Emission Properties of Oxyluciferin and Its Derivatives in Water: Revealing the Nature of the Emissive Species in Firefly Bioluminescence

Experimental and Theoretical Study for Core Excitation of Firefly Luciferin in Carbon K-Edge Spectra

Carbon (C) K-edge X-ray absorption spectra for firefly luciferin were measured and assigned using time-dependent density functional theoretical calculations for luciferin anion and dianion to elucidate the effect of hydroxy-group deprotonation. It was found that the C K-edge spectra for luciferin had four characteristic peaks. The effect of deprotonation of the hydroxy group appears in the energy difference of the first and second peaks of these spectra. This energy difference is 1.0 eV at pH 7 and 2.3 eV at pH 10. The deprotonation of the hydroxy group can be distinguished based on the soft X-ray absorption spectra.

Cloning and structural basis of fluorescent protein color variants from identical species of sea anemone, Diadumene lineata

Diadumene lineata is a colorful sea anemone with orange stripe tissue of the body column and plain tentacles with red lines. We subjected Diadumene lineata to expression cloning and obtained genes encoding orange (OFP: DiLiFP561) and red fluorescent proteins (RFPs: DiLiFP570 and DiLiFP571). These proteins formed obligatory tetramers. All three proteins showed bright fluorescence with the brightness of 58.3 mM^-1·cm^-1 (DiLiFP561), 43.9 mM^-1·cm^-1 (DiLiFP570), and 31.2 mM^-1·cm^-1 (DiLiFP571), which were equivalent to that of commonly used red fluorescent proteins. Amplitude-weighted average fluorescence lifetimes of DiLiFP561, DiLiFP570 and DiLiFP571 were determined as 3.7, 3.6 and 3.0 ns. We determined a crystal structure of DiLiFP570 at 1.63 Å resolution. The crystal structure of DiLiFP570 revealed that the chromophore has an extended π-conjugated structure similar to that of DsRed. Most of the amino acid residues surrounding the chromophore were common between DiLiFP570 and DiLiFP561, except M159 of DiLiFP570 (Lysine in DiLiFP561), which is located close to the chromophore hydroxyl group. Interestingly, a similar K-to-M substitution has been reported in a red-shifted variant of DsRed (mRFP1). It is a striking observation that the naturally evolved color-change variants are consistent with the mutation induced via protein engineering processes. The newly cloned proteins are promising as orange and red fluorescent markers for imaging with long fluorescence lifetime.

Unraveling the contributions to the spectral shape of flexible dyes in solution: insights on the absorption spectrum of an oxyluciferin analogue

We present a computational investigation of the absorption spectrum in water of 5,5-spirocyclopropyl-oxyluciferin (5,5-CprOxyLH), an analogue of the emitter compound responsible for the bioluminescence in fireflies. Several factors participate in determining the 5,5-CprOxyLH's spectral shape: (i) the contribution of the four close-energy excited states, which show significant non-adiabatic couplings, (ii) the flexible molecular structure and (iii) the specific interactions established with the surrounding environment, which strongly couple the protic solvent dynamics with the dye's spectral response. To tackle the challenge to capture and dissect the role of all these effects we preliminarily investigate the role of non-adiabatic couplings with quantum dynamics simulations and a linear vibronic coupling model in the gas phase. Then, we account for both the molecular flexibility and solvent interactions by resorting to a mixed quantum classical protocol, named Adiabatic Molecular Dynamics generalized Vertical Gradient (Ad-MD|gVG), which is built on a method recently proposed by some of us. It is rooted in the partition between stiff degrees of freedom of the dye, accounted for at the vibronic level within the harmonic approximation, and flexible degrees of freedom of the solute (and of the solvent), described classically through a sampling based on Molecular Dynamics (MD). Ad-MD|gVG avoids spurious effects arising in the excited state Hessians due to non-adiabatic couplings, and can therefore be applied to account for the contributions of the first four excited states to the 5,5-CprOxyLH absorption spectrum. The final simulated spectrum is in very good agreement with the experiment, especially when the MD is driven by a refined quantum-mechanically derived force-field. More importantly, the origin of each separate contribution to the spectral shape is appropriately accounted for, paving the way to future applications of the method to more complex systems or alternative spectroscopies, as emission or circular dichroism.

