Unleashing the shape of L-DOPA at last.

Herein, we report the first rotational study of neutral L-DOPA, an extensively used supramolecular synthon and an amino acid precursor of the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) using broadband and narrowband Fourier transform microwave spectroscopies coupled with a laser ablation vaporization system. The spectroscopic parameters derived from the analysis of the rotational spectrum conclusively identify the existence of four distinct conformers of L-DOPA in the supersonic jet, further rejecting the previously reported catechol ring-induced conformational restriction. The analysis of the 14N nuclear quadrupole coupling hyperfine structure further revealed the orientation of the N-bearing functional group, proving the existence of stabilizing N-H⋯π interactions for the observed structures.

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.

Rotational Spectrum and Conformational Analysis of Perillartine: Insights into the Structure-Sweetness Relationship.

We used high-resolution rotational spectroscopy coupled to a laser ablation source to study the conformational panorama of perillartine, a solid synthetic sweetener. Four conformers were identified under the isolation conditions of the supersonic expansion, showing that all of them present an E configuration of the C=N group with respect to the double bond of the ring. The observed structures were verified against Shallenberger-Acree-Kier's sweetness theory to shed light on the structure-sweetness relationship for this particular oxime, highlighting a deluge of possibilities to bind the receptor.

Shape-Shifting Molecules: Unveiling the Valence Tautomerism Phenomena in Bare Barbaralones.

We report a state-of-the-art spectroscopic study of an archetypical barbaralone, conclusively revealing the valence tautomerism phenomena for this bistable molecular system. The two distinct 1- and 5-substituted valence tautomers have been isolated in a supersonic expansion for the first time and successfully characterized by high-resolution rotational spectroscopy. This work provides irrefutable experimental evidence of the [3,3]-rearrangement in barbaralones and highlights the use of rotational spectroscopy to analyze shape-shifting mixtures. Moreover, this observation opens the window toward the characterization of new fluxional systems in the isolation conditions of the gas phase and should serve as a reference point in the general understanding of valence tautomerism.

Exploring the Maze of Cycloserine Conformers in the Gas Phase Guided by Microwave Spectroscopy and Quantum Chemistry.

Cycloserine has in common with isoxazolidines the saturated five-membered ring, which is an important scaffold for drug design, exhibiting diverse biological activities. The most remarkable feature of these compounds is the presence of the N-O bond framed in a cyclic moiety. The lack of an accurate characterization of this structural feature in an isolated system calls for a state-of-the-art theoretical-experimental study. A quantum-chemical investigation of cycloserine unveiled the presence of 11 local energy minima, with only two of them being separated by significant barriers. This picture has been experimentally confirmed: two species have been unequivocally detected in the gas phase by means of laser ablation microwave spectroscopy, also disentangling the complicated hyperfine structure originating from the presence of two nitrogen atoms. A thorough characterization of cycloserine and isoxazolidine, benchmarked by the semiexperimental investigation of hydroxylamine, provided the first accurate determination of their structures and pointed out that the rev-DSD-PBEP86 functional is competitive with respect to explicitly correlated coupled-cluster computations. This outcome paves the way toward accurate studies of large flexible molecules.

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.

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.

Intrinsic folding of the cysteine residue: competition between folded and extended forms mediated by the -SH group.

A dual microwave and optical spectroscopic study of a capped cysteine amino acid isolated in a supersonic expansion, combined with quantum chemistry modelling, enabled us to characterize the conformational preferences of Cys embedded in a protein chain. IR/UV double resonance spectroscopy provided evidence for the coexistence of two conformers, assigned to folded and extended backbones (with classical C7 and C5 backbone H-bonding respectively), each of them additionally stabilized by specific main-chain/side-chain H-bonding, where the sulfur atom essentially plays the role of H-bond acceptor. The folded structure was confirmed by microwave spectroscopy, which demonstrated the validity of the DFT-D methods currently used in the field. These structural and spectroscopic results, complemented by a theoretical Natural Bond Orbital analysis, enabled us to document the capacity of the weakly polar -CH2-SH side chain of Cys to adapt itself to the intrinsic local preferences of the peptide backbone, i.e., a γ-turn or a ß-sheet extended secondary structure. The corresponding local H-bonding bridges the side chain acceptor S atom to the backbone NH donor site of the same or the next residue along the chain, through a 5- or a 6-membered ring respectively.

