Marcelino Varela

The microwave spectrum of neurotransmitter serotonin

A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of serotonin for the first time. Three conformers of the neurotransmitter have been detected and characterized in the 4-10 GHz frequency range. The complicated hyperfine structure arising from the presence of two (14)N nuclei has been fully resolved for all conformers and used for their identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group probing the existence of N-Hπ interactions involving the amino group and the pyrrole unit in the Gauche-Phenyl conformer (GPh) or the phenyl unit in the Gauche-Pyrrole (GPy) ones.

Probing the γ‐Turn in a Short Proline Dipeptide Chain

The small peptide derived from proline, N-acetyl-prolinamide (Ac-Pro-NH2), has been investigated using a combination of Fourier transform microwave spectroscopy with laser ablation. Spectral signatures belonging to only one conformer have been detected in the supersonic expansion. Rotational constants and nuclear quadrupole coupling constants of the two (14)N nuclei have been used in the characterization of a γ-turn structure in the gas phase, which is stabilized by a CO···HN intramolecular hydrogen bond closing a seven-membered ring. A methyl group internal rotation barrier of 354 cm(-1) has been determined from the analysis of the A-E splittings.

Rotational spectrum of paracetamol.

In this paper we report the first rotational spectrum of isolated paracetamol. Solid samples of paracetamol were vaporized by laser ablation, expanded in a supersonic jet and characterized by Fourier transform microwave spectroscopy in the 4-10 GHz frequency range. The spectra of a total of four conformers, two with a trans peptidic -CO-NH- functional group arrangement and two with a cis peptidic group arrangement have been observed for the first time. The identification of all the conformers was based on the comparison of the experimental rotational and (14)N quadrupole coupling constants with those predicted by ab initio calculations, and relative values of their electrical dipole moment components.

Picolinic and Isonicotinic Acids: A Fourier Transform Microwave Spectroscopy Study

The rotational spectra of laser ablated picolinic and isonicotinic acids have been studied using broadband chirped pulse (CP-FTMW) and narrowband molecular beam (MB-FTMW) Fourier transform microwave spectroscopies. Two conformers of picolinic acid, s-cis-I and s-cis-II, and one conformer of isonicotinic acid have been identified through the analysis of their rotational spectra. The values of the inertial defect and the quadrupole coupling constants obtained for the most stable s-cis-I conformer of picolinic acid, evidence the formation of an O-H···N hydrogen bond between the acid group and the endocyclic N atom. The stabilization provided by this hydrogen bond compensates the destabilization energy due to the adoption of a -COOH trans configuration in this conformer. Its rs structure has been derived from the rotational spectra of several (13)C, (15)N, and (18)O species observed in their natural abundances. Mesomeric effects have been revealed by comparing the experimental values of the (14)N nuclear quadrupole coupling constants in the isomeric series of picolinic, isonicotinic, and nicotinic acids.

Conformational Analysis of Octopamine and Synephrine in the Gas Phase

Four and six conformers of the neurotransmitters octopamine and synephrine, respectively, have been identified in the gas phase using a laser ablation device in combination with a molecular-beam Fourier transform microwave spectrometer operating in the 4-10 GHz frequency range. The identification of all of the conformers was based on a comparison of the experimental rotational and (14)N quadrupole coupling constants with those predicted by ab initio calculations, as well as the relative values of their electric dipole moment components. The conformational preferences have been rationalized in terms of the various intramolecular forces operating in the different conformers of the studied molecules. All observed species are characterized by an intramolecular hydrogen bond of the type O-H···N established in the side chain of the neurotransmitters, which adopts an extended disposition in their most stable forms. For conformers with a folded side chain, an extra N-H···π hydrogen-bond-type interaction is established between the amino group and the π system of the aromatic ring.