Spectrochemistry of Firefly Bioluminescence

The chemical reactions underlying the emission of light in fireflies and other bioluminescent beetles are some of the most thoroughly studied processes by scientists worldwide. Despite these remarkable efforts, fierce academic arguments continue around even some of the most fundamental aspects of the reaction mechanism behind the beetle bioluminescence. In an attempt to reach a consensus, we made an exhaustive search of the available literature and compiled the key discoveries on the fluorescence and chemiluminescence spectrochemistry of the emitting molecule, the firefly oxyluciferin, and its chemical analogues reported over the past 50+ years. The factors that affect the light emission, including intermolecular interactions, solvent polarity, and electronic effects, were analyzed in the context of both the reaction mechanism and the different colors of light emitted by different luciferases. The collective data points toward a combined emission of multiple coexistent forms of oxyluciferin as the most probable explanation for the variation in color of the emitted light. We also highlight realistic research directions to eventually address some of the remaining questions related to firefly bioluminescence. It is our hope that this extensive compilation of data and detailed analysis will not only consolidate the existing body of knowledge on this important phenomenon but will also aid in reaching a wider consensus on some of the mechanistic details of firefly bioluminescence.

Intrinsic fluorescence from firefly oxyluciferin monoanions isolated in vacuo

Fireflies, click beetles, and railroad worms glow in the dark. The color varies from green to red among the insects and is associated with an electronically excited oxyluciferin formed catalytically by the luciferase enzyme. The actual color tuning mechanism has been, and still is, up for much debate. One complication is that oxyluciferin can occur in different charge states and isomeric forms. We present here emission spectra of oxyluciferin monoanions in vacuo at both room temperature and at 100 K recorded with a newly developed and unique mass-spectroscopy setup specially designed for gas-phase ion fluorescence spectroscopy. Ions are limited to the phenolate-keto and phenolate-enol forms that account for natural bioluminescence. At 100 K, fluorescence band maxima are at 599 ± 2 nm and 563 ± 2 nm for the keto and enol forms, respectively, and at 300 K about 5 nm further to the red. The bare-ion spectra, free from solvent effects, serve as important references as they reveal whether a protein microenvironment redshifts or blueshifts the emission, and they serve as important benchmarks for nontrivial excited-state calculations.

Selective population of triplet excited states in heavy-atom-free BODIPY-C60 based molecular assemblies

Photophysical studies on a BODIPY-fullerene-distyryl BODIPY triad (BDP-C₆₀-DSBDP) and its reference dyads (BODIPY-fullerene; BDP-C₆₀ and distyryl BODIPY-fullerene; DSBDP-C₆₀) are presented herein. In the triad, the association of the two chromophore units linked by a fullerene moiety leads to strong near UV-Visible light absorption from 300 to 700 nm. The triplet-excited state was observed upon visible excitation in all these assemblies, and shown to be localized on the C₆₀ or BODIPY moieties. Using quantitative nanosecond transient absorption, we provide a complete investigation on the lifetime and formation quantum yield of the triplet-excited state. In the BDP-C₆₀ dyad, the triplet excited state of C₆₀ (τ = 7 ± 1 μs) was obtained with a quantum yield of 40 ± 8%. For the DSBDP-C₆₀ dyad and BDP-C₆₀-DSBDP triad, a longer-lived triplet excited state with a lifetime of around 250 ± 20 μs centered on the DSBDP moiety was formed, with respective quantum yields of 37 ± 8 and 20 ± 4%. Triplet-triplet annihilation up-conversion is characterized in the BDP-C₆₀ dyad and the bichromophoric triad in the presence of perylene and DSBDP-monomer as respective annihilators. The photo-induced formation of a long-lived ³DSBDP* in the triad coupled with panchromatic light absorption offers potential applications as a heavy-atom-free organic triplet photosensitizer.

