Revealing the Structure of 6-Aminopenillanic Acid: The Active Nucleus of Penicillins.

6-Aminopenicillanic acid is a penicillanic acid compound and is the active nucleus common to all penicillins. Using laser ablation techniques, we transformed the solid into the gas phase and characterized its conformational panorama by combining supersonic expansions and Fourier transform microwave techniques. Five conformers were determined, adopting different spatial configurations. Among them, the axial and equatorial forms, which are biologically relevant, have been observed. The structural similarity to d-Ala-d-Ala and the detection of both axial and equatorial forms could explain its potential as a penicillin core and its capability as an antibiotic.

The Nicotinic Agonist Cytisine: The Role of the NH···N Interaction.

We report a detailed structural study of cytisine, an alkaloid used to help with smoking cessation, looking forward to unveiling its role as a nicotinic agonist. High-resolution rotational spectroscopy has allowed us to characterize two different conformers exhibiting axial and equatorial arrangements of the piperidinic NH group. Unexpectedly, the axial form has been found as the predominant configuration, in contrast to that observed for related molecules, such as piperidine. This anomalous behavior has been justified in terms of an intramolecular NH···N hydrogen bond. Moreover, this interaction justifies the overstabilization of the axial conformer over the equatorial one and is crucial for the mechanism of action of cytisine over the nicotinic receptor, further rationalizing its behavior as a nicotinic agonist.

A solvent-mediated conformational switch in sulfanilamide.

Sulfanilamide, a widely used antibacterial drug, has been brought into the gas phase using laser ablation techniques, and its structure has been characterized in the isolated conditions of a supersonic expansion using Fourier transform microwave techniques. A single conformer stabilized by an N-H⋯OS intramolecular interaction in an equatorial disposition has been unequivocally characterized. To emulate the microsolvation process, we studied its hydrated cluster. The results show that a single water molecule alters the conformational preference and forces sulfanilamide to switch from its initial eclipsed configuration to a staggered disposition. The observed hydrated cluster adopts a structure in which water forms three hydrogen bonds with sulfanilamide stabilizing the molecule.

Unbiased disentanglement of conformational baths with the help of microwave spectroscopy, quantum chemistry, and artificial intelligence: The puzzling case of homocysteine.

An integrated experimental-computational strategy for the accurate characterization of the conformational landscape of flexible biomolecule building blocks is proposed. This is based on the combination of rotational spectroscopy with quantum-chemical computations guided by artificial intelligence tools. The first step of the strategy is the conformer search and relative stability evaluation performed by means of an evolutionary algorithm. In this step, last generation semiempirical methods are exploited together with hybrid and double-hybrid density functionals. Next, the barriers ruling the interconversion between the low-lying conformers are evaluated in order to unravel the possible fast relaxation paths. The relative stabilities and spectroscopic parameters of the "surviving" conformers are then refined using state-of-the-art composite schemes. The reliability of the computational procedure is further improved by the inclusion of vibrational and thermal effects. The final step of the strategy is the comparison between experiment and theory without any ad hoc adjustment, which allows an unbiased assignment of the spectroscopic features in terms of different conformers and their spectroscopic parameters. The proposed approach has been tested and validated for homocysteine, a highly flexible non-proteinogenic α-amino acid. The synergism of the integrated strategy allowed for the characterization of five conformers stabilized by bifurcated N-H2⋯O=C hydrogen bonds, together with an additional conformer involving a more conventional HN⋯H-O hydrogen bond. The stability order estimated from the experimental intensities as well as the number and type of conformers observed in the gas phase are in full agreement with the theoretical predictions. Analogously, a good match has been found for the spectroscopic parameters.

An Innovative Approach for the Generation of Species of the Interstellar Medium.

The large amount of unstable species in the realm of interstellar chemistry drives an urgent need to develop efficient methods for the in situ generations of molecules that enable their spectroscopic characterizations. Such laboratory experiments are fundamental to decode the molecular universe by matching the interstellar and terrestrial spectra. We propose an approach based on laser ablation of nonvolatile solid organic precursors. The generated chemical species are cooled in a supersonic expansion and probed by high-resolution microwave spectroscopy. We present a proof of concept through a simultaneous formation of interstellar compounds and the first generation of aminocyanoacetylene using diaminomaleonitrile as a prototypical precursor. With this micro-laboratory, we open the door to generation of unsuspected species using precursors not typically accessible to traditional techniques such as electric discharge and pyrolysis.

