Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin.

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of 13 C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the rC(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at rC(1)-C(2) , rC(3)-C(4) and rC(5)-C(6) . Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm-1 , a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH3 H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V3 =67 cm-1 . Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.

Contrast-Enhanced Ultrasound: A Useful Tool to Study and Monitor Hepatic Tumors Treated With Histotripsy.

Histotripsy is a novel noninvasive nonthermal, nonionizing, and precise treatment technique for tissue destruction. Contrast-enhanced ultrasound (CEUS) improves the detection, characterization, and follow-up of hepatic lesions because it depicts accurately the vascular perfusion of both normal hepatic tissue and hepatic tumors. We present the spectrum of imaging findings of CEUS after histotripsy treatment of hepatic tumors. CEUS provides real-time information, a close approximation to the dimension of the lesion, and a clear definition of its margins. Hepatic tumors detected by ultrasound can be potentially treated using B-mode ultrasound-guided histotripsy and characterized and monitored with CEUS. CEUS has shown to be very useful after tissue treatment to monitor and assess the evolution of the treated zone. Histotripsy treated zones are practically isoechogenic and slightly heterogeneous, and their limits are difficult to establish using standard B-mode ultrasound. The use of CEUS after histotripsy showing uptake of contrast protruding into the treated zone is clinically relevant to identify residual tumors and establish the most appropriate management strategy avoiding unnecessary treatments. We here describe CEUS findings after histotripsy for hepatic tumors.

Active bleeding after kidney biopsy: Successful ultrasound-guided direct thrombin embolization into the cortical fistula.

A 27-year-old man, previously diagnosed with IgA nephropathy, was referred for native kidney biopsy. After the procedure, the patient presented active bleeding revealed by Doppler and contrast-enhanced ultrasonography at the biopsy site. Successful embolization of the cortical fistula, the focus of bleeding, was achieved using ultrasound-guided thrombin injection and confirmed by Doppler ultrasonography, contrast-enhanced ultrasonography, and CT angiography. This case report shows that contrast-enhanced ultrasonography is useful for detecting active bleeding after a solid organ biopsy. Moreover, ultrasound-guided thrombin embolization is a safe and minimally invasive treatment and an alternative to angiography-guided embolization.

Observation of the Unbiased Conformers of Putative DNA-Scaffold Ribosugars.

The constitution, configuration, and flexibility of the core sugars of DNA molecules alter their function in diverse roles. Conformational itineraries of the ribofuranosides (fs) have long been known to finely determine rates of processing, yet we also know that, strikingly, semifunctional DNAs containing pyranosides (ps) or other configurations can be created, suggesting sufficient but incompletely understood plasticity. The multiple conformers involved in such processes are necessarily influenced by context and environment: solvent, hosts, ligands. Notably, however, to date the unbiased, "naked" conformers have not been experimentally determined. Here, the inherent conformational biases of DNA scaffold deoxyribosides in unsolvated and solvated forms have now been defined using gas-phase microwave and solution-phase NMR spectroscopies coupled with computational analyses and exploitation of critical differences between natural-abundance isotopologues. Serial determination of precise, individual spectra for conformers of these 25 isotopologues in alpha (α-d) and beta (ß-d); pyrano (p) and furano (f) methyl 2-deoxy-d-ribosides gave not only unprecedented atomic-level resolution structures of associated conformers but also their quantitative populations. Together these experiments revealed that typical 2E and 3E conformations of the sugar found in complex DNA structures are not inherently populated. Moreover, while both OH-5' and OH-3' are constrained by intramolecular hydrogen bonding in the unnatural αf scaffold, OH-3' is "born free" in the "naked" lowest lying energy conformer of natural ßf. Consequently, upon solvation, unnatural αf is strikingly less perturbable (retaining 2T1 conformation in vacuo and water) than natural ßf. Unnatural αp and ßp ribosides also display low conformational perturbability. These first experimental data on inherent, unbiased conformers therefore suggest that it is the background of conformational flexibility of ßf that may have led to its emergence out of multiple possibilities as the sugar scaffold for "life's code" and suggest a mechanism by which the resulting freedom of OH-3' (and hence accessibility as a nucleophile) in ßf may drive preferential processing and complex structure formation, such as replicative propagation of DNA from 5'-to-3'.

Bond Length Alternation Observed Experimentally: The Case of 1H-Indazole.