Multivariate Curve Resolution Slicing of Multiexponential Time-Resolved Spectroscopy Fluorescence Data

Time-resolved fluorescence spectroscopy (TRFS), i.e., measurement of fluorescence decay curves for different excitation and/or emission wavelengths, provides specific and sensitive local information on molecules and on their environment. However, TRFS relies on multiexponential data fitting to derive fluorescence lifetimes from the measured decay curves and the time resolution of the technique is limited by the instrumental response function (IRF). We propose here a multivariate curve resolution (MCR) approach based on data slicing to perform tailored and fit-free analysis of multiexponential fluorescence decay curves. MCR slicing, taking as a basic framework the multivariate curve resolution-alternating least-squares (MCR-ALS) soft-modeling algorithm, relies on a hybrid bilinear/trilinear data decomposition. A key feature of the method is that it enables the recovery of individual components characterized by decay profiles that are only partially describable by monoexponential functions. For TRFS data, not only pure multiexponential tail information but also shorter time delay information can be decomposed, where the signal deviates from the ideal exponential behavior due to the limited time resolution. The accuracy of the proposed approach is validated by analyzing mixtures of three commercial dyes and characterizing the mixture composition, lifetimes, and associated contributions, even in situations where only ternary mixture samples are available. MCR slicing is also applied to the analysis of TRFS data obtained on a photoswitchable fluorescent protein (rsEGFP2). Three fluorescence lifetimes are extracted, along with the profile of the IRF, highlighting that decomposition of complex systems, for which individual isomers are characterized by different exponential decays, can also be achieved.

Absorption Spectra for Firefly Bioluminescence Substrate Analog: TokeOni in Various pH Solutions

AkaLumine hydrochloride, named TokeOni, is one of the firefly luciferin analogs, and its reaction with firefly luciferase produces near-infrared (NIR) bioluminescence. Prior to studying the bioluminescence mechanism, basic knowledge about the chemical structures, electronic states, and absorption properties of TokeOni at various pH values of solution has to be acquired. In this paper, the absorption spectra for TokeOni and AkaLumine at pH 2-10 were measured. Density functional theory (DFT) calculations, time-dependent DFT calculations, and the vibrational analyses were carried out. The absorption spectra indicate that the chemical forms of TokeOni in solutions are same as those of AkaLumine. The peaks at pH 7-10 in the absorption spectra correspond to the excitation from the ground state of a carboxylate anion of AkaLumine, the peak at pH 2 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring and N-protonated dimethylamino group of AkaLumine, and the peak at pH 4 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring of AkaLumine.

Quantum-mechanical hydration plays critical role in the stability of firefly oxyluciferin isomers: State-of-the-art calculations of the excited states

Stabilizing mechanisms of three possible isomers (phenolate-keto, phenolate-enol, and phenol-enolate) of the oxyluciferin anion hydrated with quantum explicit water molecules in the first singlet excited state were investigated using first-principles Born-Oppenheimer molecular dynamics simulations for up to 1.8 ns (or 3.7 × 10⁶ MD steps), revealing that the surrounding water molecules were distributed to form clear single-layered structures for phenolate-keto and multi-layered structures for phenolate-enol and phenol-enolate isomers. The isomers employed different stabilizing mechanisms compared to the ground state. Only the phenolate-keto isomer became attracted to the water molecules in its excited state and was stabilized by increasing the number of hydrogen bonds with nearby water molecules. The most stable isomer in the excited state was the phenolate-keto, and the phenolate-enol and phenol-enolate isomers were higher in energy by ∼0.38 eV and 0.57 eV, respectively, than the phenolate-keto. This was in contrast to the case of ground state in which the phenolate-enol was the most stable isomer.

