Identification, Quantitation, and Sensory Evaluation of Thiols in Bordeaux Red Wine with Characteristic Aging Bouquet.

Great Bordeaux red wines are known for their distinctive aging bouquet. However, the nature of volatile chemicals underpinning this sensory expression is not fully understood. This work investigated the empyreumatic aging bouquet of a collection of premium Bordeaux red wines using silver-ion (Ag+) solid-phase extraction, cryogenic heart-cutting multidimensional gas chromatography mass spectrometry/olfactometry, and comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry. In doing so, a substantial number of "meaty" odors were revealed. Three detected "meaty" notes were tentatively or unequivocally attributed to furan thiols. Among them, 2-methyltetrahydrofuran-3-thiol (1) with a pleasant "meaty" aroma was reported in wine for the first time. Its trans isomer (trans-1a) was resolved from its racemate by chemical modification, which confirmed its presence in wine. The odor detection threshold of trans-1a in the model wine was determined at 55 ng/L. Moreover, an additive effect between 1 and literature-known 2-methyl-3-furanthiol was observed. By a new ultra high-performance liquid chromatography quadrupole Orbitrap high-resolution mass spectrometry method, the concentration of trans-1a, in addition to those of 2-methyl-3-furanthiol and 2-furfuryl thiol, was measured in the wines at ng/L levels.

Asymmetric Allenylation of Alkynes mediated by Chiral Organoselenated Reagents under Oxidative Conditions.

The reactivity of novel chiral lactamide-substituted diselenide-based reagents under oxidative conditions was exploited to develop a metal-free method for the preparation of enantioenriched allenylamides from simple alkynes in good yields, and with enantiomeric excesses up to 99 %. The key of the success in this method is attributed to the hydrogen-bonded lactamide appendages that ensure configurational stability of chiral vinyl selenoxide intermediates for an optimal enantiotopic ß-syn-elimination step.

Synthesis and Chiroptical Properties of a Chiral Isotopologue of syn-Cryptophane-B.

We report the synthesis and absolute configuration (AC) of a chiral isotopologue of syn-cryptophane-B. Low chiral signatures were measured by polarimetry and electronic circular dichroism, whereas most significant chiroptical effects were observed by vibrational circular dichroism (VCD) and Raman optical activity (ROA). The comparison of experimental VCD and ROA spectra with those predicted by DFT calculations allows the determination of the AC of the two enantiomers as (-)589-MP-syn-2 and (+)589-PM-syn-2.

Chiroptical and potential in vitro anti-inflammatory properties of viniferin stereoisomers from grapevine (Vitis vinifera L.).

Determination of stereochemistry and enantiomeric excess in chiral natural molecules is a research of great interest because enantiomers can exhibit different biological activities. Viniferin stilbene dimers are natural molecules present in grape berries and wine but also, in larger amount, in stalks of grapevine. Four stereoisomers of viniferin stilbene dimers (7aS,8aS)-E-ε-viniferin (1a), (7aR,8aR)-E-ε-viniferin (1b), (7aS,8aR)-E-ω-viniferin (2a), and (7aR,8aS)-E-ω-viniferin (2b) were isolated from grapevine stalks of Cabernet Sauvignon, Merlot and Sauvignon Blanc, using a combination of centrifugal partition chromatography (CPC), preparative and chiral HPLC. The structure elucidation of these molecules was achieved by NMR whereas the absolute configurations of the four stereoisomers were investigated by vibrational circular dichroism spectroscopy in combination with density functional theory (DFT) calculations. This study unambiguously established the (+)-(7aS,8aS) and (+)-(7aR,8aS) configurations for E-ε-viniferin and E-ω-viniferin, respectively. Finally, we show that Cabernet Sauvignon provided the quasi enantiopure (+)-(7aS,8aS)-E-ε-viniferin compound which presents the best anti-inflammatory and anti-oxidant activities.

Are the Physical Properties of Xe@Cryptophane Complexes Easily Predictable? The Case of syn- and anti-Tris-aza-Cryptophanes.

