Synthesis and bioluminescence of electronically modified and rotationally restricted colour-shifting infraluciferin analogues

Bioluminescence, photophysical, computational and molecular docking studies of fully conformationally restricted enamine infraluciferin

A new rationally designed fully rotationally restricted luciferin has been synthesised. This synthetic luciferin, based upon the structure of infraluciferin, has two intramolecular H-bonds to reduce degrees of freedom, an amine group to enhance ICT process, and an alkenyl group to increase π-conjugation. In the spectroscopic measurements and computational calculations, enamine luciferin showed more red-shifted absorption and fluorescence emission than LH₂ and iLH₂. With PpyWT luciferase enamine luciferin gave bioluminescence at 564 nm which is similar to LH₂ at 561 nm. Further investigation by docking studies revealed that the emission wavelength of enamine luciferin might be attributed to the unwanted twisted structure caused by Asp531 within the enzyme. With mutant luciferase FlucRed, the major emission peak was shifted to 606 nm, a distinct shoulder above 700 nm, and 21% of its spectrum located in the nIR range.

Systematic Comparison of Beetle Luciferase-Luciferin Pairs as Sources of Near-Infrared Light for In Vitro and In Vivo Applications

Luciferases catalyze light-emitting reactions that produce a rainbow of colors from their substrates (luciferins), molecular oxygen, and often additional cofactors. These bioluminescence (BL) systems have afforded an incredible variety of basic research and medical applications. Driven by the importance of BL-based non-invasive animal imaging (BLI) applications, especially in support of cancer research, new BL systems have been developed by engineering beetle luciferase (Luc) variants and synthetic substrate combinations to produce red to near-infrared (nIR) light to improve imaging sensitivity and resolution. To stimulate the application of BLI research and advance the development of improved reagents for BLI, we undertook a systematic comparison of the spectroscopic and BL properties of seven beetle Lucs with LH₂ and nine substrates, which included two new quinoline ring-containing analogs. The results of these experiments with purified Luc enzymes in vitro and in live HEK293T cells transfected with luc genes have enabled us to identify Luc/analog combinations with improved properties compared to those previously reported and to provide live cell BL data that may be relevant to in vivo imaging applications. Additionally, we found strong candidate enzyme/substrate pairs for in vitro biomarker applications requiring nIR sources with minimal visible light components. Notably, one of our new substrates paired with a previously developed Luc variant was demonstrated to be an excellent in vitro source of nIR and a potentially useful BL system for improved resolution in BLI.

A higher spectral range of beetle bioluminescence with infraluciferin

Coleopteran bioluminescence is unique in that beetle luciferases emit colors ranging between green (ca.550 nm) and red (ca.600 nm), including intermediate colors such as yellow and orange, allowing up to 3 simultaneous parameters to be resolved in vitro with natural luciferin (D-LH₂). Here, we report a more than doubling of the maximum bioluminescence wavelength range using a single synthetic substrate, infraluciferin (iLH₂). We report that different luciferases can emit colors ranging from visible green to near-infrared (nIR) with iLH_2, including in human cells. iLH₂ was designed for dual color far-red to nIR bioluminescence imaging (BLI) in small animals and has been utilized in different mouse models of cancer (including a metastatic hepatic model showing detailed hepatic morphology) and for robust dual parameter imaging in vivo (including in systemic hematological models). Here, we report the properties of different enzymes with iLH₂: Lampyrid wild-type (WT) Photinus pyralis (Ppy) firefly luciferase, Ppy-based derivatives previously engineered to be thermostable with D-LH₂, and also color-shifted Elaterid-based enzymes: blue-shifted Pyrearinus termitilluminans derivative Eluc (reported D-LH₂ λmax = 538 nm) and red-shifted Pyrophorus plagiopthalamus derivative click beetle red (CBR) luciferase (D-LH₂ λmax = 618 nm). As purified enzyme, in bacteria or in human cells, Eluc emitted green light (λmax = 536 nm) with DL-iLH₂ whereas Ppy Fluc (λmax = 689 nm), x2 Fluc (λmax = 704 nm), x5 Fluc (λmax = 694 nm), x11 Fluc (λmax = 694 nm) and CBR (λmax = 721 nm) produced far-red to nIR peak wavelengths. Therefore, with iLH_2, enzyme λmaxes can be separated by ca.185nm, giving almost non-overlapping spectra. This is the first report of single-substrate bioluminescence color emission ranging from visible green to nIR in cells and may help shed light on the color tuning mechanism of beetle luciferases. We also report on the reason for the improvement in activity of x11 Fluc with iLH₂ and engineer an improved infraluciferase (iluc) based on this mutant.

Coumarin luciferins and mutant luciferases for robust multi-component bioluminescence imaging

Multi-component bioluminescence imaging requires an expanded collection of luciferase-luciferin pairs that emit far-red or near-infrared light. Toward this end, we prepared a new class of luciferins based on a red-shifted coumarin scaffold. These probes (CouLuc-1s) were accessed in a two-step sequence via direct modification of commercial dyes. The bioluminescent properties of the CouLuc-1 analogs were also characterized, and complementary luciferase enzymes were identified using a two-pronged screening strategy. The optimized enzyme-substrate pairs displayed robust photon outputs and emitted a significant portion of near-infrared light. The CouLuc-1 scaffolds are also structurally distinct from existing probes, enabling rapid multi-component imaging. Collectively, this work provides novel bioluminescent tools along with a blueprint for crafting additional fluorophore-derived probes for multiplexed imaging.

Applications of bioluminescence in biotechnology and beyond

Bioluminescent probes have hugely benefited from the input of synthetic chemistry and protein engineering. Here we review the latest applications of these probes in biotechnology and beyond, with an eye on current limitations and future directions.

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.

Conformationally rigid pyrazoloquinazoline α-amino acids: one- and two-photon induced fluorescence

Conformationally rigid unnatural α-amino acids bearing a pyrazoloquinazoline ring system that are amenable to both one- and two-photon excitation have been developed as new fluorescent probes.

Mutagenesis and Structural Studies Reveal the Basis for the Activity and Stability Properties That Distinguish the Photinus Luciferases scintillans and pyralis

The dazzling yellow-green light emission of the common North American firefly Photinus pyralis and other bioluminescent organisms has provided a wide variety of prominent research applications like reporter gene assays and in vivo imaging methods. While the P. pyralis enzyme has been extensively studied, only recently has a second Photinus luciferase been cloned from the species scintillans. Even though the enzymes share very high sequence identity (89.8%), the color of the light they emit, their specific activity and their stability to heat, pH, and chemical denaturation are quite different with the scintillans luciferase being generally more resistant. Through the construction and evaluation of the properties of chimeric domain swapped, single point, and various combined variants, we have determined that only six amino acid changes are necessary to confer all of the properties of the scintillans enzyme to wild-type P. pyralis luciferase. Altered stability properties were attributed to four of the amino acid changes (T214N/S276T/H332N/E354N), and single mutations each predominantly changed emission color (Y255F) and specific activity (A222C). Results of a crystallographic study of the P. pyralis enzyme containing the six changes (Pps6) provide some insight into the structural basis for some of the documented property differences.