Carlos Cabezas

The conformational behaviour of free D-glucose—at last

The conformational behaviour of isolated D-glucose has been revealed in this work using Fourier transform microwave spectroscopy coupled with laser ablation of crystalline α- and β-glucopyranose samples. Four conformers of α-D-glucopyranose and three of β-D-glucopyranose have been unequivocally identified on the basis of the spectroscopic rotational parameters in conjunction with ab initio predictions. Stereoelectronic hyperconjugative factors, like those associated with anomeric or gauche effects, as well as the cooperative OH⋯O chains extended along the entire molecule, are the main factors driving the conformational behaviour. The most abundant conformers exhibit a counter-clockwise arrangement (cc) of the network of intramolecular hydrogen bonds.

Accurate Characterization of the Peptide Linkage in the Gas Phase: A Joint Quantum-Chemical and Rotational Spectroscopy Study of the Glycine Dipeptide Analogue

Accurate structures of aminoacids in the gas phase have been obtained by joint microwave and quantum-chemical investigations. However, the structure and conformational behavior of α-aminoacids once incorporated into peptide chains are completely different and have not yet been characterized with the same accuracy. To fill this gap, we present here an accurate characterization of the simplest dipeptide analogue (N-acetyl-glycinamide) involving peptidic bonds. State-of-the-art quantum-chemical computations are complemented by a comprehensive study of the rotational spectrum using a combination of Fourier transform microwave spectroscopy with laser ablation. The coexistence of the C7 and C5 conformers has been proved and energetically as well as spectroscopically characterized. This joint theoretical-experimental investigation demonstrated the feasibility of obtaining accurate structures for flexible small biomolecules, thus paving the route to the elucidation of the inherent behavior of peptides.

Six pyranoside forms of free 2-deoxy-D-ribose.

Carbohydrates are one of the most versatile biochemical building blocks, widely acting in energetic, structural, or recognition processes. The interpretation of the biological activity of saccharides is based on the structure and relative stability of their conformers. One of the obstacles to resolving the basic structure issues arises from their ability to form strong intermolecular hydrogen bonds with polar solvents, which in turn can result in conformational changes. A clear picture of the conformational panorama of isolated 2-deoxyd-ribose has been revealed using Fourier-transform microwave spectroscopy in conjunction with a UV ultrafast laser ablation source. Additionally, the availability of rotational data has been the main bottle-neck for examining the presence of these building blocks in interstellar space, so these studies could also be useful to the astrochemistry community. 2-Deoxy-d-ribose (2DR, C5H10O4; Figure 1a) is an important naturally occurring monosaccharide, present in nucleotides, which are the building blocks for DNA. In DNA, 2DR is present in the furanose (five-membered) ring form, whereas free in aqueous solution it cyclizes into fiveor six-membered rings, with the latter—the pyranoid form—being dominant. By closing the chain into a six-membered ring, the C1 carbon atom is converted into an asymmetric center, yielding two possible stereochemical a and b anomeric species (Figure 1b). In aqueous solution, 2DR primarily exists as a mixture of nearly equal amounts of a and b pyranose forms, present in their low-energy chair conformations, C1 and C4 (Figure 1c). [4] Such configurations are connected through ring inversion, thus establishing the axial or equatorial position of the OH group for each conformer. In addition, the monossacharides exhibit an unusual preferential stabilization of pyranose rings containing an axial OH group at the C1 carbon over the equatorial orientation, widely known as the anomeric effect, although its physical origin remains controversial. Nevertheless, structural analysis of 2DRmust take into consideration the intramolecular hydrogen bonding between adjacent OH groups. The formation of hydrogenbond networks reinforces their stability owing to hydrogenbond cooperativity effects. Such networks are fundamental to the molecular recognition of carbohydrates. By dissecting all these factors we can determine the most stable conformers of 2DR and the relative arrangement of the different hydroxy groups under isolated conditions, such as in the gas phase. In vacuo theoretical calculations, carried out on a-/bpyranoses, a-/b-furanoses, and open-chain conformations, predict 15 furanose and pyranose forms (Figure 1d, Table 1) in an energy window of 12 kJmol 1 above the predicted cc-apyr C1 global minimum. The notation used to label the different conformers include the symbols a and b to denote the anomer type, C1 and C4 to denote the pyranose chair form, C2-endo or C3-endo to denote the furanose envelope forms, and “c” or “cc” to indicate a clockwise or counterclockwise configuration of the adjacent OH bonds, respectively. A number is added to provide theMP2 energy ordering within the same family. To validate the predicted conformational behavior, comparison with precise experimental data of 2DR is needed. Previous experiments to determine the conformation of monosaccharides were based on X-ray and NMR measurements. However, these data are influenced by environmental effects associated with the solvent or crystal lattice. Recently, an IR spectrum of 2DR in an inert matrix in

