We report the first syntheses—from commercially available 3-chloro-2-fluoroprop-1-ene (9)—of key garlic-derived compounds containing sp2-fluorine. We also report synthesis of fluoro-5,6-dihydrothiopyrans by trapping 2-fluorothioacrolein (15). Thus, difluoroallicin (12, S-(2-fluoro-2-propenyl) 2-fluoroprop-2-ene-1-sulfinothioate) is prepared by peracid oxidation of 1,2-bis(2-fluoro-2-propenyl)disulfane (11). S-2-Fluoro-2-propenyl-l-cysteine (2-fluorodeoxyalliin, 13), synthesized from cysteine and characterized by X-ray crystallography, is oxidized to its S-oxide, 2-fluoroalliin (22). The latter, with alliinase-containing powdered fresh garlic, gives a mixture of 12, allicin (1), and isomers of monofluoroallicin (23), indicating that 22 serves as a substrate for garlic alliinase. Upon heating, 12 generates transient 15, which dimerizes giving difluoro vinyl dithiins 6 and 7. Ethyl acrylate trapping of 15 affords 5- and 6-substituted 3-fluoro-5,6-dihydro-4H-thiopyrans (19 and 20). In 1,1,1,3,3,3-hexafluoro-2-propanol (HEFP) as solvent, 12 is converted into trifluoroajoene ((E,Z)-1-(2-fluoro-3-((2-fluoro-2-propenyl)sulfinyl)prop-1-en-1-yl)-2-(2-fluoro-2-propenyl)disulfane; 18). Liquid sulfur converts 11 to a (CH2=CFCH2)2Sn mixture (n = 4–15), characterized by UPLC-(Ag+)-coordination ion spray-mass spectrometry.

The Journal retracts the article “Fluorinated Analogs of Organosulfur Compounds from Garlic (Allium sativum): Synthesis, Chemistry and Anti-Angiogenesis and Antithrombotic Studies” [...]

Re-examination of the claimed isolation and X-ray characterization of di-p-tolyl and dimesityl 1,2-disulfoxides from thermolysis of the corresponding aryl sulfinimines and thiosulfinates showed that the isolated disulfide dioxides are instead the well-known isomeric thiosulfonates, as confirmed by XAS, DART-MS, X-ray, IR and NMR methods. Concerns with the original X-ray structures are addressed. Our results agree with the DFT prediction of very weak diaryl 1,2-disulfoxide S-S bond dissociation enthalpies. For now, room-temperature-stable noncyclic 1,2-disulfoxides remain unknown.

Allicin is the main flavour component of crushed raw garlic. This plant defence molecule has strong antibiotic properties. While measurements in the liquid phase using LC-MS are established, accessing reactive organosulfur compounds in the gas phase is still a challenge due to heat-degradation in the gas chromatograph. Using a gentle secondary electrospray ionisation coupled Orbitrap mass spectrometry procedure (SESI-Orbitrap MS), we measured gas phase concentrations of allicin evaporating from a pure solution. Despite the mild conditions, two quantitatively major allicin-derived breakdown products were found. The SESI-Orbitrap MS technique was used to follow the known chemistry of alliin, isoallin and methiin conversion in garlic, onion and ramsons. Allicin and its metabolites were also measured over two hours in human breath after garlic consumption. These results demonstrate the utility of SESI-Orbitrap MS for analysis of sulfur-containing volatiles from plants in the genus Allium and potentially for capturing volatilomes of foodstuffs in general.

Understanding how genes and experience work in concert to generate phenotypic variability will provide a better understanding of individuality. Here, we considered this in the main olfactory epithelium, a chemosensory structure with over a thousand distinct cell types in mice. We identified a subpopulation of olfactory sensory neurons, defined by receptor expression, whose abundances were sexually dimorphic. This subpopulation of olfactory sensory neurons was overrepresented in sex-separated mice and robustly responsive to sex-specific semiochemicals. Sex-combined housing led to an attenuation of the dimorphic representations. Single-cell sequencing analysis revealed an axis of activity-dependent gene expression amongst a subset of the dimorphic OSN populations. Finally, the pro-apoptotic gene Baxwas necessary to generate the dimorphic representations. Altogether, our results suggest a role of experience and activity in influencing homeostatic mechanisms to generate a robust sexually dimorphic phenotype in the main olfactory epithelium.

