Iciar Uriarte

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

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 ).

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.

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

The conformational landscapes of β-ionone and two mutants (α-ionone and β-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 β-ionone due to its flatter potential energy surface. This feature confers on β-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.

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.

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.

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.

SOLVING THE TAUTOMERIC EQUILIBRIUM OF PURINE THROUGH THE ANALYSIS OF THE COMPLEX HYPERFINE STRUCTURE OF THE FOUR 14N NUCLEI

EMILIO J. COCINERO, ICIAR URIARTE, PATRICIA ECIJA, Physical Chemistry Department, Universidad del Paı́s Vasco (UPV/EHU), Bilbao, Spain; LAURA B. FAVERO, Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche (ISMN-CNR), Bologna, Italy; LORENZO SPADA, CAMILLA CALABRESE, WALTHER CAMINATI, Dep. Chemistry ’Giacomo Ciamician’, University of Bologna, Bologna, Italy.

Rotational spectroscopy of cf2clccl3 and analysis of hyperfine structure from four quadrupolar nuclei

CF2ClCCl3 has recently been identified among several new ozonedepleting substances in the atmosphere.a There are no literature reports concerning rotational spectroscopy of this molecule, although we were recently able to report its first chirped pulse, supersonic expansion spectrum.b CF2ClCCl3 has a rather small dipole moment so that the spectrum is weak and each transition displays very complex nuclear quadrupole hyperfine structure resulting from the presence of four chlorine nuclei.

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.