We report the synthesis and optical resolution of C3-symmetrical tris-aza-cryptophanes anti-3 and syn-4, as well as the study of their interaction with xenon via hyperpolarized 129Xe NMR. These molecular cages are close structural analogues of the two well-known cryptophane-A (1; chiral) and cryptophane-B (2; achiral) diastereomers since these new compounds differ only by the presence of three nitrogen atoms grafted onto the same cyclotribenzylene unit. The assignment of their relative (syn vs anti) and absolute configurations was made possible, thanks to the combined use of quantum calculations at the density functional theory level and vibrational circular dichroism spectroscopy. More importantly, our results show that despite the large structural similarities with cryptophane-A (1) and -B (2), these two new compounds show a very different behavior in the presence of xenon in organic solutions. These results demonstrate that prediction of the physical properties of the xenon@cryptophane complexes, only based on structural parameters, remains extremely difficult.

Resolution, structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes.

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co3 (dpa)4 (CH3 CN)2 ]2+ (dpa = the anion of 2,2'-dipyridylamine) can be resolved using the chiral [As2 (tartrate)2 ]2- anion (AsT) and that these complexes demonstrate a strong chiroptical response in the ultraviolet-visible and X-ray energy regions. Here we report that the nickel congener, [Ni3 (dpa)4 (CH3 CN)2 ]2+ , can likewise be resolved using AsT. Depending on the stereochemistry of the enantiopure AsT anion, one or the other of the trinickel enantiomers crystallize from CH3 CN and diethyl ether in space group P421 2 as the (NBu4 )2 [Ni3 (dpa)4 (CH3 CN)2 ](AsT)2 ·[solvent] salt. After resolution, the AsT salts were converted into the PF6 - salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni3 (dpa)4 (CH3 CN)2 ](PF6 )2 ·2CH3 CN and [Co3 (dpa)4 (CH3 CN)2 ](PF6 )2 ·CH3 CN·C4 H10 O compounds crystallize in space groups C2 and P21 , respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH3 CN. Vibrational circular dichroism (VCD) data obtained in CD3 CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with Δε values up to 6 Lmol-1 cm-1 and vibrational dissymmetry factors on the order of 10-3 . Density functional theory calculations well reproduce the experimental spectra, showing little difference between the peak position, sign, and intensity in the VCD for the cobalt and nickel complexes. These results suggest that VCD enhancement of these peaks is unlikely, and their remarkable intensity may be due to their rigid helicoidal structure.

Chiroptical study of cryptophanes subjected to self-encapsulation.

In 1,1,2,2-tetrachloroethane-d2 , the 129 Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a 1 H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The 1 H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d6 and 1,1,2,2-tetrachloroethane-d2 solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)254 ]-1 and [CD(+)254 ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.

Chiroptical properties of cryptophane-111.

The two enantiomers of cryptophane-111 (1), which possesses the most simplified chemical structure of cryptophane derivatives and exhibits the highest binding constant for xenon encapsulation in organic solution, were separated by HPLC using chiral stationary phases. The chiroptical properties of [CD(+)254]-1 and [CD(-)254]-1 were determined in CH2Cl2 and CHCl3 solutions by polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of cryptophane-222 (2) derivative. Synchroton Radiation Circular Dichroism (SRCD) spectra were also recorded for the two enantiomers of 1 to investigate low-lying excited states in the 1Bb region. Time-dependent density functional theory (TDDFT) calculations of the ECD and SRCD as well as DFT calculations of the VCD and ROA allowed the [CD(-)254]-PP-1 and [CD(+)254]-MM-1 absolute configurations for 1 in CH2Cl2 and CHCl3 solutions. Similar configurations were found in the solid state from X-ray crystals of the two enantiomers but the chemical structures are significantly different from the one calculated in solution. In addition, the chiroptical properties of the two enantiomers of 1 were independent of the nature of the solvent, which is significantly different to that observed for cryptophane-222 compound. The lack of solvent molecule (CH2Cl2 or CHCl3) within the cavity of 1 can explain this different behaviour between 1 and 2. Finally, we show in this article that the encapsulation of xenon by 1 can be evidenced by ROA following the symmetric breathing mode of the cryptophane-111 skeleton at 150 cm-1.

