in algae in the absence of phototropins. Aureochromes comprise a light-, oxygen-, or voltage-sensitive (LOV) domain as a sensory module binding the flavin chromophore and a basic region leucine zipper (bZIP) domain as an effector. The domain arrangement in aureochromes with an N-terminal effector is inversed to other LOV proteins. To clarify the role of the linking A'α helix in signaling, we have investigated the LOV domain of aureochrome1a from the diatom alga Phaeodactylum tricornutum without the N-terminal A'α helix but with the C-terminal Jα helix. Results were analyzed in comparison to those previously obtained on the LOV domain with both flanking helices and on the LOV domain with the A'α helix but without the Jα helix. Fourier transform infrared difference spectroscopy provides evidence by a band at 1656 cm(-1) that the A'α helix unfolds in response to light. This unfolding takes place only in the presence and as a consequence of the unfolding of the Jα helix, which points to an allosteric regulation. Size exclusion chromatography shows the LOV domain to be dimeric in the absence and monomeric in the presence of the A'α helix, implying that the folded helix covers the dimerization site. Therefore, the A'α helix directly modulates the oligomerization state of the LOV domain, whereas the Jα helix acts as an allosteric regulator. Both the allosteric control and the light-induced dimerization have not been observed in phototropin-LOV2 and point to a different signaling mechanism within the full-length proteins.

Aureochrome 1 illuminated: structural changes of a transcription factor probed by molecular spectroscopy

Aureochrome 1 from Vaucheria frigida is a recently identified blue-light receptor that acts as a transcription factor. The protein comprises a photosensitive light-, oxygen- and voltage-sensitive (LOV) domain and a basic zipper (bZIP) domain that binds DNA rendering aureochrome 1 a prospective optogenetic tool. Here, we studied the photoreaction of full-length aureochrome 1 by molecular spectroscopy. The kinetics of the decay of the red-shifted triplet state and the blue-shifted signaling state were determined by time-resolved UV/Vis spectroscopy. It is shown that the presence of the bZIP domain further prolongs the lifetime of the LOV₃₉₀ signaling state in comparison to the isolated LOV domain whereas bound DNA does not influence the photocycle kinetics. The light-dark Fourier transform infrared (FTIR) difference spectrum shows the characteristic features of the flavin mononucleotide chromophore except that the S-H stretching vibration of cysteine 254, which is involved in the formation of the thio-adduct state, is significantly shifted to lower frequencies compared to other LOV domains. The presence of the target DNA influences the light-induced FTIR difference spectrum of aureochrome 1. Vibrational bands that can be assigned to arginine and lysine side chains as well to the phosphate backbone, indicate crucial changes in interactions between transcription factor and DNA.

FTIR spectroscopy of flavin-binding photoreceptors

Light-induced difference Fourier transform infrared (FTIR) spectroscopy is a powerful, sensitive, and informative method to study structure-function relationships in photoreceptive proteins. Strong absorption of water in the IR region is always problematic in this method, but if water content in the sample is controlled during measurements, this method can provide useful information on a single protein-bound water molecule. We established three kinds of sample preparations: hydrated film, redissolved sample, and concentrated solution. Hydrated films were used for the measurements of LOV and BLUF domains, where accurate difference FTIR spectra were obtained in the whole mid-IR region (4,000-800 cm(-1)). Vibrations of S-H stretch of cysteine, O-H stretch of water, and O-H stretch of tyrosine provide important information on hydrogen bonds in these proteins. Redissolved samples were used for the measurements of (6-4) photolyase, in which enzymatic turnover takes place. From the illumination time-dependence of excess amount of substrate, it is possible to isolate the signal originating from the binding of enzyme to substrate. If proteins are less tolerant to drying, as for example cryptochromes of the DASH type, concentrated solution is used. Detailed methodological aspects in light-induced difference FTIR spectroscopy are reviewed by mainly focusing on our results.