The elusive relationship between structure and colour emission in beetle luciferases

In beetles, luciferase enzymes catalyse the conversion of chemical energy into light through bioluminescence. The principles of this process have become a fundamental biotechnological tool that revolutionized biological research. Different beetle species can emit different colours of light, despite using the same substrate and highly homologous luciferases. The chemical reasons for these different colours are hotly debated yet remain unresolved. This Review summarizes the structural, biochemical and spectrochemical data on beetle bioluminescence reported over the past three decades. We identify the factors that govern what colour is emitted by wild-type and mutant luciferases. This topic is controversial, but, in general, we note that green emission requires cationic residues in a specific position near the benzothiazole fragment of the emitting molecule, oxyluciferin. The commonly emitted green-yellow light can be readily changed to red by introducing a variety of individual and multiple mutations. However, complete switching of the emitted light from red to green has not been accomplished and the synergistic effects of combined mutations remain unexplored. The minor colour shifts produced by most known mutations could be important in establishing a 'mutational catalogue' to fine-tune emission of beetle luciferases, thereby expanding the scope of their applications.

pH-Dependent absorption spectrum of oxyluciferin analogues in the active site of firefly luciferase

In the quest for the identification of the light emitter(s) responsible for the firefly bioluminescence, the study of oxyluciferin analogues with controlled chemical and electronic structures is of particular importance. In this article, we report the results of our experimental and computational investigation of the pH-dependent absorption spectra characterizing three analogues bound into the luciferase cavity, together with adenosine-monophosphate (AMP). While the analogue microscopic pKa values do not differ much from their reference values, it turns out that the AMP protonation state is analogue-dependent and never doubly-deprotonated. A careful analysis of the interactions evidences the main role of E344 glutamic acid, as well as the flexibility of the cavity which can accommodate any oxyluciferin analogue. The consideration of the absorption spectra suggests that the oxyluciferin enolate form has to be excluded from the list of the bioluminescence reaction products.

Shining light on the electronic structure and relaxation dynamics of the isolated oxyluciferin anion

Firefly bioluminescence is exploited widely in imaging in the biochemical and biomedical sciences; however, our fundamental understanding of the electronic structure and relaxation processes of the oxyluciferin that emits the light is still rudimentary. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate the electronic structure and relaxation of a series of model oxyluciferin anions. We find that changing the deprotonation site has a dramatic influence on the relaxation pathway following photoexcitation of higher lying electronically excited states. The keto form of the oxyluciferin anion is found to undergo internal conversion to the fluorescent S₁ state, whereas we find evidence to suggest that the enol and enolate forms undergo internal conversion to a dipole bound state, possibly via the fluorescent S₁ state. Partially resolved vibrational structure points towards the involvement of out-of-plane torsional motions in internal conversion to the dipole bound state, emphasising the combined electronic and structural role that the microenvironment plays in controlling the electronic relaxation pathway in the enzyme.

Excited-State Proton Transfer in Oxyluciferin and Its Analogues

One of the most characterized bioluminescent reactions involves the firefly luciferase that catalyzes the oxidation of the luciferin producing oxyluciferin in its first excited state. While relaxing to the ground state, oxyluciferin emits visible light with an emission maximum that can vary from green to red. Oxyluciferin exists under six different chemical forms resulting from a keto/enol tautomerization and the deprotonation of the phenol or enol moieties. The optical properties of each chemical form have been recently characterized by the investigations of a variety of oxyluciferin derivatives, indicating unresolved excited-state proton transfer (ESPT) reactions. In this work, femtosecond pump-probe spectroscopy and time-resolved fluorescence spectroscopy are used to investigate the picosecond kinetics of the ESPT reactions and demonstrate the excited state keto to enol conversion of oxyluciferin and its derivatives in aqueous buffer as a function of pH. A comprehensive photophysical scheme is provided describing the complex luminescence pathways of oxyluciferin in protic solution.

