Novel Peptide Chemistry in Terrestrial Animals: Natural Luciferin Analogues from the Bioluminescent Earthworm Fridericia heliota

We report isolation and structure elucidation of AsLn5, AsLn7, AsLn11 and AsLn12: novel luciferin analogs from the bioluminescent earthworm Fridericia heliota. They were found to be highly unusual modified peptides, comprising either of the two tyrosine‐derived chromophores, CompX or CompY and a set of amino acids, including threonine, gamma‐aminobutyric acid, homoarginine, and unsymmetrical N,N‐dimethylarginine. These natural compounds represent a unique peptide chemistry found in terrestrial animals and rise novel questions concerning their biosynthetic origin.     

The Chemical Basis of Fungal Bioluminescence

Many species of fungi naturally produce light, a phenomenon known as bioluminescence, however, the fungal substrates used in the chemical reactions that produce light have not been reported. We identified the fungal compound luciferin 3‐hydroxyhispidin, which is biosynthesized by oxidation of the precursor hispidin, a known fungal and plant secondary metabolite. The fungal luciferin does not share structural similarity with the other eight known luciferins. Furthermore, it was shown that 3‐hydroxyhispidin leads to bioluminescence in extracts from four diverse genera of luminous fungi, thus suggesting a common biochemical mechanism for fungal bioluminescence.     

A Novel Type of Luciferin from the Siberian Luminous Earthworm Fridericia heliota: Structure Elucidation by Spectral Studies and Total Synthesis

The structure elucidation and synthesis of the luciferin from the recently discovered luminous earthworm Fridericia heliota is reported. This luciferin is a key component of a novel ATP‐dependent bioluminescence system. UV, fluorescence, NMR, and HRMS spectroscopy studies were performed on 0.005 mg of the isolated substance and revealed four isomeric structures that conform to spectral data. These isomers were chemically synthesized and one of them was found to produce light when reacted with a protein extract from F. heliota. The novel luciferin was found to have an unusual extensively modified peptidic nature, thus implying an unprecedented mechanism of action.     

Oxidative Decarboxylation of Short‐Chain Fatty Acids to 1‐Alkenes

The enzymatic oxidative decarboxylation of linear short‐chain fatty acids (C4:0–C9:0) employing the P450 monooxygenase OleT, O₂ as the oxidant, and NAD(P)H as the electron donor gave the corresponding terminal C₃ to C₈ alkenes with product titers of up to 0.93 g L^?1 and TTNs of >2000. Key to this process was the construction of an efficient electron‐transfer chain employing putidaredoxin CamAB in combination with NAD(P)H recycling at the expense of glucose, formate, or phosphite. This system allows for the biocatalytic production of industrially important 1‐alkenes, such as propene and 1‐octene, from renewable resources for the first time.     

Mechanism of 3-Methylglutaconyl CoA Decarboxylase AibA/AibB: Pericyclic Reaction versus Direct Decarboxylation

The enzyme 3-methylglutaconyl coenzyme A (CoA) decarboxylase (called AibA/AibB) catalyzes the decarboxylation of 3-methylglutaconyl CoA to generate 3,3-dimethylacrylyl-CoA, representing an important step in the biosynthesis of isovaleryl-coenzyme A in Myxococcus xanthus when the regular pathway is blocked. A novel mechanism involving a pericyclic transition state has previously been proposed for this enzyme, making AibA/AibB unique among decarboxylases. Herein, density functional calculations are used to examine the energetic feasibility of this mechanism. It is shown that the intramolecular pericyclic reaction is associated with a very high energy barrier that is similar to the barrier of the same reaction in the absence of the enzyme. Instead, the calculations show that a direct decarboxylation mechanism has feasible energy barriers that are in line with the experimental observations.     

QM/MM Study on the Light Emitters of Aequorin Chemiluminescence,Bioluminescence, and Fluorescence: A General Understanding of the Bioluminescence of Several Marine Organisms

Aequorea victoria is a type of jellyfish that is known by its famous protein, green fluorescent protein (GFP), which has been widely used as a probe in many fields. Aequorea has another important protein, aequorin, which is one of the members of the EF‐hand calcium‐binding protein family. Aequorin has been used for intracellular calcium measurements for three decades, but its bioluminescence mechanism remains largely unknown. One of the important reasons is the lack of clear and reliable knowledge about the light emitters, which are complex. Several neutral and anionic forms exist in chemiexcited, bioluminescent, and fluorescent states and are connected with the H‐bond network of the binding cavity in the protein. We first theoretically investigated aequorin chemiluminescence, bioluminescence, and fluorescence in real proteins by performing hybrid quantum mechanics and molecular mechanics methods combined with a molecular dynamics method. For the first time, this study reported the origin and clear differences in the chemiluminescence, bioluminescence and fluorescence of aequorin, which is important for understanding the bioluminescence not only of jellyfish, but also of many other marine organisms (that have the same coelenterazine caved in different coelenterazine‐type luciferases).     