Photoactivation mechanisms of flavin-binding photoreceptors revealed through ultrafast spectroscopy and global analysis methods

Flavin-binding photoreceptor proteins use the isoalloxazine moiety of flavin cofactors to absorb light in the blue/UV-A wavelength region and subsequently translate it into biological information. The underlying photochemical reactions and protein structural dynamics are delicately tuned by the protein environment and represent fundamental reactions in biology and chemistry. Due to their photo-switchable nature, these proteins can be studied efficiently with laser-flash induced transient absorption and emission spectroscopy with temporal precision down to the femtosecond time domain. Here, we describe the application of both visible and mid-IR ultrafast transient absorption and time-resolved fluorescence methods in combination with sophisticated global analysis procedures to elucidate the photochemistry and signal transduction of BLUF (Blue light receptors using FAD) and LOV (Light oxygen voltage) photoreceptor domains.

Protonated triplet-excited flavin resolved by step-scan FTIR spectroscopy: implications for photosensory LOV domains

Among many other functions, flavin serves as a chromophore in LOV (light-, oxygen-, or voltage-sensitive) domains of blue light sensors. These sensors regulate central responses in many organisms such as the growth of plants towards light. The triplet-excited state of flavin ((3)Fl) has been identified as a key intermediate in the photocycle of LOV domains, either in its neutral or protonated state. Even time-resolved infrared spectroscopy could not resolve unambiguously whether (3)Fl becomes protonated during the photoreaction, because the protonated triplet-excited state (3)FlH(+) has not been characterized before. Here, the step-scan Fourier transform infrared (FTIR) technique was applied to the flavin mononucleotide (FMN) in aqueous solution at different pH values to resolve laser-induced changes in the time range from 1.5 μs to 860 μs. A high-pressure-resistant flow cell system was established to account for the irreversibility of the photoreaction and the small path length. Several marker bands were identified in the spectrum of (3)Fl in water and assigned by quantum chemical calculations. These bands exhibit a solvent-induced shift as compared with previous spectra of (3)Fl in organic solvents. The marker bands undergo a further distinct shift upon formation of (3)FlH(+). Band patterns can be clearly separated from those of the anion radical or the fully reduced state resolved in the presence of an electron donor. A comparison to spectra of (3)Fl in LOV domains leads to the conclusion that (3)FlH(+) is not formed in the photoreaction of these blue light sensors.

Light-induced structural changes of the LOV2 domains in various phototropins revealed by FTIR spectroscopy

Phototropin (Phot), a blue-light photoreceptor in plants, consists of two FMN-binding domains (named LOV1 and LOV2) and a serine/threonine (Ser/Thr) kinase domain. We have investigated light-induced structural changes of LOV domains, which lead to the activation of the kinase domain, by means of light-induced difference FTIR spectroscopy. FTIR spectroscopy revealed that the reactive cysteine is protonated in both unphotolyzed and triplet-excited states, which is difficult to detect by other methods such as X-ray crystallography. In this review, we describe the light-induced structural changes of hydrogen-bonding environment of FMN chromophore and protein backbone in Adiantum neo1-LOV2 in the C=O stretching region by use of ¹³C-labeled samples. We also describe the comprehensive FTIR analysis of LOV2 domains among Arabidopsis phot1, phot2, and Adiantum neo1 with and without Jα helix domain.

