Susana Blanco

Seven conformers of L-threonine in the gas phase: a LA-MB-FTMW study

Rotational spectroscopy in combination with molecular beams and laser ablation (laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy) has proved to be successful in characterizing the conformers of natural amino acids. The procedure usually followed to assign and identify the different conformers of an amino acid from the rotational spectrum is described through the study of the natural amino acid L-threonine. The solid sample of L-threonine was vaporized by laser pulses, diluted in Ne and supersonically expanded between the mirrors of a Fabry-Pérot resonator where it was spectroscopically probed by microwave radiation. The rotational and nuclear quadrupole coupling constants extracted from the analysis of the rotational spectrum are directly compared with those predicted by ab initio methods to achieve the conclusive identification of seven different conformers. A complex hydrogen bonding network arises as a consequence of the polar side chain of threonine.

Revealing the multiple structures of serine

We explored the conformational landscape of the proteinogenic amino acid serine [CH2OHCH(NH2)COOH] in the gas phase. Solid serine was vaporized by laser ablation, expanded in a supersonic jet, and characterized by Fourier transform microwave spectroscopy. In the isolation conditions of the jet there have been discovered up to seven different neutral (non-zwitterionic) structures of serine, which are conclusively identified by the comparison between the experimental values of the rotational and quadrupole coupling constants with those predicted by ab initio calculations. These seven forms can serve as a basis to represent the shape of serine in the gas phase. From the postexpansion abundances we derived the conformational stability trend, which is controlled by the subtle network of intramolecular hydrogen bonds formed between the polar groups in the amino acid backbone and the hydroxy side chain. It is proposed that conformational cooling perturbs the equilibrium conformational distribution; thus, some of the lower-energy forms are “missing” in the supersonic expansion.

Conformations of γ‐Aminobutyric Acid (GABA): The Role of the n→π* Interaction

g-Aminobutyric acid (GABA) is arguably the most important inhibitory neurotransmitter in the brain and brainstem/spinal cord. Owing to the high torsional flexibility of its heavyatom backbone, this molecular system has a large number of low-energy conformers (see Figure 1). Identifying the stable conformers of GABA can be relevant to understanding the selectivity of the biological processes in which the neurotransmitter participates. This can be done by placing GABA on a supersonic jet such that conformers are cooled down and trapped in their energy minima. In such an isolated environment the conformers with sufficient population can be detected and studied by spectroscopic methods. In this context, several methods have made great contributions to the elucidation of the structures of biomolecules in the gas phase. In the present work we have observed and characterized nine conformers of GABA using Fourier transform microwave spectroscopy in supersonic jets combined with laser ablation. Microwave spectroscopy, considered the most definitive gas-phase structural probe, can distinguish between different conformational structures since they have unique moments of inertia and give separate rotational spectra. In general, large molecules, in particular those of biological importance, have low vapor pressures and tend to degrade upon heating, making them unsuited for structural studies in the gas phase. Recently, rotational studies of biomolecules have entered in a new stage with the LAMB-FTMW experiment, which combines laser ablation (LA) with molecular beam Fourier transform microwave spectroscopy (MB-FTMW), an approach that overcomes the problems of thermal decomposition associated with conventional heating methods. To date, different aand b-amino acids have been studied using this technique, making it possible to characterize their preferred conformations. Even in conformationally challenging systems these can be identified by rotational spectroscopy, as has been illustrated with the assignment of seven low-energy conformers of serine and threonine, six of cysteine, and four of b-alanine and proline. In the present work, we have examined the conformations of GABA. In this system the separation of the polar amino and carboxylic groups, characteristic of many families of neurotransmitters, opens new conformational possibilities with respect to a-amino acids if one considers the balance of intramolecular forces that contribute to stabilizing the different conformations. The five hindered rotations around the single bonds generate a plethora of conformational species (Figure 1). An overall picture of the conformational landscape obtained from theoretical predictions at the MP2/6-311++G(d,p) level confirms the conformational richness of GABA: the 30 feasible conformers shown in Figure 1 were localized with relative energies below 900 cm . These conformers are labeled by two letters (a, G, or g) followed by a number. The first letter refers to the configuration at Ca and the second one to the configuration at Cg : a means anti conformers, G gauche conformers with positive value of the torsional angle CCOOH-Ca-Cb-Cg or N-Cg-Cb-Ca, and g gauche conformers Figure 1. Predicted low-energy conformers of GABA. The nine observed conformers are circled.