Bond length alternation is a chemical phenomenon in benzene rings fused to other rings, which has been mainly predicted theoretically. Its physical origin is still not clear and has generated discussion. Here, by using a strategy that combines microwave spectroscopy, custom-made synthesis and high-level ab initio calculations, we demonstrate that this phenomenon is clearly observed in the prototype indazole molecule isolated in the gas phase. The 1H-indazole conformer was detected by rotational spectroscopy, and its 17 isotopologues resulting from single and double heavy atom substitution (13 C and 15 N) were also unambiguously observed. Several experimental structures were determined and, in particular, the most useful semi-experimental equilibrium structure (re SE ), allowed determination of the heavy atom bond lengths to milli-Ångstrom precision. The experimentally determined bond length alternation is estimated to correspond to 60:40 contributions from the two resonant forms of 1H-indazole.

A General Treatment to Study Molecular Complexes Stabilized by Hydrogen-, Halogen-, and Carbon-Bond Networks: Experiment and Theory of (CH2 F2 )n ⋠⋠⋠(H2 O)m.

Rotational spectra of several difluoromethane-water adducts have been observed using two broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers. The experimental structures of (CH2 F2 )⋅⋅⋅(H2 O)2 , (CH2 F2 )2 ⋅⋅⋅(H2 O), (CH2 F2 )⋅⋅⋅(H2 O)3 , and (CH2 F2 )2 ⋅⋅⋅(H2 O)2 were unambiguously identified with the aid of 18 isotopic substituted species. A subtle competition between hydrogen, halogen, and carbon bonds is observed and a detailed analysis was performed on the complex network of non-covalent interactions which stabilize each cluster. The study shows that the combination of stabilizing contact networks is able to reinforce the interaction strength through a cooperative effect, which can lead to large stable oligomers.

Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid-Base Theory.

The correct identification of all gases released during hybrid perovskite degradation is of great significance to develop strategies to extend the lifespan of any device based on this semiconductor. CH3X (X = Br/I) is a released degradation gas/low boiling point liquid arising from methylammonium (MA+) based perovskites, which has been largely overlooked in the literature focusing on stability of perovskite solar cells. Herein, we present an unambiguous identification of CH3I release using microwave (rotational) spectroscopy. An experimental back-reaction test demonstrates that the well-known CH3NH2/HX degradation route may not be the ultimate degradation pathway of MAPbX3 in thermodynamic closed systems. Meanwhile, the CH3X/NH3 route cannot back-react selectively to MAX formation as occurred for the former back-reaction. Metadynamics calculations uncover the X halide effect on energy barriers for both degradation reactions showing a better stability of Br based perovskite ascribed to two aspects: (i) lower Brönsted-Lowry acidity of HBr compared to HI and (ii) higher nucleophilic character of CH3NH2 compared to NH3. The latter property makes CH3NH2 molecules stay preferentially attached on the electrophilic perovskite surface (Pb2+) during the dynamic simulation instead of being detached as observed for the NH3 molecule.

Interactions between azines and alcohols: a rotational study of pyridine-tert-butyl alcohol.

We assigned the rotational spectra of the parent and the OD isotopologues of the intermolecular complex pyridine-tert-butyl alcohol. The rotational and 14N quadrupole coupling constants are in agreement with a σ-type shape and a Cs symmetry of the complex. The two subunits are held together by a "classical" O-HN intermolecular hydrogen bond. Structural features of these hydrogen bonds are given and compared to those of similar molecular adducts. The ON distance decreases by 4 mÅ upon deuteration of the hydroxyl group, denoting a marked reverse Ubbelohde effect of the O-HN hydrogen bond.

Competing Dispersive Interactions: From Small Energy Differences to Large Structural Effects in Methyl Jasmonate and Zingerone.

Modern structural studies of biologically relevant molecules require an exhaustive interplay between experiment and theory. In this work, we present two examples where a poor choice of the theoretical method led to a misinterpretation of experimental results. We do that by performing a rotational spectroscopy study on two large and flexible biomolecules: methyl jasmonate and zingerone. The results show the enormous potential of rotational spectroscopy as a benchmark to evaluate the performance of theoretical methods.

Shapes, Dynamics, and Stability of ß-Ionone and Its Two Mutants Evidenced by High-Resolution Spectroscopy in the Gas Phase.