Signals of LOV1: a computer simulation study on the wildtype LOV1-domain of Chlamydomonas reinhardtii and its mutants

Phototropins are photoreceptors regulating the blue-light response in plants and bacteria. They consist of two LOV (light oxygen voltage sensitive) domains each containing a non-covalently bound flavin-mononucleotide (FMN) chromophore, which are connected to a serine/threonine-kinase. Upon illumination, the LOV-domains undergo conformational changes, triggering a signal cascade in the organism through kinase activation. Here, we present results from molecular dynamics simulations in which we investigate the signal transduction pathway of the wildtype LOV1-domain of Chlamydomonas reinhardtii and a methyl-mercaptan (MM) adduct of its Cys57Gly-mutant at the molecular level. In particular, we analyzed the effect of covalent-bond formation between the reactive cysteine Cys57 and the FMN-reaction center, as well as the subsequent charge redistribution, on the spatio-dynamical behavior of the LOV1-domain. We compare the calculation results with experimental data and demonstrate that these adduct state characteristics have an important influence on the response of this photosensor. The light-induced changes implicate primarily an alteration of the surface charge distribution through rearrangement of the highly flexible Cα-, Dα- and Eα-helices including the Glu51-Lys91-salt bridge on the hydrophilic side of the protein domain and a β-sheet tightening process via coupling of the Aβ- and Bβ-strands. Our findings confirm the aptitude of the LOV1-domain to function as a dimerization partner, allowing the green alga to adapt its reproduction and growth speed to the environmental conditions.

Photoreaction of plant and DASH cryptochromes probed by infrared spectroscopy: the neutral radical state of flavoproteins

Flavoprotein radicals are important intermediates in many biochemical processes. In the blue light sensor plant cryptochrome, the radical state acts as a signaling state. An isolation and assignment of infrared bands of flavin radicals in the most relevant spectral region of carbonyl stretches is missing because of their overlap with absorption of water and the protein moiety. In this study, the neutral radical state of flavoproteins was investigated by Fourier transform infrared difference spectroscopy. The light-induced conversion of oxidized to neutral radical state was monitored in a plant cryptochrome and that of radical to fully reduced state in a DASH cryptochrome. A pure difference spectrum of flavin radical minus oxidized state was obtained from a point mutant of a phototropin LOV (light-, oxygen-, or voltage-sensitive) domain. The analysis of the spectra revealed a correlation between the frequencies of carbonyl vibrations of the flavin radical state and those of its visible absorption. Plant cryptochrome shows a very low frequency of the carbonyl stretch in the radical state. It is postulated that the downshift is caused by the charge of an adjacent aspartate, which donated its proton to flavin N(5). Contributions from the protein moiety to the spectra were isolated for DASH and plant cryptochromes. As a conclusion, the photosensitive domain of plant cryptochromes shows changes in secondary structure upon illumination, which might be related to signaling.

Key dynamics of conserved asparagine in a cryptochrome/photolyase family protein by fourier transform infrared spectroscopy

Cryptochromes (Crys) and photolyases (Phrs) are flavoproteins that contain an identical cofactor (flavin adenine dinucleotide, FAD) within the same protein architecture but whose physiological functions are entirely different. In this study, we investigated light-induced conformational changes of a cyanobacterium Cry/Phr-like protein (SCry-DASH) with UV-visible and Fourier transform infrared (FTIR) spectroscopy. We developed a system for measuring light-induced difference spectra under the concentrated conditions. In the presence of a reducing agent, SCry-DASH showed photoreduction to the reduced form, and we identified a signal unique for an anionic form in the process. Difference FTIR spectra enabled us to assign characteristic FTIR bands to the respective redox forms of FAD. An asparagine residue, which anchors the FAD embedded within the protein, is conserved not only in the cyanobacterial protein but also in Phrs and other Crys, including the mammalian clock-related Crys. By characterizing an asparagine-to-cysteine (N392C) mutant of SCry-DASH, which mimics an insect specific Cry, we identified structural changes of the carbonyl group of this conserved asparagine upon light irradiation. We also found that the N392C mutant is stabilized in the anionic form. We did not observe a signal from protonated carboxylic acid residues during the reduction process, suggesting that the carboxylic acid moiety would not be directly involved as a proton donor to FAD in the system. These results are in contrast to plant specific Crys represented by Arabidopsis thaliana Cry1 that carry Asp at the position. We discuss potential roles for this conserved asparagine position and functional diversity in the Cry/Phr frame.