Rotational Probes of Six Conformers of Neutral Cysteine

has made possible the gas-phase study of solidbiomolecules with high melting points. In this approach,solidsarevaporizedbyahigh-energylaserpulse,supersoni-callyexpandedintoavacuumchamber,andcharacterizedbytheirrotationalspectrum.Ofthebiomoleculesthathavebeenstudiedbythistechnique,aliphaticaminoacidshavereceivedspecial attention because of the lack of experimental infor-mationandtheirbiologicalrelevance.Aminoacidsarewellknowntoexistaszwitterions(NH

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.

Pure rotational spectrum and model calculations of indole–water

The molecular beam Fourier transform microwave spectra of two isotopomers of the 1:1 complex between indole and water have been measured. The water molecule has been reliably located in the complex from these experimental data. The complex has a Cs symmetry with an N–H⋯O hydrogen bond and the plane of the H2O molecule perpendicular to the indole plane. The two-dimensional potential energy surface of the internal rotation and inversion of water in the complex, evaluated with B3LYP/6-31G** or MP2/6-31G** quantum chemical calculations, suggests the tunneling motion of water to take place with the contribute of both motions. The experimental evidence combined with flexible model calculations, indicate, however, that the tunneling motion is mainly an internal rotation of water around its C2 symmetry axis.

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

How water links to cis and trans peptidic groups: the rotational spectrum of N-methylformamide–water

We investigated the Fourier transform microwave spectra of the hydrated forms of N-methylformamide (NMF) in a supersonic expansion and assigned the rotational spectra of two mono-hydrated species. The conformation of each molecular complex was reliably determined on the basis of the values of the rotational constants, of the relative intensities of mu(a)- and mu(b)-type transitions, and of the features of the (14)N quadrupole hyperfine structure of the rotational transitions. In both complexes water acts as a proton donor and NMF has a trans configuration of the peptidic group. In the most stable of these conformers, water is also weakly bound to the methyl group.

A rotational study of laser ablated thiourea

A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of solid thiourea for the first time. The sensitivity reached in the experiment allowed the observation of the isotopomers (34)S, (13)C, and (15)N in their natural abundance. The spectrum of D(4)-thiourea was also analyzed from an enriched sample. The complicated hyperfine structure arising from the presence of two (14)N quadrupolar nuclei has been fully resolved and analyzed. The substitution r(s) structure has been derived from the experimental moments of inertia. Thiourea in gas phase presents a planar heavy atom skeleton. Experimental inertial defect values and high-level ab initio calculations reveal that the amino groups hydrogen atoms lie out-of-plane with a C(2) symmetry configuration and are involved in large amplitude inversion motions.

Jet-cooled rotational spectrum of laser-ablated 1,3,5-trithiane ☆

Abstract The jet-cooled rotational spectra of the parent and three isotopic species (13CC2H6S3, C3H634SS2 and C3H633SS2) in natural abundance of 1,3,5-trithiane have been observed by laser ablation combined with molecular-beam Fourier transform microwave spectroscopy. The r0 and rs structures of the molecule have been determined from the rotational data. The sensitivity of the experimental set-up proved to be high enough to observe the 33S isotopomer in natural abundance. An analysis of the nuclear quadrupole hyperfine structure due to the 33S nucleus allowed the determination of the quadrupole coupling tensor, giving some insight about the electronic distribution around this atom.