The conformational landscapes of ß-ionone and two mutants (α-ionone and ß-damascone) have been analyzed by means of state-of-the-art rotational spectroscopy and quantum-chemical calculations. The experiments performed at high resolution and sensitivity have provided a deep insight into their conformational spaces, assigning more than 8000 transitions corresponding to the rotational structures of 54 different species (3 isomers, 14 conformers, and 40 isotopologues). Methyl internal rotation dynamics were also observed and analyzed. The work proved the great flexibility of ß-ionone due to its flatter potential energy surface. This feature confers on ß-ionone a wider ability to interconvert between conformers with rather similar energies with respect to its mutants, allowing the retinal ligand to better adapt inside the binding pocket.

Investigating the Conformation of the Bridged Monosaccharide Levoglucosan.

Levoglucosan is one of the main products of the thermal degradation of glucose and cellulose and is commonly used as a tracer for biomass burning. Herein we report a conformational analysis of levoglucosan under isolation conditions, by means of microwave spectroscopy coupled with ultrafast laser vaporization in supersonic expansions. We observed three different conformations of levoglucosan in the gas phase. They all share a common heavy atom rigid bicyclic structure. The difference between the three of them lies in the network of intramolecular hydrogen bonds that arises from the OH groups at positions 2, 3 and 4. The different combinations of H-bonds give richness to the conformational landscape of levoglucosan. The gas phase conformers obtained in this work are compared to the crystal structure of levoglucosan previously reported. Although the heavy atom frame remains unchanged, there are significant differences in the positions of the H-atoms. In addition, the levoglucosan structure can be compared to the related glucose, for which gas phase conformational studies exist in the literature. In this case, in going from glucose to levoglucosan, there is an inversion in the chair conformation of the pyranose ring. This forces the OH groups to adopt axial positions (instead of the more favorable equatorial positions in glucose) and completely changes the pattern of intramolecular H-bonds.

Structural Studies of Nicotinoids: Cotinine versus Nicotine.

Nicotinoids are agonists of the acetylcholine receptor (nAChR) and play important biochemical and pharmacological roles. Herein, we report on the structure and conformation of cotinine, and compare its molecular properties with the nicotine prototype, from which it only differs in the addition of a carbonyl group. This investigation included a theoretical survey of the effects of rotamerization of the pyridine moiety, the puckering of the pyrrolidinone ring and the internal rotation of the methyl group. The experimental work examined the rotational spectrum of the molecule in a supersonic expansion, using both broadband chirped-pulse excitation techniques and cavity microwave spectrometers. Two conformers were observed for cotinine, and the fine and hyperfine structures arising from the two quadrupolar 14 N nuclei and the methyl internal rotor were fully analyzed. The two observed conformers share the same twisted conformation of the five-membered ring, but differ in a roughly 180° rotamerization around the C-C bond connecting the two rings. The energy barriers for the internal rotation of the methyl group in cotinine (4.55(4) and 4.64(3) kJ mol-1 , respectively) are much lower than in nicotine (estimated in 16.5 kJ mol-1 ). The combination of different intramolecular electronic effects, hydrogen bonding and possible binding differences to receptor molecules arising from the carbonyl group could explain the lower affinity of cotinine for nAChRs.

Effects of Chlorination on the Tautomeric Equilibrium of 2-Hydroxypyridine: Experiment and Theory.

The effects of halogenation on the tautomeric and conformational equilibria of the model system 2-hydroxypyridine/2-pyridone have been investigated through chlorine substitution at positions 3, 4, 5, and 6. In the gas phase, the lactim syn-periplanar tautomer (OHs ) was the predominant species for all compounds over the lactam form (C=O) and the less abundant anti-periplanar lactim (OHa ). However, the population of the three species was shown to be dependent on the position of the chlorine substitution. Chlorination in position 5 or 6 strongly stabilizes the OHs tautomer, whereas the C=O form has a significant population when the ring is chlorinated in positions 3 or 4. Overall, the OHa form is the least favourable form, although the 3-substitution favours the population of this tautomer. In addition, the C=O tautomer is strongly stabilized in the solvent, which makes it the dominant form in some substituted species. This study has been performed by means of rotational spectroscopy in the gas phase and/or theoretical calculations in the isolated phase and in solution. Both the OHs and C=O forms of 5-chloro-2-hydroxypyridine and the OHs form of 6-chloro-2-hydroxypyridine were experimentally observed. All transitions displayed a complex nuclear hyperfine structure owing to the presence of the chlorine and nitrogen nuclei. For all species, a full quadrupolar hyperfine analysis has been performed. This has provided crucial information for the unambiguous identification of tautomers.

Scopine Isolated in the Gas Phase.