Photophysical characterisation and photo-cycle dynamics of LOV1-His domain of phototropin from Chlamydomonas reinhardtii with roseoflavin monophosphate cofactor

The wild-type phototropin protein phot from the green alga Chlamydomonas reinhardtii with the blue-light photoreceptor domains LOV1 and LOV2 has flavin mononucleotide (FMN) as cofactor. For the LOV1-His domain from phot of C. reinhardtii studied here, the FMN chromophore was replaced by roseoflavin monophosphate (8-dimethylamino-8-demethyl-FMN, RoFMN) during heterologous expression in a riboflavin auxotropic Escherichia coli strain. An absorption and emission spectroscopic characterisation of the cofactor exchanged-LOV1-His (RoLOV1) domain was carried out in aqueous pH 8 phosphate buffer. The fluorescence of RoLOV1 is quenched by photo-induced charge transfer at room temperature. The photo-cyclic dynamics of RoLOV1 was observed by blue-light induced hypochromic and bathochromic absorption changes which recover on a minute timescale in the dark. Photo-excited RoFMN is thought to cause reversible protein and cofactor structural changes. Prolonged intense blue-light exposure caused photo-degradation of RoFMN in RoLOV1 to fully reduced flavin and lumichrome derivatives. Photo-cycle schemes of RoLOV1 and LOV1 are presented, and the photo-degradation dynamics of RoLOV1 is discussed.

Photoreaction of mutated LOV photoreceptor domains from Chlamydomonas reinhardtii with aliphatic mercaptans: implications for the mechanism of wild type LOV

Irradiation of the LOV1 domain from the blue-light photoreceptor phototropin of the green alga Chlamydomonas reinhardtii leads to the formation of a covalent adduct of the sulfur atom of cysteine 57 to the carbon C(4a) in the chromophore FMN. This reaction is not possible in the mutant LOV1-C57G in which this cysteine is replaced by glycine. Irradiation of LOV1-C57G in the absence of oxygen but in the presence of aliphatic mercaptans or thioethers leads to the formation of a species with an absorption maximum at 615 nm, which is identified as the neutral radical FMNH . When oxygen is admitted, the reaction is completely reversible. Irradiation of LOV1-C57G in the presence of methylmercaptan CH(3)SH under oxygen-free conditions yields, in addition to FMNH , a third species with a single absorption maximum at 379 nm. This species is stable against oxygen and is also formed when the irradiation is performed in the presence of oxygen. This species is assigned to the adduct between CH(3)SH and FMN. In aqueous solution the photoreaction of CH(3)SH with FMN leads to the fully reduced hydroquinone form FMNH(2) or its anion FMNH(-). Adduct formation apparently requires the protein cage. After formation, the adduct is stable for hours inside the protein, but decomposes immediately upon denaturation. The implications of these observations for the mechanism of adduct formation in wild type LOV domains are discussed.

Conformational heterogeneity and propagation of structural changes in the LOV2/Jalpha domain from Avena sativa phototropin 1 as recorded by temperature-dependent FTIR spectroscopy