Shape of Testosterone.

We have successfully characterized the structure of testosterone, one of the essential steroids, through high-resolution rotational spectroscopy. A single conformer has been detected, and a total of 404 transitions have been fitted, allowing a precise determination of the rotational constants. It allowed us to unravel that the isolated structure of testosterone adopts an extended disposition. The results obtained in this work highlight how using laser ablation techniques in combination with Fourier transform microwave techniques allow the study of large biomolecules or common pharmaceuticals. It is an important step toward studying relevant biomolecules and developing new analytical techniques with unprecedented sensitivity and resolution.

Looking for the Elusive Imine Tautomer of Creatinine: Different States of Aggregation Studied by Quantum Chemistry and Molecular Spectroscopy.

New spectroscopic experiments and state-of-the-art quantum-chemical computations of creatinine in different aggregation states unequivocally unveiled a significant tuning of tautomeric equilibrium by the environment: from the exclusive presence of the amine tautomer in the solid state and aqueous solution to a mixture of amine and imine tautomers in the gas phase. Quantum-chemical calculations predict the amine species as the most stable tautomer by about 30 kJ mol-1 in condensed phases. On the contrary, moving to the isolated forms, both Z and E imine isomers become more stable by about 7 kJ mol-1 . Since the imine isomers and one amine tautomer are separated by significant energy barriers, all of them should be present in the gas phase. This prediction has indeed been confirmed by high-resolution rotational spectroscopy, which provides the first experimental characterization of the elusive imine tautomer. The interpretation of the complicated hyperfine structure of the rotational spectrum, originated by three 14 N nuclei, makes it possible to use the spectral signatures as a sort of fingerprint for each individual tautomer in the complex sample.

Unveiling Five Naked Structures of Tartaric Acid.

The unbiased, naked structures of tartaric acid, one of the most important organic compounds existing in nature and a candidate to be present in the interstellar medium, has been revealed in this work for the first time. Solid samples of its naturally occurring (R,R) enantiomer have been vaporized by laser ablation, expanded in a supersonic jet, and characterized by Fourier transform microwave spectroscopy. In the isolation conditions of the jet, we have discovered up to five different structures stabilized by intramolecular hydrogen-bond networks dominated by O-H⋅⋅⋅O=C and O-H⋅⋅⋅O motifs extended along the entire molecule. These five forms, two with an extended (trans) disposition of the carbon chain and three with a bent (gauche) disposition, can serve as a basis to represent the shape of tartaric acid. This work also reports the first set of spectroscopy data that can be used to detect tartaric acid in the interstellar medium.

Unveiling the Neutral Forms of Glutamine.

Neutral glutamine has been evaporated by laser ablation of its solid sample to seed a rare gas carrier prior to a supersonic expansion and proved by Fourier transform microwave techniques. We report on three distinct neutral conformers that show a singular non-interacting and flexible amide sidechain in contrast with the other proteinogenic aliphatic amino acids. It could explain the essential biological role of glutamine as a nitrogen source, and its unique ability to form a variety of hydrogen bonds with peptide backbones. Common computational methods fail to predict the delicate balance of intramolecular interactions controlling the geometry of the most stable conformer. The spectroscopic data here reported can be used to benchmark novel computational methods in quantum chemistry.

Laser Ablation Assists Cyclization Reactions of Hydantoic Acid: A Proof for the Near-Attack Conformation Theory?

In the course of the investigation of the rotational spectrum of prebiotic hydantoic acid by Fourier transform microwave spectroscopy coupled to a laser ablation source in a supersonic expansion, rotational signatures of two cyclic molecules, hydantoin and 2,5-oxazolidinedione, have been unexpectedly observed along with the four most stable conformers of hydantoic acid. Interestingly, two of them presented folded geometric arrangements that might act as precursors in the cyclization reactions assisted by laser ablation. They could play the role of near-attack conformations (NACs) in the framework of the NAC theory for intramolecular reactions. A detailed analysis of the spectrum further revealed the simultaneous formation of other species in the jet, showing that the laser ablation of solid organic precursors constitutes an alternative tool in the generation of new chemical species.