The rotational spectrum of the tropane alkaloid scopine is detected by Fourier transform microwave spectroscopy in a pulsed supersonic jet. A nonconventional method for bringing the molecules intact into the gas phase is used in which scopine syrup is mixed with glycine powder and the solid mixture is vaporized with an ultrafast UV laser beam. Laser vaporization prevents the easy isomerization to scopoline previously observed with conventional heating methods. A single conformer is unambiguously observed in the supersonic jet and corresponds to the energetically most stable species according to quantum chemical calculations. Rotational and centrifugal distortion constants are accurately determined. The spectrum shows fine and hyperfine structure due to the hindered rotation of the methyl group and the presence of a quadrupolar nucleus (14 N), respectively. This additional information allows the angle of N-methyl inversion between the N-CH3 bond and the bicyclic C-N-C plane to be determined (131.8-137.8°), as well as the internal rotation barrier of the methyl group (6.235(1) kJ mol-1 ).

Furanosic forms of sugars: conformational equilibrium of methyl ß-d-ribofuranoside.

The investigation of an isolated ribofuranose unit in the gas phase reveals the intrinsic conformational landscape of the biologically active sugar form. We report the rotational spectra of two conformers of methyl ß-d-ribofuranoside in a supersonic jet expansion. Both conformers adopt a near twisted ((3)T2) ring conformation with the methoxy and hydroxymethyl substituents involved in various intramolecular hydrogen bonds.

Solving the Tautomeric Equilibrium of Purine through Analysis of the Complex Hyperfine Structure of the Four (14)N Nuclei.

The rotational spectra of two tautomers of purine have been measured by pulsed jet Fourier transform microwave spectroscopy coupled to a UV ultrafast vaporization system. The population ratio of the two main tautomers [N(7)H]/[N(9)H] is about 1/40 in the gas phase. It contrasts with the solid state where only the N(7)H species is present, or in solution where a mixture of both tautomers is observed. For both species, a full quadrupolar hyperfine analysis has been performed. This has led to the determination of the full sets of diagonal quadrupole coupling constants of the four (14)N atoms, which have provided crucial information for the unambiguous identification of both species. This work shows the great potential of microwave spectroscopy to study isolated biomolecules in the gas phase. All the work was supported by theoretical calculations.

Potential energy surface of fluoroxene: experiment and theory.

The potential energy surface (PES) of the general anesthetic fluoroxene (2,2,2-trifluoroethyl vinyl ether) was probed in a supersonic jet expansion using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy and theoretical calculations. The PES is dominated by a single conformation, as other stable minima are shown to kinetically relax in the expansion to the global minimum. Consistently, the rotational spectrum reveals a single conformation. Fluoroxene adopts a CS heavy-atom planar skeleton structure in the gas phase, with a cis-trans conformation (cis for the CH2=CH-O-CH2- and trans for the =CH-O-CH2-CF3 part). The sensitivity of a recently-built CP-FTMW spectrometer at the UPV/EHU is demonstrated by the detection of five isotopologues of fluoroxene in natural abundance, corresponding to the (13)C and (18)O monosubstituted species. The rS and r0 structures were determined and are in good agreement with theoretical predictions using the MP2, B3LYP and M06-2X methods.

Structural distortion of the epoxy groups in norbornanes: a rotational study of exo-2,3-epoxynorbornane.

Exo-2,3-epoxynorbornane is studied in the gas phase by pulsed jet Fourier transform microwave spectroscopy in the 4-18 GHz region. Six isotopologues were observed and characterized in their natural abundance. The experimental substitution and effective structures were obtained. Comparison with the structure of norbornane shows significant differences in several bond lengths and valence angles upon introduction of the epoxy group. All the work is supported by quantum chemical calculations.

Static and dynamic properties of 1,1'-bi-2-naphthol and its conjugated acids and bases.

Several convergent techniques were used to characterize 1,1'-bi-2-naphthol (BINOL) and some of its properties. Its acidity in the gas-phase, from neutral species to monoanion, was measured by mass spectrometry. The conformation and structure of BINOL in the gas phase was determined by microwave rotational spectroscopy. NMR experiments in fluorosulfonic acid established that BINOL was monoprotonated on one of the hydroxyl oxygen atoms. The enantiomerization barriers reported in the literature for BINOL under neutral, basic, and acid conditions were analyzed with regard to the species involved. Finally, DFT calculations allowed all of these results to be gathered in a coherent picture of the BINOL structure.