Phototropins control phototropism, chloroplast movement, stomatal opening, and leaf expansion in plants. Phototropin 1 (phot1) is composed of a kinase domain linked to two blue light-sensing domains, LOV2 and LOV1, which bind flavin mononucleotide. Disruption of the interaction between the LOV2 domain and a helical segment named Jalpha, joining LOV to the kinase domain, induces the subsequent kinase activity of phototropin 1 and further-downstream signal transduction. Here we study the effects of temperature and hydration on the light-triggered signal propagation in the phot1 LOV2 domain of Avena sativa (AsLOV2/Jalpha), using Fourier transform infrared spectroscopy to unravel part of the molecular mechanism of phototropin 1. We report that AsLOV2/Jalpha shows an intense signal in the amide I and II regions, arising mainly from beta-sheet changes and the unbinding of the Jalpha helix from the Per-ARNT-Sim core and its subsequent partial unfolding. Importantly, these structural changes only occur under conditions of full hydration and at temperatures above 280 K. We characterized a newly isolated low-hydration intermediate that shows a downshift of high-frequency amide I signals and that possibly corresponds to loop tightening, without large beta-sheet or Jalpha structural changes. In addition, we report a heterogeneity in AsLOV2/Jalpha involving two different C(4)=O conformer populations, coexisting in the dark state and characterized by C(4)=O carbonyl frequencies at 1712 cm(-1) and 1694 cm(-1) that are attributable to a single H-bond and two H-bonds at this site, respectively. Such conformers display slightly shifted absorption spectra and cause a splitting of the 475-nm band in the ultraviolet/visible spectra of LOV domains at low temperature.

Vibrational assignment of the flavin-cysteinyl adduct in a signaling state of the LOV domain in FKF1

LOV domains belong to the PAS domain superfamily, which are found in a variety of sensor proteins in organism ranging from archaea to eukaryotes, and they noncovalently bind a single flavin mononucleotide as a chromophore. We report the Raman spectra of the dark state of LOV domain in FKF1 from Arabidopsis thaliana. Spectra have been also measured for the signaling state, where a cysteinyl-flavin adduct is formed upon light irradiation. Most of the observed Raman bands are assigned on the basis of normal mode calculations using a density functional theory. We also discuss implication for the analysis of the infrared spectra of LOV domains. The comprehensive assignment provides a satisfactory framework for future investigations of the photocycle mechanism in LOV domains by vibrational spectroscopy.

Primary reactions of the LOV2 domain of phototropin studied with ultrafast mid-infrared spectroscopy and quantum chemistry

Phototropins, major blue-light receptors in plants, are sensitive to blue light through a pair of flavin mononucleotide (FMN)-binding light oxygen and voltage (LOV) domains, LOV1 and LOV2. LOV2 undergoes a photocycle involving light-driven covalent adduct formation between a conserved cysteine and the FMN C(4a) atom. Here, the primary reactions of Avena sativa phototropin 1 LOV2 (AsLOV2) were studied using ultrafast mid-infrared spectroscopy and quantum chemistry. The singlet excited state (S1) evolves into the triplet state (T1) with a lifetime of 1.5 ns at a yield of approximately 50%. The infrared signature of S1 is characterized by absorption bands at 1657 cm(-1), 1495-1415 cm(-1), and 1375 cm(-1). The T1 state shows infrared bands at 1657 cm(-1), 1645 cm(-1), 1491-1438 cm(-1), and 1390 cm(-1). For both electronic states, these bands are assigned principally to C=O, C=N, C-C, and C-N stretch modes. The overall downshifting of C=O and C=N bond stretch modes is consistent with an overall bond-order decrease of the conjugated isoalloxazine system upon a pi-pi* transition. The configuration interaction singles (CIS) method was used to calculate the vibrational spectra of the S1 and T1 excited pipi* states, as well as respective electronic energies, structural parameters, electronic dipole moments, and intrinsic force constants. The harmonic frequencies of S1 and T1, as calculated by the CIS method, are in satisfactory agreement with the evident band positions and intensities. On the other hand, CIS calculations of a T1 cation that was protonated at the N(5) site did not reproduce the experimental FMN T1 spectrum. We conclude that the FMN T1 state remains nonprotonated on a nanosecond timescale, which rules out an ionic mechanism for covalent adduct formation involving cysteine-N(5) proton transfer on this timescale. Finally, we observed a heterogeneous population of singly and doubly H-bonded FMN C(4)=O conformers in the dark state, with stretch frequencies at 1714 cm(-1) and 1694 cm(-1), respectively.