The Six Isomers of the Cyclohexanol Dimer: A Delicate Test for Dispersion Models.

The cyclohexanol homodimer acts as a delicate test model of the role of dispersion forces in intermolecular association. Whereas phenol produces a single dimer, the suppression of π interactions and the larger conformational flexibility in cyclohexanol results in multiple isomerism, with six competing dimers of the free molecule being observed in a supersonic jet expansion. Rotational spectroscopy reveals accurate structural data, specifically the formation of homo- and heterochiral diastereoisomers and the presence of both equatorial and axial forms in the dimers. Four dispersion-corrected density-functional molecular orbital calculations were tested against the experiment, with B3LYP-D3(BJ) offering good structural reproducibility with an Alrich's triple-ζ basis set. However, the prediction of the dimer energetics is largely model-dependent, thus offering a testbed for the validation of dispersion-corrected computational models.

The Shape of the Archetypical Oxocarbon Squaric Acid and Its Water Clusters.

Herein, a full structural description is presented for the archetypical supramolecular synthone squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione), placed in the gas phase by laser ablation and characterized by chirped pulse Fourier transform microwave technique. Free from natural environmental disturbances, two different anti-anti and syn-anti planar forms and the corresponding water clusters have been revealed in a supersonic expansion. The substitution structure of the most stable anti-anti conformer has also been extracted from the analysis of the rotational spectra of the 13 C and 18 O isotopic species in their natural abundance. The interplay between inter- and intramolecular interactions involving hydroxy and carbonyl groups has been analyzed by QTAIM (quantum theory of atoms in molecules) methods for squaric acid and its water clusters to understand their chemical behavior and further rationalize their role in the stabilization of these molecular systems.

Excited state dynamics of aniline homoclusters.

We have investigated the relaxation, following excitation in the 290-235 nm region, of neutral aniline homoclusters (An)n formed in a supersonic expansion by femtosecond time resolved ionization. The applied method permits isolation of the dynamics of the dimer from that originated in bigger species of the generated distribution. Interestingly, and differently from the monomer and (An)n≥3 clusters, the dimer does not present a N-H dissociative 1πσ* channel. This fact can be explained in terms of the symmetric structure adopted, in which each molecule establishes two N-Hπ interactions, destabilizing the H dissociation channel. The observations permit relating the photophysics to the interactions established by the aniline units and confirming previous observations and theoretical predictions on the structure of aniline aggregates.

Rotational Spectrum of Saccharin: Structure and Sweetness.

We present the first high-resolution rotational study of the artificial sweetener saccharin. By combining laser ablation (LA), narrow- and broadband Fourier transform microwave techniques (FTMW), and supersonic expansions, we have transferred the solid of saccharin (mp 229 °C) to a supersonic jet and captured its rotational spectrum. The rotational constants were accurately determined by fitting more than 60 rotational transitions for the parent and 34S isotopic species in the 6.4-10.4 GHz frequency range. Experiment and complementary quantum-chemical calculations provide accurate geometrical parameters for saccharin, the first artificial sweetener investigated by high-resolution microwave spectroscopy. The detailed structural information extracted from the rotational and 14N nuclear quadrupole coupling constants provided useful data in the context of the old theories of sweetness.

Influence of the solvent in the electronic excitation of aromatic alcohols: Excited state IR-UV of propofol(H2O)8.

