Our studies are focused in molecules of biological interest which have very different sizes, shapes and functional groups, directly conditioning their biological functionality. Understanding the relationship between structure and function is thus a major goal in Chemistry. Connected with this objective there is a growing interest in the study of biological molecules in gas-phase. Gas-phase studies specifically benefit from dissaggregatino and solvation control. For bare moleculas isolation from solvation, crystal or matrix partners reveals its intrinsic structural propierties, distginguishing between the molecular properties and those imposed by the environment. For molecular aggregates it is possible to select particular solvation partners or specific interactions, producing information on At influence of non-covalent interactions.
All the experimental studies are performed in the gas phase using Courier transform microwave spectrometer in pulsed supersonic expansions. Supersonic expansions provide a unique environment characterized by virtual isolation conditions (due to the absence of intermolecular collisions) and strong rovibrational cooling (Trot < 5 K). At the same time supersonic jets provide an effective method for the production in situ of a high density of weakly-bound complexes, amenable to multiple spectroscopic studies. The molecules of biological interest are introduced in the supersonic expansion diluted in a carrier gas by gentle warming through a custom-made heating nozzle.
To date we have studied tropane alkaloids like tropinone. The analysis of tropinone evidenced an important degree of flexibility in the N-methyl stereochernisuy of the tropane motif, which might be present in other drug agents.
Species containg 13C(1.1%), 15N (0.4%) and 18O (0.2%) were detected in natural abundance.
A section of the microwave spectrum (in MHz) and sorne typical rotational transitions of tropinone (quantum numbers YraK’c J”K”aK”c). Hyperfine components (F,–F”, F=I+J) are due to 14N nuclear quadrupole coupling to the molecular rotation. All transitions appear as a doublet by the Doppler effect. The ab initio I I predictions are shown in the two lower traces.
The conformational landscape of the anabasine nicotinoid was also analyzed recently. The results allow a detailed comparison of the structural properties of the prototype piperidinic and pyrrolidinic nicotinoids (anabasine vs. nicotine). Anabasine adopts two most stable conformations in isolation conditions, for which we determined accurate rotational and nuclear quadrupole coupling pararneters. The preferred conformations are characterized by an equatorial pyridine moiety and additional N-H equatorial stereochemistry at the piperidine ring (eq-eq; eq= equatorial). The two rings of anabasine are close to a bisecting arrangernent, with the observed conformations differing by an approximately 180. rotation of the pyridine subunit, denoted either syn or anti. The preference of anabasine for the eq-eq-syn conformation has been established by relative intensity measurements (syn/anti-5(2)).
The conformational preferences of free anabasine are directed by a weak N•••H-C hydrogen bond interaction between the nitrogen lone pair at piperidine and the closest C-H bond in pyridine, with N.••H distances ranging from 2.686 (syn) to 2.667 A (anti).
The 6(3,4)<–5(2,3) rotational transition of eq-eq-syn anabasine illustrates the complex hyperfine patterns arising frorn the two-nuclei 14N nuclear quadrupole coupling (hyperfine components labelled as l’,F,–1″,F”; 1=1,12, F=1+1) . All transitions appear as a doublet by the Doppler Effect.
Vanillin and derivates
The conformational equilibria of vanillin and ethylvanillin have been investigated in a supersonic jet expansion using rotational spectroscopy. Two conformers have been detected for each molecule, with a dominant 0-H…0 intramolecular hydrogen bond locking the local conformation of the hydroxyl and methoxy/ethoxy groups. As a consequence, the observed conformers of vanillin differ only in the orientation of the aldehyde gmup, either cis or trans respect to the methoxy group. For ethylvanil lin the ethoxy group would plausibly generate additional trans (in-plane) or gauche (AA-of-plane) orientations. However, the two detected conformations exhibit only planar ethoxy trans arrangements, with the gauche forms depopulated by collisional relaxation in the jet.
The intrinsic conformational and structural properties of the .4 bicycle exo-2-arninonorbornane have been probed.The rotational , spectrum revealed two different conformers arising from the -7. 2 internal rotation of the arnino group, exhibiting small (MHz) hyperfine patterns originated by the 14N nuclear quadrupole e coupling interaction. Due to the similarity of the rotational congants, the structural assignment of the observed rotamers and the .5 calculation of the torsion angles of the amino group were based on the conformatuonal dependence of the 14N ks 2 nuclear quadrupole coupling hyperfine tensor.
Sweet truth: The search for sugars in interstellar space is harnpered by a lack of spectroscopic information. D-ribose is now the first C-5 sugar observed in gas-phase using microwave spectroscopy. The rotational spectrum revealed six conforrnations of free ribose, adopting prefemtially betta-pyranose rings and higher-energy alpha-pyranose forms. No evidence was found of alpha-/betta-furanoses or linear forrns, unlike biological systerns, where betta-furanoses are found in RNA.