Unusual Chiroptical Properties of the Cryptophane-222 Skeleton.

Enantiopure cryptophane-222 derivative (1) devoid of substituents was obtained via high-performance liquid chromatography (HPLC) using chiral stationary phases. The chiroptical properties of 1 were determined from polarimetry, electronic circular dichroism (ECD), synchrotron radiation circular dichroism (SRCD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of the cryptophane-A (2) derivative. Unusual polarimetric results were obtained for 1 in CHCl3 solvent as the sign of the optical rotation (OR) values changes in the nonresonance region above 365 nm, whereas no change was observed in the CH2Cl2 solvent. ECD spectra in the 1La and 1Lb regions were very similar for the two solutions and could not explain these unusual polarimetric properties. In contrast, SRCD spectra in the 1Bb region revealed spectral differences for the two solutions, which have been associated with conformational changes of the three linkers by time-dependent density functional theory (TDDFT) calculations. DFT calculations of the OR support that conformational changes may explain the polarimetric results obtained for the two solvents. Finally, TDDFT calculations of the ECD as well as DFT calculations of the VCD and ROA allowed the attribution of the (-)589-PP absolute configuration for 1 in solution, as determined from the X-ray structures of 1.

How stereochemistry influences the taste of wine: Isolation, characterization and sensory evaluation of lyoniresinol stereoisomers.

Wine expresses its beauty by sending a sensory message to the taster through molecules coming from grapes, yeast metabolism or oak wood. Among the compounds released during barrel aging, lyoniresinol has been recently reported as a relevant contributor to wine bitterness. As this lignan contains three stereogenic carbons, this work aimed at investigating the influence of stereochemistry on wine taste by combining analytical and sensorial techniques. First, an oak wood extract was screened by Liquid Chromatography-High Resolution Mass Spectrometry to target isomers separable in a symmetric environment and a diastereoisomer called epi-lyoniresinol was isolated for the first time. Then, an original racemic resolution based on natural xylose-derivatives was carried out to obtain lyoniresinol enantiomers. Chiroptical spectroscopic measurements associated with theoretical calculations allowed the unambiguous determination of their absolute configuration. The taste properties of all these stereoisomers revealed that only one lyoniresinol enantiomer is strongly bitter whereas the other one is tasteless and the diastereoisomer is slightly sweet. The presence of these three compounds was established in an oaked Bordeaux wine by chiral and non-chiral chromatography, suggesting the significant influence of stereochemistry on wine taste.

Chiroptical Properties of Cryptophane-223 and -233 Investigated by ECD, VCD, and ROA Spectroscopy.

Enantiopure cryptophane derivatives 1 and 2, possessing linkers of different nature (ethylenedioxy and propylenedioxy) connecting the two cyclotribenzylenes (CTB) units, were separated by HPLC using chiral stationary phases. X-ray crystallographic structures of the four enantiomers (+)-1, (-)-1, (+)-2, and (-)-2 have been obtained, allowing the unambiguous determination of their absolute configuration (AC) in the solid state. The chiroptical properties of compounds 1 and 2 were determined from polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of cryptophane-A (3) derivative. VCD, ROA and ECD spectra of 1 and 2 were calculated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, respectively, to confirm the AC of the cryptophane derivatives in solution. The (+)-PP and (-)-MM configurations were established for compounds 1 and 2 in chloroform solution, as already reported for the two enantiomers of 3. This result is in agreement with the X-ray structures of the two enantiomers of 1 and 2.

In situ helicity inversion of self-assembled nano-helices.

The handedness of nanometrical helices based on surfactant assemblies was inverted when these helices were in contact with an excess solution of chiral anions with opposite enantiomers. An important difference in the kinetics of chirality inversion at the molecular level and mesoscopic level was observed.

Raman optical activity of enantiopure cryptophanes.