Light signal transduction pathway from flavin chromophore to the J alpha helix of Arabidopsis phototropin1

In the plant blue-light sensor phototropin, illumination of the chromophoric LOV domains causes activation of the serine/threonine kinase domain. Flavin mononucleotide (FMN) is a chromophore molecule in the two LOV domains (LOV1 and LOV2), but only LOV2 is responsible for kinase activation. Previous studies reported an important role of an additional helix connected to the C-terminal of LOV2 (Jalpha helix) for the function of phototropin; however, it remains unclear how the Jalpha helix affects light-induced structural changes in LOV2. In this study we compared light-induced protein structural changes of the LOV2 domain of Arabidopsis phot1 in the absence (LOV2-core) and presence (LOV2-Jalpha) of the Jalpha helix by Fourier-transform infrared spectroscopy. Prominent peaks were observed only in the amide-I region (1650 (-)/1625 (+) cm(-1)) of LOV2-Jalpha at physiological temperatures (>/=260 K), corresponding to structural perturbation of the alpha-helix. The peaks were diminished by point mutation of functionally important amino acids such as Phe-556 between FMN and the beta-sheet, Gln-575 being hydrogen-bonded with FMN, and Ile-608 on the Jalpha helix. We thus conclude that a light signal is relayed from FMN through these amino acids and eventually changes the interaction between LOV2-core and the Jalpha helix in Arabidopsis phot1.

Time-resolved Fourier transform infrared study on photoadduct formation and secondary structural changes within the phototropin LOV domain

Phototropins are plant blue-light photoreceptors containing two light-, oxygen-, or voltage-sensitive (LOV) domains and a C-terminal kinase domain. The two LOV domains bind noncovalently flavin mononucleotide as a chromophore. We investigated the photocycle of fast-recovery mutant LOV2-I403V from Arabidopsis phototropin 2 by step-scan Fourier transform infrared spectroscopy. The reaction of the triplet excited state of flavin with cysteine takes place with a time constant of 3 micros to yield the covalent adduct. Our data provide evidence that the flavin is unprotonated in the productive triplet state, disfavoring an ionic mechanism of bond formation. An intermediate adduct species was evident that displayed changes in secondary structure in the helix or loop region, and relaxed with a time constant of 120 micros. In milliseconds, the final adduct state is formed by further alterations of secondary structure, including beta-sheets. A comparison with wild-type adduct spectra shows that the mutation does not interfere with the functionality of the domain. All signals originate from within the LOV domain, because the construct does not comprise the adjacent Jalpha helix required for signal transduction. The contribution of early and late adduct intermediates to signal transfer to the Jalpha helix outside of the domain is discussed.

Palladacycles catalyse the oxidation of critical thiols of the mitochondrial membrane proteins and lead to mitochondrial permeabilization and cytochrome c release associated with apoptosis

Permeabilization of the mitochondrial membrane has been extensively associated with necrotic and apoptotic cell death. Similarly to what had been previously observed for B16F10-Nex2 murine melanoma cells, PdC (palladacycle compounds) obtained from the reaction of dmpa (N,N-dimethyl-1-phenethylamine) with the dppe [1,2-ethanebis(diphenylphosphine)] were able to induce apoptosis in HTC (hepatoma, tissue culture) cells, presenting anticancer activity in vitro. To elucidate cell site-specific actions of dmpa:dppe that could respond to the induction of apoptosis in cancer cells in the present study, we investigated the effects of PdC on isolated RLM (rat liver mitochondria). Our results showed that these palladacycles are able to induce a Ca2+-independent mitochondrial swelling that was not inhibited by ADP, Mg2+ and antioxidants. However, the PdC-induced mitochondrial permeabilization was partially prevented by pre-incubation with CsA (cyclosporin A), NEM (N-ethylmaleimide) and bongkreic acid and totally prevented by DTT (dithiothreitol). A decrease in the content of reduced thiol groups of the mitochondrial membrane proteins was also observed, as well as the presence of membrane protein aggregates in SDS/PAGE without lipid and GSH oxidation. FTIR (Fourier-transform IR) analysis of PdC-treated RLM demonstrated the formation of disulfide bonds between critical thiols in mitochondrial membrane proteins. Associated with the mitochondrial permeabilization, PdC also induced the release of cytochrome c, which is sensitive to inhibition by DTT. Besides the contribution to clarify the pro-apoptotic mechanism of PdC, this study shows that the catalysis of specific protein thiol cross-linkage is enough to induce mitochondrial permeabilization and cytochrome c release.