Here, I will address the evolutionary and molecular basis for the exquisite sensitivity of the human nose to sulfur compounds. With a focus on low molecular weight thiols, I will illustrate how olfactory receptors for specific thiols were identified and how ionic copper was found to dramatically enhance human sensitivity to thiols. I will also discuss several theories proposed for the molecular mechanism of olfaction, and will consider the plausibility of the vibration theory of olfaction as an explanation for thiol detection by the human nose.

Sulfur is sometimes called a spectroscopically silent element because there are so few tools for probing its chemistry. This paper will review how sulfur K-edge X-ray absorption spectroscopy and related methods can be used to overcome this limitation, using examples from the authors' research. X-rays from synchrotron radiation sources provide the means to study essentially any element in a variety of systems, ranging from pure compounds, to complex mixtures and even structured systems such as biological tissues. We show that sulfur K-edge X-ray absorption spectroscopy provides an important spectroscopic probe of sulfur chemistry in a range of systems, including those containing reactive species. Using a micro-focused X-ray beam, chemically-dependent X-ray fluorescence imaging can be used to image different sulfur species at a variety of spatial resolutions from microscopic to macroscopic. Increased sensitivity and deeper insights into electronic structure will be available in future studies using a suite of methods that are collectively known as advanced X-ray spectroscopy.

Sulfur-containing compounds within a physiological relevant, natural odor space, such as the key food odorants, typically constitute the group of volatiles with the lowest odor thresholds. The observation that certain metals, such as copper, potentiate the smell of sulfur-containing, metal-coordinating odorants led to the hypothesis that their cognate receptors are metalloproteins. However, experimental evidence is sparse-so far, only one human odorant receptor, OR2T11, and a few mouse receptors, have been reported to be activated by sulfur-containing odorants in a copper-dependent way, while the activation of other receptors by sulfur-containing odorants did not depend on the presence of metals. Here we identified an evolutionary conserved putative copper interaction motif CC/CSSH, comprising two copper-binding sites in TMH5 and TMH6, together with the binding pocket for 3-mercapto-2-methylpentan-1-ol in the narrowly tuned human receptor OR2M3. To characterize the copper-binding motif, we combined homology modeling, docking studies, site-directed mutagenesis, and functional expression of recombinant ORs in a cell-based, real-time luminescence assay. Ligand activation of OR2M3 was potentiated in the presence of copper. This effect of copper was mimicked by ionic and colloidal silver. In two broadly tuned receptors, OR1A1 and OR2W1, which did not reveal a putative copper interaction motif, activation by their most potent, sulfur-containing key food odorants did not depend on the presence of copper. Our results suggest a highly conserved putative copper-binding motif to be necessary for a copper-modulated and thiol-specific function of members from three subfamilies of family 2 ORs.

Humans have 396 unique, intact olfactory receptors (ORs), G-protein coupled receptors (GPCRs) containing receptor-specific binding sites; other mammals have more. Activation of these transmembrane proteins by an odorant initiates a signaling cascade, evoking an action potential leading to perception of a smell. Because the number of distinguishable odorants vastly exceeds the number of ORs, research has focused on mechanisms of recognition and signaling processes for classes of odorants. In this review, selected recent examples will be presented of "deorphaned" mammalian receptors, where the OR ligands (odorants) as well as key aspects of receptor-odorant interactions were identified using odorant-mediated receptor activation data together with site-directed mutagenesis and molecular modeling. Based on cumulative evidence from OR deorphaning and olfactory receptor neuron activation studies, a receptor-ligand docking model rather than an alternative bond vibration model is suggested to best explain the molecular basis of the exquisitely sensitive odor discrimination in mammals.