Raman optical activity (ROA) and density functional theory (DFT) calculations were used to determine the absolute configuration of enantiopure cryptophane molecules and to obtain conformational information about their three ethylenedioxy linkers. ROA spectra recorded in chloroform solution for the two resolved enantiomers of cryptophanes derivatives bearing five (2), six (1), nine (3 and 4), and 12 (5) methoxy substituents are presented for the first time. The number of methoxy substituents (cryptophanes 1, 3, and 5) and the arrangement of the three linkers (anti for 3 and syn for 4) are two important parameters that significantly affect the ROA spectra. DFT calculations, at the B3PW91/6-31G** level, for cryptophane bearing six methoxy substituents establish, besides the absolute configuration, the preferential all-trans conformation of the ethylenedioxy linkers of the chloroform-cryptophane complex. This study shows that the ROA/DFT approach exhibits a higher selectivity for the conformation of the linkers than vibrational circular dichroism (VCD) associated with theoretical calculations.

Quantitative label-free RNA detection using surface-enhanced Raman spectroscopy.

Surface-Enhanced Raman Spectroscopy (SERS) was performed to detect label-free RNA. We defined conditions which make it possible to probe the four bases of RNA, in single strands of polyadenosine (pA), polyuridine (pU), polycytosine (pC) and polyguanosine (pG). We therefore present below a quantitative analysis of mixtures of non-hybridized single strands, based on the deconvolution of the SERS mixture spectrum into the relative contributions of the SERS spectra of each constituent.

PM-IRRAS investigation of self-assembled monolayers grafted onto SiO2/Au substrates.

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize self-assembled monolayers (SAMs). Novel ester-terminated organosilicon coupling agents possessing a trialkoxysilyl headgroup and a urea group in the linear alkyl chains (4) were synthesized and grafted onto SiO(2)/Au substrates (SiO(2) film of 200 Å thickness deposited on gold mirror). This composite substrate allowed the anchoring of SAMs and preserved the high reflectivity for infrared radiation. PM-IRRAS spectra with very high signal-to-noise ratios have been obtained in the mid-infrared spectral range allowing monitoring of the grafted SAMs. Quantitative analysis of the measured signal is described to compare PM-IRRAS and conventional IRRAS spectra. This quantitative analysis has been validated since the band intensities in the corrected PM-IRRAS and conventional IRRAS spectra are identical. Orientation information on the different functional groups has been obtained comparing the corrected PM-IRRAS spectrum with the one calculated using isotropic optical constants of ester-terminated organosilicon coupling agents 4. The carbonyls of the urea groups are preferentially parallel to the substrate surface favoring intermolecular hydrogen bonding and consequently a close packing of the molecules attached to the surface. By contrast, the alkyl chains present gauche defects and are poorly oriented.

Quantitative determination of band distortions in diamond attenuated total reflectance infrared spectra.

Due to its unmatched hardness and chemical inertia, diamond offers many advantages over other materials for extreme conditions and routine analysis by attenuated total reflection (ATR) infrared spectroscopy. Its low refractive index can offer up to a 6-fold absorbance increase compared to germanium. Unfortunately, it also results for strong bands in spectral distortions compared to transmission experiments. The aim of this paper is to present a methodological approach to determine quantitatively the degree of the spectral distortions in ATR spectra. This approach requires the determination of the optical constants (refractive index and extinction coefficient) of the investigated sample. As a typical example, the optical constants of the fibroin protein of the silk worm Bombyx mori have been determined from the polarized ATR spectra obtained using both diamond and germanium internal reflection elements. The positions found for the amide I band by germanium and diamond ATR are respectively 6 and 17 cm(-1) lower than the true value dtermined from the k(nu) spectrum, which is calculated to be 1659 cm(-1). To determine quantitatively the effect of relevant parameters such as the film thickness and the protein concentration, various spectral simulations have also been performed. The use of a thinner film probed by light polarized in the plane of incidence and diluting the protein sample can help in obtaining ATR spectra that are closer to their transmittance counterparts. To extend this study to any system, the ATR distortion amplitude has been evaluated using spectral simulations performed for bands of various intensities and widths. From these simulations, a simple empirical relationship has been found to estimate the band shift from the experimental band height and width that could be of practical use for ATR users. This paper shows that the determination of optical constants provides an efficient way to recover the true spectrum shape and band frequencies of distorted ATR spectra.