Fungal Light Emitter: Understanding Its Chemical Nature and pH-Dependent Emission in Water Solution

Fungal bioluminescence is a fascinating natural process, standing out for the continuous conversion of chemical energy into light. The structure of fungal oxyluciferin (light emitter) was proposed in 2017, being different and more complex than other oxyluciferins. The complexity of fungal oxyluciferin arises from diverse equilibria such as keto/enol tautomerization or deprotonation equilibria of four titratable groups. For this reason, still some crucial details of its structure remain unexplored. To obtain further structural information, a combined experimental and computational study of natural and three synthetic fungal oxyluciferin analogues has been performed. Here, we state the most stable chemical form of fungal oxyluciferin regarding its keto and enol tautomers, in the ground and excited states. We propose the (3Z,5E)-6-(3,4-dihydroxyphenyl)-4-hydroxy-2-oxohexa-3,5-dienoic acid form as the light emitter (fluorescent state) in water solution. Moreover, we show that chemical modifications on fungal oxyluciferin can affect the relative stability of the conformers. Furthermore, we show the clear effect of pH on emission. General conclusions about the role of these titratable groups in emission modulation have been drawn, such as the key role of dihydroxyphenyl deprotonation. This study is key to further analyze the properties of fungal bioluminescence and propose novel synthetic analogues.

Relative Order of Acidity among Hydroxyl Groups of Oxyluciferin and Emission Light Colors in Aqueous Solution

The magnitude of the acidity of the oxyluciferin in water in the ground and excited state is investigated, and it is found for the first time using computational approach that the enol group of the phenol-enol species is the most acidic in the ground state, but the deprotonation of the phenol of the phenol-keto form is the most favored in the excited state. The relative order of the acidity among the hydroxyl groups in the oxyluciferin is attributed to the sequence of the O-H bond lengths in the enol and phenol group of the phenol-enol form, and the phenol group of the phenol-keto species. The mechanism of determining the dominant emissive species in the excited state is proposed, and the dependence of emission light colors on the photoexcitation energy is elucidated by the high relative concentration of six chemical forms in the ground state and the absorption efficiency.

Theoretical Study of the Wavelength Selection for the Photocleavage of Coumarin-caged D-luciferin

The equilibrium structures and optical properties of the photolabile caged luciferin, (7-diethylaminocoumarin-4-yl)methyl caged D-luciferin (DEACM-caged D-luciferin), in aqueous solution were investigated via quantum chemical calculations. The probable conformers of DEACM-caged D-luciferin were determined by potential energy curve scans and structural optimizations. We identified 40 possible conformers of DEACM-caged D-luciferin in water by comparing the Gibbs free energy of the optimized structures. Despite the difference in their structures, the conformers were similar in terms of assignments, oscillator strengths and energies of the three low-lying excited states. From the concentrations of the conformers and their oscillator strengths, we obtained a theoretical UV/Vis spectrum of DEACM-caged D-luciferin that has two main bands of shape nearly identical to the experimental UV/Vis spectrum. The absorption bands with maxima ~ 384 and 339 nm were attributed to the electronic excitations of the caged group and the luciferin moiety, respectively, by analysis of the theoretical UV/Vis spectrum. Furthermore, the analysis showed that DEACM-caged D-luciferin is excited in the caged group only by light of wavelength ranging within 400-430 nm, which is in the long-wavelength tail of the 384 nm band. This should be tested to lower damage upon photocleavage.

Phrixotrix luciferase and 6'-aminoluciferins reveal a larger luciferin phenolate binding site and provide novel far-red combinations for bioimaging purposes