Conformational Map of Phenolic Acids.

The benefits of vaporization by laser ablation and the high resolution and sensitivity attained by the chirped pulse Fourier transform microwave spectroscopy CP-FTMW have provided the first conformational map of the simplest phenolic acids of trans-cinnamic and p-coumaric. Two conformers of trans-cinnamic acid and four conformers of trans-p-coumaric acid have been characterized under the isolation conditions of a supersonic expansion. The spectroscopic constants derived from the analysis of the rotational spectra compared with those predicted theoretically provide an unmatched means to achieve an unambiguous identification of the observed species.

Sweet Structural Signatures Unveiled in Ketohexoses.

The conformational behaviour of naturally occurring ketohexoses has been revealed in a supersonic expansion by Fourier transform microwave spectroscopy coupled with a laser ablation source. Three, two and one conformers of d-tagatose, d-psicose and l-sorbose, respectively, have been identified by their rotational constants extracted from the analysis of the spectra. Singular structural signatures involving the hydroxyl groups OH(1) and OH(2) have been disentangled from the intricate intramolecular hydrogen bond networks stabilising the most abundant conformers. The present results place the old Shallenberger and Kier sweetness theories on a firmer footing.

The rotational spectrum of tyrosine.

In this work neutral tyrosine has been generated in the gas phase by laser ablation of solid samples, and its most abundant conformers characterized through their rotational spectra. Their identification has been made by comparison between the experimental and ab initio values of the rotational and quadrupole coupling constants. Both conformers are stabilized by an O-H•••N hydrogen bond established within the amino acid skeleton chain and an additional weak N-H•••π hydrogen bond. The observed conformers differ in the orientation of the phenolic -OH group.

Analysis of illicit drugs by direct ablation of solid samples.

Analysis of illicit drugs arises as an important field of work given the high social impacts presented by drugs in the modern society. Direct laser ablation of solid compounds allows their analysis without sampling or preparation procedures. For that purpose, an experimental set-up that combines laser ablation with time-of- flight mass spectrometry has been constructed very recently to perform studies on the mass spectra of such drugs as 3,4-methylenedioxy-N-methylamphetamine, commonly known as MDMA or ecstasy. Analysis of the observed fragmentation pattern in mass spectra may elucidate the ablation-induced photofragmentation phenomena produced, which differ from those previously observed with conventional ionization methods.

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.

Rotational spectrum of tryptophan.

The rotational spectrum of the natural amino acid tryptophan has been observed for the first time using a combination of laser ablation, molecular beams, and Fourier transform microwave spectroscopy. Independent analysis of the rotational spectra of individual conformers has conducted to a definitive identification of two different conformers of tryptophan, with one of the observed conformers never reported before. The analysis of the (14)N nuclear quadrupole coupling constants is of particular significance since it allows discrimination between structures, thus providing structural information on the orientation of the amino group. Both observed conformers are stabilized by an O-H···N hydrogen bond in the side chain and a N-H···π interaction forming a chain that reinforce the strength of hydrogen bonds through cooperative effects.

Alanine water complexes.

Two complexes of alanine with water, alanine-(H2O)n (n = 1,2), have been generated by laser ablation of the amino acid in a supersonic jet containing water vapor and characterized using Fourier transform microwave spectroscopy. In the observed complexes, water molecules bind to the carboxylic group of alanine acting as both proton donors and acceptors. In alanine-H2O, the water molecule establishes two intermolecular hydrogen bonds forming a six-membered cycle, while in alanine-(H2O)2 the two water molecules establish three hydrogen bonds forming an eight-membered ring. In both complexes, the amino acid moiety is in its neutral form and shows the conformation observed to be the most stable for the bare molecule. The microsolvation study of alanine-(H2O)n (n = 1,2) can be taken as a first step toward understanding bulk properties at a microscopic level.

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.

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.