Conformational equilibria of TADDOL-s in solution investigated by vibrational circular dichroism.

Six enantiomeric pairs of TADDOL-s gathered in two series with either methyl (series A) or phenyl (series B) substituent in 2-position of the dioxolane ring were studied by vibrational circular dichroism (VCD). Experimental IR and VCD spectra associated with density functional theory (DFT) calculations showed that the two series exhibit quite different conformations in solution. In series A, the conformer with anti C-O bonds and stabilized by intramolecular OH...OH hydrogen bonding prevails, whereas in series B the conformer with gauche C-O bonds and intramolecular OH...π hydrogen bonding is favored. The shape and sign of the VCD bands in the O-H stretching region revealing the nature of the intramolecular hydrogen bonding were clearly identified. Polarimetric measurements showed that, within the same absolute configuration, compounds in series A and in series B have opposite signs of optical rotation.

A low temperature investigation of the N(4S degrees) + NO reaction.

The kinetics of the N((4)S degrees) + NO(X(2)Pi) reaction have been studied in a continuous supersonic flow reactor over the range 48 K

Dynamics of the reactions of C(3P(J)) atoms with ethylene, allene, and methylacetylene at low energy revealed by Doppler-Fizeau spectroscopy.

The dynamics of the H-atom elimination reactions of C((3)P(J)) atoms with ethylene, allene, and methylacetylene have been investigated in experiments conducted with pulsed supersonic beams using a variable beam crossing angle configuration at relative translational energies, E(T), in the range of 0.7 to 5.5 kJ mol(-1). H((2)S(1/2)) atoms were detected by time-of-flight mass spectrometry after sequential excitation to the (2)P(o)(J) state using a laser beam tuned to the Lyman-alpha transition around 121.57 nm and ionization by a second laser beam at 364.7 nm. Doppler-Fizeau spectra of the recoiling H atoms were recorded in two configurations, with the Lyman-alpha laser beam oriented either parallel or perpendicular to the relative velocity vector of the reagents. A mathematical model developed to account for the density-to-flux transformation and to extract angular and recoil energy distribution functions from the experimental spectra by a forward convolution procedure is fully described. The model, applied to the C + C(2)H(4) reaction, gives an excellent agreement with differential cross sections already determined in a previous combined study, thus providing a good test for its validity. All three processes are seen to pass through single pathways, identified by the comparison of the recoil energy distribution functions with the calculated reaction enthalpies, yielding H(2)CCCH + H (for the C + ethylene reaction) and H(2)CCCCH + H (for the C + allene and methylacetylene reactions). These results are discussed in the context of earlier experimental measurements performed at much higher collision energies.

Reaction kinetics to low temperatures. Dicarbon + acetylene, methylacetylene, allene and propene from 77 < or = T < or = 296 K.

The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X1Sigma(g)+) and first excited triplet (a 3Pi(u)) states with acetylene, methylacetylene, allene and propene has been studied using a recently constructed continuous supersonic flow reactor. Four Laval nozzles have been designed to access specified temperatures over the range of 77 < or = T < or = 220 K and measurements have been performed at 296 K under subsonic flow conditions. C2 was produced in its two lowest electronic states via the in situ multiphoton dissociation of C2Br4 at 266 nm. The time dependent losses of C2 in these two states in the presence of an excess of co-reagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C2, respectively. The rate coefficients were measured to be very fast, with values larger than 10(-10) cm3 molecule(-1) s(-1) and up to 5 x 10(-10) cm3 molecule(-1) s(-1). The reactions of 1C2 are seen to be essentially temperature independent from 77 < or = T < or = 296 K whereas the rate coefficients for the 3C2 reactions are seen to increase until they are equivalent to the 1C2 values at 77 K.