Tautomerism in Neutral Histidine

Histidine is an important natural amino acid, involved in many relevant biological processes, which, because of its physical properties, proved difficult to characterize experimentally in its neutral form. In this work, neutral histidine has been generated in the gas phase by laser ablation of solid samples and its N(ε)H tautomeric form unraveled through its rotational spectrum. The quadrupole hyperfine structure, arising from the existing three (14)N nuclei, constituted a site-specifically probe for revealing the tautomeric form as well as the side chain configuration of this proteogenic amino acid.

LABORATORY AND ASTRONOMICAL DISCOVERY OF HYDROMAGNESIUM ISOCYANIDE

We report on the detection of hydromagnesium isocyanide, HMgNC, in the laboratory and in the carbon-rich evolved star IRC+10216. The J = 1-0 and J = 2-1 lines were observed in our microwave laboratory equipment in Valladolid with a spectral accuracy of 3 KHz. The hyperfine structure produced by the nitrogen atom was resolved for both transitions. The derived rotational constants from the laboratory data are B 0 = 5481.4333(6) MHz, D 0 = 2.90(8) KHz, and eQq(N) = –2.200(2) MHz. The predicted frequencies for the rotational transitions of HMgNC in the millimeter domain have an accuracy of 0.2-0.7 MHz. Four rotational lines of this species, J = 8-7, J = 10-9, J = 12-11, and J = 13-12, have been detected toward IRC+10216. The differences between observed and calculated frequencies are <0.5 MHz. The rotational constants derived from space frequencies are B 0 = 5481.49(3) MHz and D 0 = 3.2(1) KHz, i.e., identical to the laboratory ones. A merged fit to the laboratory and space frequencies provides B 0 = 5481.4336(4) MHz and D 0 = 2.94(5) KHz. We have derived a column density for HMgNC of (6 ± 2) × 1011 cm–2. From the observed line profiles the molecules have to be produced in the layer where other metal-isocyanides have been already found in this source. The abundance ratio between MgNC and its hydrogenated variety, HMgNC, is 20.

Disentangling the Puzzle of Hydrogen Bonding in Vitamin C.

Fast-passage Fourier transform microwave spectroscopy in combination with a laser ablation source has been successfully applied to probe vitamin C (l-ascorbic acid) in the gas phase. Its ethyldiol side chain and two hydroxyl groups around the γ-lactone ring provide five internal rotation axes, enabling vitamin C to assume a wide variety of nonplanar 3D cooperative hydrogen bond networks that can also include the keto and ether functions. The rotational constants extracted from the analysis of the spectrum unequivocally identify the existence of three dominant conformers stabilized by different intramolecular hydrogen bonding motifs forming five-, six-, or seven-membered rings.

Seven Conformers of Neutral Dopamine Revealed in the Gas Phase

The rotational spectrum of neutral dopamine has been investigated for the first time using a combination of Fourier transform microwave spectroscopy with laser ablation. The parameters extracted from the analysis of the spectrum unequivocally identify the existence of seven conformers of dopamine. (14)N nuclear quadrupole coupling interactions have been used to determine the orientation of the amino group probing the existence of stabilizing N-H···π interactions for all observed conformers.

Conformations of D-xylose: the pivotal role of the intramolecular hydrogen-bonding

Crystalline samples of D-xylose have been vaporized by laser ablation and probed in the gas phase using Fourier transform microwave spectroscopy. The rotational spectrum revealed the existence of two α-D-xylopyranose conformers stabilized by the anomeric effect and cooperative hydrogen bond networks. The experiment spectroscopically tracked fine structural changes upon clockwise and counterclockwise arrangements of the OH groups in the observed conformers. The five monosubstituted (13)C species of the most abundant conformer cc-α-(4)C1 have also been observed in their natural abundance, and its structure has been derived. This work demonstrates the pivotal role that the intramolecular hydrogen-bonding network plays in the conformational behavior of free monosaccharides.

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.

Observation of dihydrated glycine

The complex of glycine with two water molecules glycine-(H2O)2 has been generated by laser ablation in a supersonic expansion and characterised using rotational spectroscopy. The water molecules bind to the carboxylic group of glycine and to each other through three intermolecular hydrogen bonds, closing an eight-membered ring. In the complex, glycine adopts the conformation found to be the most stable for bare glycine.