How the unique luciferase of Phrixothrix hirtus (PxRE) railroad worm catalyzes the emission of red bioluminescence using the same luciferin of fireflies, remains a mystery. Although PxRE luciferase is a very attractive tool for bioanalysis and bioimaging in hemoglobin rich tissues, it displays lower quantum yield (15%) when compared to green emitting luciferases (>40%). To identify which parts of PxRE luciferin binding site (LBS) determine bioluminescence color, and to develop brighter and more red-shifted emitting luciferases, we compared the effects of site-directed mutagenesis and of larger 6′-substituted aminoluciferin analogues (6′-morpholino- and 6′-pyrrolidinyl-LH) on the bioluminescence properties of PxRE and green-yellow emitting beetle luciferases. The effects of mutations in the benzothiazolyl and thiazolyl parts of PxRE LBS on the K_M and catalytic efficiencies, indicated their importance for luciferin binding and catalysis. However, the absence of effects on the bioluminescence spectrum indicated a less interactive LBS in PxRE during light emission. Mutations at the bottom of LBS of PxRE blue-shifted the spectra and increased catalytic efficiency, suggesting that lack of interactions of this part of LBS with excited oxyluciferin phenolate underlie red light emission. The much higher bioluminescence activity and red-shifted spectra of PxRE luciferase with 6′-morpholino- (634 nm) and 6′-pyrrolidinyl-luciferins (644 nm), when compared to other beetle luciferases, revealed a larger luciferin phenolate binding pocket. The size and orientation of the side-chains of L/I/H348 are critical for amino-analogues accommodation and modulate bioluminescence color, affecting the interactions and mobility of excited oxyluciferin phenolate. The PxRE luciferase and 6′-aminoluciferins provide potential far-red combinations for bioimaging applications.

Effect of Protein Conformation and AMP Protonation State on Fireflies' Bioluminescent Emission

The emitted color in fireflies’ bioluminescent systems depends on the beetle species the system is extracted from and on different external factors (pH, temperature…) among others. Controlling the energy of the emitted light (i.e., color) is of crucial interest for the use of such bioluminescent systems. For instance, in the biomedical field, red emitted light is desirable because of its larger tissue penetration and lower energies. In order to investigate the influence of the protein environment and the AMP protonation state on the emitted color, the emission spectra of the phenolate-keto and phenolate-enol oxyluciferin forms have been simulated by means of MD simulations and QM/MM calculations, considering: two different protein conformations (with an open or closed C-terminal domain with respect to the N-terminal) and two protonation states of AMP. The results show that the emission spectra when considering the protein characterized by a closed conformation are blue-shifted compared to the open conformation. Moreover, the complete deprotonation of AMP phosphate group (AMP²⁻) can also lead to a blue-shift of the emission spectra but only when considering the closed protein conformation (open form is not sensitive to changes of AMP protonation state). These findings can be reasoned by the different interactions (hydrogen-bonds) found between oxyluciferin and the surrounding (protein, AMP and water molecules). This study gets partial insight into the possible origin of the emitted color modulation by changes of the pH or luciferase conformations.

The role of solvation models on the computed absorption and emission spectra: the case of fireflies oxyluciferin

Absorption and emission energies calculation covering both implicit and explicit solvation models using oxyluciferin as the case of study.

Towards broadband two-Dimensional electronic spectroscopy with ~8 fs phase-locked pulses at 400 nm

We introduce a two-dimensional spectroscopy setup based on a pair of near-UV-blue (360 - 430 nm), 8.4-fs, phase-locked, collinear excitation pulses and a nearly collinear UV-Vis supercontinuum probe pulse.

pH-Dependent fluorescence from firefly oxyluciferin in agarose thin films

The emitter of the firefly bioluminescence, oxyluciferin, and its derivatives were incorporated in agarose matrix to obtain self-supporting, lightweight fluorescent acidochromic thin films. This study demonstrates an alternative approach to investigating environmental effects on bioluminescent molecules.