Significance While natural musk has been used for 2,000 years in perfumery, and in traditional medicine for its cardioprotective effects, its mode of activating odorant receptors (ORs) is unknown. ORs, G protein-coupled receptors (GPCRs), which constitute 40% of all pharmacophore receptors, are also expressed in nonolfactory tissues. Understanding the activation of ORs at the molecular level is challenging due to lack of crystallographic models. By combining site-directed mutagenesis with computational studies of human musk ORs involving 35 chiral and achiral muscone analogues, we propose structural models, including binding site prediction and responsible amino acid residues identification. Our studies of musk-responsive ORs should assist the study of the pharmacological effects of musks involving non-OR GPCRs.

Humans have 396 unique, intact olfactory receptors (ORs), G-protein coupled receptors (GPCRs) containing receptor-specific binding sites; other mammals have more. Activation of these transmembrane proteins by an odorant initiates a signaling cascade, evoking an action potential leading to perception of a smell. Because the number of distinguishable odorants vastly exceeds the number of ORs, research has focused on mechanisms of recognition and signaling processes for classes of odorants. In this review, selected recent examples will be presented of "deorphaned" mammalian receptors, where the OR ligands (odorants) as well as key aspects of receptor odorant interactions were identified using odorant-mediated receptor activation data together with site-directed mutagenesis and molecular modeling. Based on cumulative evidence from OR deorphaning and olfactory receptor neuron activation studies, a receptor ligand docking model rather than an alternative bond vibration model is suggested to best explain the molecular basis of the exquisitely sensitive odor discrimination in mammals.

Stereoisomers of 5-(2-allylsulfinyl)-3,4-dimethylthiolane-2-ol, a family of 3,4-dimethylthiolanes of formula C9H16O2S2 we name ajothiolanes, were isolated from garlic (Allium sativum) macerates and characterized by a variety of analytical and spectroscopic techniques, including ultraperformance liquid chromatography (UPLC), direct analysis in real time-mass spectrometry (DART-MS), and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Ajothiolanes were found to be spectroscopically identical to a family of previously described compounds named garlicnins B1-4 (C9H16O2S2), whose structures we demonstrate have been misassigned. 2D C-13-C-13 NMR incredible natural abundance double quantum transfer experiments (INADEQUATE) were used to disprove the claim of nine contiguous carbons in these compounds, while X-ray absorption spectroscopy (XAS) along with computational modeling was used to disprove the claim that these compounds were thiolanesulfenic acids. On the basis of the similarity of their NMR spectra to those of the ajothiolanes, we propose that the structures of previously described, biologically active onionins A(1-3) (C9H16O2S2), from extracts of onion (Allium cepa) and Allium fistulosum, and garlicnin A (C12H20O2S4), from garlic extracts, should also be reassigned, in each case as isomeric mixtures of 5-substituted-3,4-dimethylthiolane-2-ols. We conclude that 3,4-dimethylthiolanes may be a common motif in Allium chemistry. Finally, we show that another garlic extract component, garlicnin D (C7H12O2S3), claimed to have an unprecedented structure, is in fact a known compound from garlic with a structure different from that proposed, namely, 2(E)-3-(methylsulfinyl)-2-propenyl 2-propenyl disulfide.

Metal-coordinating compounds are generally known to have strong smells, a phenomenon that can be attributed to the fact that odorant receptors for intense-smelling compounds, such as those containing sulfur, may be metalloproteins. We previously identified a mouse odorant receptor (OR), Olfr1509, that requires copper ions for sensitive detection of a series of metal-coordinating odorants, including (methylthio)methanethiol (MTMT), a strong-smelling component of male mouse urine that attracts female mice. By combining mutagenesis and quantum mechanics/molecular mechanics (QM/MM) modeling, we identified candidate binding sites in Olfr1509 that may bind to the copper-MTMT complex. However, whether there are other receptors utilizing metal ions for ligand-binding and other sites important for receptor activation is still unknown. In this study, we describe a second mouse OR for MTMT with a copper effect, namely Olfr1019. In an attempt to investigate the functional changes of metal-coordinating ORs in multiple species and to decipher additional sites involved in the metal effect, we cloned various mammalian orthologs of the 2 mouse MTMT receptors, and a third mouse MTMT receptor, Olfr15, that does not have a copper effect. We found that the function of all 3 MTMT receptors varies greatly among species and that the response to MTMT always co-occurred with the copper effect. Furthermore, using ancestral reconstruction and QM/MM modeling combined with receptor functional assay, we found that the amino acid residue R260 in Olfr1509 and the respective R261 site in Olfr1019 may be important for receptor activation.