Perturbation of the ground-state electronic structure of FMN by the conserved cysteine in phototropin LOV2 domains

In LOV2, the blue-light sensitive domain of phototropin, the primary photophysical event involves intersystem crossing (ISC) from the singlet-excited state to the triplet state. The ISC rate is enhanced in LOV2 as compared to flavin mononucleotide (FMN) in solution, which likely results from a heavy-atom effect of a nearby conserved cysteine, C450. Here, we applied fluorescence line narrowing (FLN), resonance Raman (RR) and Fourier-transform infrared (FTIR) spectroscopy to investigate the electronic structure of FMN bound to Avena sativa LOV2 (AsLOV2), its C450A mutant and Adiantum LOV2 (Phy3LOV2). We demonstrate that FLN is the method of choice to obtain accurate vibrational spectra on highly fluorescent flavoproteins. The vibrational spectrum of AsLOV2-C450A showed small but significant shifts with respect to those of wild type AsLOV2 and Phy3LOV2, with a systematic down-shift of Ring I vibrations, upshifts of Ring II and III vibrations and an upshift of the C2=O mode. These trends are similar to those in FMN model systems with an electron-donating group substituted at Ring I, known to induce a quinoid character to the electronic structure of oxidized flavin. Thus, enhancement of the ISC rate in LOV2 is induced through weak electron donation by the cysteine which mixes the FMN pi-electrons with the heavy sulfur orbitals, manifesting itself in a quinoid character of the ground electronic state of oxidized FMN. The proximity of the cysteine to FMN thus not only enables formation of a covalent adduct between FMN and cysteine, but also facilitates the rapid electronic formation of the reactive FMN triplet state.

Hydrogen bond switching among flavin and amino acid side chains in the BLUF photoreceptor observed by ultrafast infrared spectroscopy

BLUF domains constitute a recently discovered class of photoreceptor proteins found in bacteria and eukaryotic algae. BLUF domains are blue-light sensitive through a FAD cofactor that is involved in an extensive hydrogen-bond network with nearby amino acid side chains, including a highly conserved tyrosine and glutamine. The participation of particular amino acid side chains in the ultrafast hydrogen-bond switching reaction with FAD that underlies photoactivation of BLUF domains is assessed by means of ultrafast infrared spectroscopy. Blue-light absorption by FAD results in formation of FAD(*-) and a bleach of the tyrosine ring vibrational mode on a picosecond timescale, showing that electron transfer from tyrosine to FAD constitutes the primary photochemistry. This interpretation is supported by the absence of a kinetic isotope effect on the fluorescence decay on H/D exchange. Subsequent protonation of FAD(*-) to result in FADH(*) on a picosecond timescale is evidenced by the appearance of a N-H bending mode at the FAD N5 protonation site and of a FADH(*) C=N stretch marker mode, with tyrosine as the likely proton donor. FADH(*) is reoxidized in 67 ps (180 ps in D(2)O) to result in a long-lived hydrogen-bond switched network around FAD. This hydrogen-bond switch shows infrared signatures from the C-OH stretch of tyrosine and the FAD C4=O and C=N stretches, which indicate increased hydrogen-bond strength at all these sites. The results support a previously hypothesized rotation of glutamine by approximately 180 degrees through a light-driven radical-pair mechanism as the determinant of the hydrogen-bond switch.