The proton and metal binding sites responsible for the pH-dependent green-red bioluminescence color tuning in firefly luciferases

Firefly luciferases produce yellow-green light under physiological and alkaline conditions, however at acidic pH, higher temperatures or in the presence of heavy metals the color changes to red, a property called pH-sensitivity. Despite many decades of studies, the proton and metal binding sites responsible for pH-sensitivity remain enigmatic. Previously we suggested that the salt bridge E311/R337 keeps a closed conformation of the luciferin phenolate binding site. Here we further investigated the effect of this salt bridge and mutations of the neighbor residues H310 and E/N354, on metal and pH-sensitivity of firefly luciferases emitting distinct bioluminescence colors (Cratomorphus distinctus: 548 nm; Macrolampis sp2: 569 nm). The substitutions of H310 and E/N354 modulate metal sensitivity, whereas the carboxylate of E311 may work as the catalytic base essential for green bioluminescence and pH-sensitivity. Modeling studies showed that H310, E311 and E354 side-chains coordinate Zinc, constituting the metal binding site and the pH-sensor. Electrostatic potential and pKa calculations suggest that the external couple H310/E354 is affected by pH, whereas E311/R337 make a stabilized internal pair which retains excited oxyluciferin ejected proton near its phenolate group into a high energy state, promoting yellow-green bioluminescence. Protonation or metal binding weaken these electrostatic gates and their ability to retain the excited oxyluciferin released proton near its phenolate, promoting red light emission.

Chemi- and Bioluminescence of Cyclic Peroxides

Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

β-Deuterium Isotope Effects on Firefly Luciferase Bioluminescence

A 5,5-d2 -luciferin was prepared to measure isotope effects on reactions of two intermediates in firefly bioluminescence: emission by oxyluciferin and elimination of a putative luciferyl adenylate hydroperoxide to dehydroluciferin. A negligible isotope effect on bioluminescence provides further support for the belief that the emitting species is the keto-phenolate of oxyluciferin and rules out its excited-state tautomerization, one potential contribution to a bioluminescence quantum yield less than unity. A small isotope effect on dehydroluciferin formation supports a single-electron-transfer mechanism for reaction of the luciferyl adenylate enolate with oxygen to form the hydroperoxide or dehydroluciferin. Partitioning between the dioxetanone intermediate (en route to oxyluciferin) and dehydroluciferin is determined, not by the fate of the hydroperoxide, but by that of the radical formed from luciferyl adenylate, and the kinetic isotope effect (KIE) reflects H-atom abstraction by superoxide.

Oxyluciferin Derivatives: A Toolbox of Environment-Sensitive Fluorescence Probes for Molecular and Cellular Applications

In this work, we used firefly oxyluciferin (OxyLH₂) and its polarity-dependent fluorescence mechanism as a sensitive tool to monitor biomolecular interactions. The chromophores, OxyLH₂, and its two analogues, 4-MeOxyLH and 4,6'-DMeOxyL, were modified trough carboxylic functionalization and then coupled to the N-terminus part of Tat and NCp7 peptides of human immunodeficiency virus type-1 (HIV-1). The photophysical properties of the labeled peptides were studied in live cells as well as in complex with different oligonucleotides in solution. By monitoring the emission properties of these derivatives we were able, for the first time, to study in vitro biomolecular interactions using oxyluciferin as a sensor. As an additional application, cyclopropyl-oxyluciferin (5,5-Cpr-OxyLH) was site-specifically conjugated to the thiol group (Cys-232) of the human protein α-1 antytripsin to investigate its interaction with porcine pancreatic elastase. Our data demonstrate that OxyLH₂ and its derivatives can be used as fluorescence reporters for monitoring biomolecular interactions.

Cloning of the Orange Light-Producing Luciferase from Photinus scintillans-A New Proposal on how Bioluminescence Color is Determined

Unlike the enchanting yellow-green flashes of light produced on warm summer evenings by Photinus pyralis, the most common firefly species in North America, the orange lights of Photinus scintillans are infrequently observed. These Photinus species, and likely all bioluminescent beetles, use the same substrates beetle luciferin, ATP and oxygen to produce light. It is the structure of the particular luciferase enzyme that is the key to determining the color of the emitted light. We report here the molecular cloning of the P. scintillans luc gene and the expression and characterization of the corresponding novel recombinant luciferase enzyme. A comparison of the amino acid sequence with that of the highly similar P. pyralis enzyme and subsequent mutagenesis studies revealed that the single conservative amino acid change tyrosine to phenylalanine at position 255 accounted for the entire emission color difference. Additional mutagenesis and crystallographic studies were performed on a H-bond network, which includes the position 255 residue and five other stringently conserved beetle luciferase residues, that is proximal to the substrate/emitter binding site. The results are interpreted in the context of a speculative proposal that this network is key to the understanding of bioluminescence color determination.