Sulfur-based thiyl radicals are known to be involved in a wide range of chemical and biological processes, but they are often highly reactive, which makes them difficult to observe directly. We report herein X-ray absorption spectra and analysis that support the direct observation of two different thiyl species generated photochemically by X-ray irradiation. The thiyl radical sulfur K-edge X-ray absorption spectra of both species are characterized by a uniquely low energy transition at about 2465 eV, which occurs at a lower energy than any previously observed feature at the sulfur K-edge and corresponds to a 1s→3p transition to the singly occupied molecular orbital of the free radical. Our results constitute the first observation of substantial levels of thiyl radicals generated by X-ray irradiation and detected by sulfur K-edge X-ray absorption spectroscopy.

Covering: up to the end of 2017While suggestions concerning the possible role of metals in olfaction and taste date back 50 years, only recently has it been possible to confirm these proposals with experiments involving individual olfactory receptors (ORs). A detailed discussion of recent experimental results demonstrating the key role of metals in enhancing the response of human and other vertebrate ORs to specific odorants is presented against the backdrop of our knowledge of how the sense of smell functions both at the molecular and whole animal levels. This review emphasizes the role of metals in the detection of low molecular weight thiols, sulfides, and other organosulfur compounds, including those found in strong-smelling animal excretions and plant volatiles, and those used in gas odorization. Alternative theories of olfaction are described, with evidence favoring the modified "shape" theory. The use of quantum mechanical/molecular modeling (QM/MM), site-directed mutagenesis and saturation-transfer-difference (STD) NMR is discussed, providing support for biological studies of mouse and human receptors, MOR244-3 and OR OR2T11, respectively. Copper is bound at the active site of MOR244-3 by cysteine and histidine, while cysteine, histidine and methionine are involved with OR2T11. The binding pockets of these two receptors are found in different locations in the three-dimensional seven transmembrane models. Another recently deorphaned human olfactory receptor, OR2M3, highly selective for a thiol from onions, and a broadly-tuned thiol receptor, OR1A1, are also discussed. Other topics covered include the effects of nanoparticles and heavy metal toxicants on vertebrate and fish ORs, intranasal zinc products and the loss of smell (anosmia).

We describe the synthesis, reactivity, and antithrombotic and anti-angiogenesis activity of difluoroallicin (S-(2-fluoroallyl) 2-fluoroprop-2-ene-1-sulfinothioate) and S-2-fluoro-2-propenyl-l-cysteine, both easily prepared from commercially available 3-chloro-2-fluoroprop-1-ene, as well as the synthesis of 1,2-bis(2-fluoroallyl)disulfane, 5-fluoro-3-(1-fluorovinyl)-3,4-dihydro-1,2-dithiin, trifluoroajoene ((E,Z)-1-(2-fluoro-3-((2-fluoroallyl)sulfinyl)prop-1-en-1-yl)-2-(2-fluoroallyl)disulfane), and a bis(2-fluoroallyl)polysulfane mixture. All tested organosulfur compounds demonstrated effective inhibition of either FGF or VEG-mediated angiogenesis (anti-angiogenesis activity) in the chick chorioallantoic membrane (CAM) or the mouse Matrigel® models. No embryo mortality was observed. Difluoroallicin demonstrated greater inhibition (p < 0.01) versus organosulfur compounds tested. Difluoroallicin demonstrated dose-dependent inhibition of angiogenesis in the mouse Matrigel® model, with maximal inhibition at 0.01 mg/implant. Allicin and difluoroallicin showed an effective antiplatelet effect in suppressing platelet aggregation compared to other organosulfur compounds tested. In platelet/fibrin clotting (anti-coagulant activity), difluoroallicin showed concentration-dependent inhibition of clot strength compared to allicin and the other organosulfur compounds tested.

We highlight the role that copper and other metals play in mammalian olfaction of low molecular weight organosulfur compounds from both natural and non-natural sources.