Primary processes during the light-signal transduction of phototropin

Phototropin is a blue-light photoreceptor in plants that mediates phototropism, chloroplast relocation, stomata opening and leaf expansion. Phototropin molecule has two photoreceptive domains named LOV1 (light-oxygen-voltage) and LOV2 in the N-terminus and a serine/threonine kinase domain in the C-terminus, and acts as a blue light-regulated kinase. Each LOV domain binds a flavin mononucleotide as a chromophore and undergoes unique cyclic reactions upon blue-light absorption that comprises a cysteinyl-flavin adduct formation through a triplet-excited state and a successive adduct break to revert to the initial ground state. The molecular reactions underlying the photocycle are reviewed and one of the probable molecular schemes is presented. Adduct formation alters the secondary protein structure of the LOV domains. This structural change could be transferred to the linker between the kinase domain and involved in the photoregulation of the kinase activity. The structural changes as well as the oligomeric structures seem to differ between LOV1 and LOV2, which may explain the proposed roles of each domain in the photoregulation of the kinase activity. The photoregulation mechanism of phototropin kinase is reviewed and discussed in reference to the regulation mechanism of protein kinase A, which it resembles.

When light falls in LOV: a quantum mechanical/molecular mechanical study of photoexcitation in Phot-LOV1 of Chlamydomonas reinhardtii

Plants use sophisticated photosensing mechanisms to maximize their utilization of the available sunlight and to control developmental processes. The plant blue-light receptors of the Phot family mediate plant phototropism and contain two light, oxygen, and voltage (LOV)-sensitive domains as photoactive elements. Here, we report combined quantum mechanical/molecular mechanical simulations of the photocycle of a complete Phot-LOV1 domain from Chlamydomonas reinhardtii. We have investigated the electronic properties and structural changes that follow blue-light absorption. This permitted us to characterize the pathway for flavin-cysteinyl adduct formation, which was found to proceed via a neutral radical state generated by hydrogen atom transfer from the reactive cysteine residue, Cys57, to the chromophore flavin mononucleotide. Interestingly, we find that adduct formation does not cause any larger scale conformational changes in Phot-LOV1, which suggests that dynamic effects mediate signal transmission following the initial photoexcitation event.

Hydration and temperature similarly affect light-induced protein structural changes in the chromophoric domain of phototropin

Phototropin is a blue-light sensor protein in plants, and LOV domain binds a flavin mononucleotide (FMN) as a chromophore. A photointermediate state, S390, is formed by light-induced adduct formation between FMN and an S-H group of nearby cysteine, which triggers protein structural changes for kinase activation in phototropin. We previously studied the low-temperature Fourier transform infrared (FTIR) spectra between the S390 and unphotolyzed states for a LOV2 domain of a phototropin from Adiantum (neo1-LOV2), and found that the protein structures of the S390 intermediate are highly temperature dependent (Iwata, T., Nozaki, D., Tokutomi, S., Kagawa, T., Wada, M., and Kandori, H. (2003) Biochemistry 42, 8183-8191). At physiological temperature, amide-I vibration at 1640-1620 cm-1 is significantly changed, implying structural alteration of beta-sheet region. Such changes are largely suppressed at low temperatures, though S390 is formed. This observation suggested the presence of progressive protein structural changes in the unique active state (S390). Here we report that the hydration dependence of the amide-I vibrational bands in neo1-LOV2 is similar to the temperature dependence. As hydration of the sample is lowered, amide-I vibration at 1640-1620 cm-1 is significantly reduced. Instead, amide-I vibration at 1694 cm-1 newly emerged at low hydration as well as at low temperature, which shows a weakened hydrogen bond in the loop region. Spectral coincidence between low hydrations and temperatures strongly suggested that protein structural changes are similarly restricted under such conditions. It is likely that protein fluctuations are prerequisite for formation of the active state of neo1-LOV2.