The effect of dynamical fluctuations of hydration structures on the absorption spectra of oxyluciferin anions in an aqueous solution

Photo-absorption spectra of firefly oxyluciferin anions reflecting different hydration structure formed on different isomers, as revealed by ab initio simulations.

Excited-State Dynamics of Oxyluciferin in Firefly Luciferase

The color variations of light emitted by some natural and mutant luciferases are normally attributed to collective factors referred to as microenvironment effects; however, the exact nature of these interactions between the emitting molecule (oxyluciferin) and the active site remains elusive. Although model studies of noncomplexed oxyluciferin and its variants have greatly advanced the understanding of its photochemistry, extrapolation of the conclusions to the real system requires assumptions about the polarity and proticity of the active site. To decipher the intricate excited-state dynamics, global and target analysis is performed here for the first time on the steady-state and time-resolved spectra of firefly oxyluciferin complexed with luciferase from the Japanese firefly (Luciola cruciata). The experimental steady-state and time-resolved luminescence spectra of the oxyluciferin/luciferase complex in solution are compared with the broadband time-resolved firefly bioluminescence recorded in vivo. The results demonstrate that de-excitation of the luminophore results in a complex cascade of photoinduced proton transfer processes and can be interpreted by the pH dependence of the emitted light. It is confirmed that proton transfer is the central event in the spectrochemistry of this system for which any assignment of the pH-dependent emission to a single chemical species would be an oversimplification.

Perspectives on Bioluminescence Mechanisms

The molecular mechanisms of the bioluminescence systems of the firefly, bacteria and those utilizing imidazopyrazinone luciferins such as coelenterazine are gradually being uncovered using modern biophysical methods such as dynamic (ns-ps) fluorescence spectroscopy, NMR, X-ray crystallography and computational chemistry. The chemical structures of all reactants are well defined, and the spatial structures of the luciferases are providing important insight into interactions within the active cavity. It is generally accepted that the firefly and coelenterazine systems, although proceeding by different chemistries, both generate a dioxetanone high-energy species that undergoes decarboxylation to form directly the product in its S₁ state, the bioluminescence emitter. More work is still needed to establish the structure of the products completely. In spite of the bacterial system receiving the most research attention, the chemical pathway for excitation remains mysterious except that it is clearly not by a decarboxylation. Both the coelenterazine and bacterial systems have in common of being able to employ "antenna proteins," lumazine protein and the green-fluorescent protein, for tuning the color of the bioluminescence. Spatial structure information has been most valuable in informing the mechanism of the Ca²⁺ -regulated photoproteins and the antenna protein interactions.

Bioinspired Molecular Lantern: Tuning the Firefly Oxyluciferin Emission with Host-Guest Chemistry

Fireflies generate flashes of visible light via luciferase-catalyzed chemiexcitation of the substrate (luciferin) to the first excited state of the emitter (oxyluciferin). Microenvironment effects are often invoked to explain the effects of the luciferase active pocket on the emission; however, the exceedingly complex spectrochemistry and synthetic burdens have precluded elucidation of the nature of these interactions. To decipher the effects of microenvironment on the light emission, here the hydrophobic interior of cucurbit[7]uril (CB7) is used to mimic the nonpolar active pocket of luciferase. The hydrophobic interior of CB7 induces shifts of the ground-state pKas by 1.9-2.5 units to higher values. Upon sequestration, the emission maxima of neutral firefly oxyluciferin and its conjugate monodeprotonated base are blue-shifted by 40 and 39 nm, respectively, resulting in visual color changes of the emitted light.