Understanding Bacterial Bioluminescence: A Theoretical Study of the Entire Process,from Reduced Flavin to Light Emission

Bacterial bioluminescence (BL) has been successfully applied in water‐quality monitoring and in vivo imaging. The attention of researchers has been attracted for several decades, but the mechanism of bacterial BL is still largely unknown due to the complexity of the multistep reaction process. Debates mainly focus on three key questions: How is the bioluminophore produced? What is the exact chemical form of the bioluminophore? How does the protein environment affect the light emission? Using quantum mechanics (QM), combined QM and molecular mechanics (QM/MM) and molecular dynamic (MD) calculations in gas‐phase, solvent and protein environments, the entire process of bacterial BL was investigated, from flavin reduction to light emission. This investigation revealed that: 1) the chemiluminescent decomposition of flavin peroxyhemiacetal does not occur through the intramolecular chemical initiated electron exchange luminescence (CIEEL) or the “dioxirane” mechanism, as suggested in the literature. Instead, the decomposition occurs according to the charge‐transfer initiated luminescence (CTIL) mechanism for the thermolysis of dioxetanone. 2) The first excited state of 4a‐hydroxy‐4a,5‐dihydroFMN (HFOH) was affirmed to be the bioluminophore of bacterial BL. This study provides details regarding the mechanism by which bacterial BL is produced and is helpful in understanding bacterial BL in general.     

Peptide‐Templated Acyl Transfer: A Chemical Method for the Labeling of Membrane Proteins on Live Cells

The development of a method is described for the chemical labeling of proteins which occurs with high target specificity, proceeds within seconds to minutes, and offers a free choice of the reporter group. The method relies upon the use of peptide templates, which align a thioester and an N‐terminal cysteinyl residue such that an acyl transfer reaction is facilitated at nanomolar concentrations. The protein of interest is N‐terminally tagged with a 22 aa long Cys‐E3 peptide (acceptor), which is capable of forming a coiled‐coil with a reporter‐armed K3 peptide (donor). This triggers the transfer of the reporter to the acceptor on the target protein. Because ligation of the two interacting peptides is avoided, the mass increase at the protein of interest is minimal. The method is exemplified by the rapid fluorescent labeling and fluorescence microscopic imaging of the human Y₂ receptor on living cells.     

Front Cover: Employing Click Chemistry to Expand the Scope of the Metal-Free Donor-Acceptor-Type Photocatalyst 4CzIPN (Eur. J. Org. Chem. 25/2023)

The design of a versatile alkyne-bearing derivative of the donor-acceptor fluorophore 2,4,5,6-tetrakis(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) suitable for further targeted modifications by copper-catalysed Azide Alkyne Cycloaddition (CuAAC) is reported. The newly synthesised photoredox catalyst notably exhibits analogously unique photoelectronic and steric features as the well-established carbazolyl dicyanobenzene motif and performs equally in a C−C-Coupling model reaction. Furthermore, the variability of this donor-acceptor system was demonstrated by the generation of a library of fourteen photoredox catalysts with electron-withdrawing and -donating groups as well as residues with high steric demands in one simple and selective CuAAC reaction. The modified catalysts feature a broad scope of reducing and oxidising powers in their excited states (span of reducing powers over more than 1 V) and were also successfully applied in the photoredox and nickel-catalysed decarboxylative cross coupling model reaction. This work represents a method to overcome the limitations in the flexibility of metal-free donor-acceptor fluorophores required for the targeted application in organic synthesis and paves the way for the design of customised catalysts with multiple functionalities.     

Mechanism of Thyroxine Deiodination by Naphthyl‐Based Iodothyronine Deiodinase Mimics and the Halogen Bonding Role: A DFT Investigation

This paper deals with a systematic density functional theory (DFT) study aiming to unravel the mechanism of the thyroxine (T4) conversion into 3,3′,5‐triiodothyronine (rT3) by using different bio‐inspired naphthyl‐based models, which are able to reproduce the catalytic functions of the type‐3 deiodinase ID‐3. Such naphthalenes, having two selenols, two thiols, and a selenol–thiol pair in peri positions, which were previously synthesized and tested in their deiodinase activity, are able to remove iodine selectively from the inner ring of T4 to produce rT3. Calculations were performed including also an imidazole ring that, mimicking the role of the His residue, plays an essential role deprotonating the selenol/thiol moiety. For all the used complexes, the calculated potential energy surfaces show that the reaction proceeds via an intermediate, characterized by the presence of a X?I?C (X=Se, S) halogen bond, whose transformation into a subsequent intermediate in which the C?I bond is definitively cleaved and the incipient X?I bond is formed represents the rate‐determining step of the whole process. The calculated trend in the barrier heights of the corresponding transition states allows us to rationalize the experimentally observed superior deiodinase activity of the naphthyl‐based compound with two selenol groups. The role of the peri interactions between chalcogen atoms appears to be less prominent in determining the deiodination activity.     

A new efficient route to chiral 1,3-disubstituted ferrocenes: application to the syntheses of (R(p))- and (S(p))-17alpha-[(3'-formylferrocenyl)ethynyl]estradiol

Starting from (2S,4S)-2-ferrocenyl-4-(methoxymethyl)-1,3-dioxane (4), use of the stereogenic ortho-directing menthyl para-tolyl sulfoxide group, which occupies the 2' position in the ferrocenyl ring and redirects subsequent lithiation to the 3' position, allowed the synthesis of optically pure (S(p))-1-formyl-3-iodoferrocene (8), that was characterized by single-crystal X-ray diffraction. Combination of this method with a protection-deprotection strategy, using trimethylsilyl as a temporary blocking group, yielded (R(p))-1-formyl-3-iodoferrocene (13). Separate Sonogashira coupling of each of the enantiomeric iodoformylferrocenes 8 and 13 with 17alpha-ethynyl-estradiol produced (R(p))-17alpha-[(3'-formylferrocenyl)ethynyl]estradiol (14) and (S(p))-17alpha-[(3'-formylferrocenyl)ethynyl]estradiol (15), respectively.