It is well known that water plays an important role in the reactivity and dynamics in a solution of molecules in electronic excited states. For example, electronic excitation is usually accompanied by a solvent rearrangement that may also influence the redistribution of the excitation energy. However, there is a lack of experimental data on such processes. Here, we explore the structural changes that follow electronic excitation in aggregates of propofol (2,6-diisopropylphenol) with up to eight water molecules, using a combination of mass-resolved excitation spectroscopy and density functional theory calculations. The molecules of water form a polyhedron around the hydroxyl group of propofol, also interacting with the π cloud of the aromatic ring. Electronic excitation produces a strong structural change in the water superstructure, which moves to an interaction with one of the carbon atoms of the aromatic ring, producing its distortion into a prefulvenic structure. Such deformation is not observed in smaller water clusters or in propofol-phenol aggregates highlighting the decisive role played by the solvent.

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.

Stepwise Nucleation of Aniline: Emergence of Spectroscopic Fingerprints of the Liquid Phase.

This work deals with the controlled nucleation of aniline from the isolated molecule until formation of a moderately large aggregate: aniline nonamer. The structure of the cluster at each step of the nucleation was unravelled combining mass-resolved IR spectroscopy and computational chemistry, demonstrating that aggregation is primarily guided by formation of extensive N-H⋅⋅⋅N hydrogen-bond networks that give the aggregates a sort of branched backbone, in clear competition with multiple N-H/C-H⋅⋅⋅π and π⋅⋅⋅π interactions. The result is the co-existence of close nucleation paths connecting relational aggregates. The delicate balance of molecular forces makes the aniline clusters a challenge for molecular orbital calculations and an ideal system to refine the present nucleation models. Noticeably, spectroscopic signatures characteristic of the condensed phase are apparent in the nanometer-size aggregates formed in this work.

The Structural Signs of Sweetness in Artificial Sweeteners: A Rotational Study of Sorbitol and Dulcitol.

A gas-phase study on the artificial sweeteners sorbitol and dulcitol has been carried out for the first time by using a combination of chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy and laser ablation (LA). The isolation conditions provided by the supersonic expansion reveal the intrinsic conformational structures of these sweeteners. The three and five observed conformers for sorbitol and dulcitol, respectively, are stabilized by networks of cooperative intramolecular hydrogen bonds between vicinal hydroxyl groups in clockwise or counterclockwise arrangements. Suitable places in the structure of seven out of eight conformers identified for both polyalcohols meet the requirements of the glucophore proposed by Shallenberger and Acree's molecular theory of sweet taste. Present results provide the first linkage between sweetness and structure in sugar alcohols.

Probing the structures and bonding of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging.

We report an investigation of a series of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging and ab initio calculations. Vibrationally resolved photoelectron spectra are obtained, allowing the electron affinities of Au-(C≡C) n -Au to be accurately measured as 1.651(1), 1.715(1), and 1.873(1) eV for n = 1-3, respectively. Both the Au-C symmetric stretching and a bending vibrational frequency are observed for each neutral auropolyyne. Theoretical calculations find that the ground state of Au2C2 - has a linear acetylenic Au-C≡C-Au- structure, whereas the asymmetric Au-Au-C≡C- structure is a low-lying isomer. However, for Au2C4 - and Au2C6 -, our calculations show that the asymmetric Au-Au-(C≡C) n - isomers are the global minima and the Au-(C≡C) n -Au- symmetric structures become low-lying isomers. All the asymmetric Au-Au-(C≡C) n - isomers are found computationally to have much higher electron binding energies and are not accessible at the detachment photon energies used in the current study. For neutral Au2C2n , the Au-(C≡C) n -Au auropolyyne structures are found to be the global minima for n = 1-3. The electronic structures and bonding for Au-(C≡C) n -Au (n = 1-3) are compared with the corresponding Au-(C≡C) n and Au-(C≡C) n -H species.

Isomerism of the Aniline Trimer.

Weaker intermolecular forces expand the isomerization alternatives for molecular aggregation, as observed for the prototype models of the aniline trimer (An3 ) and the monohydrated aniline dimer (An2 -W) when compared to the phenol trimer. In this experiment the aniline clusters were generated in a jet-cooled expansion and probed using broadband (chirped-pulsed) microwave spectroscopy. Three isomers of the aniline trimer and two isomers of the hydrated dimer were detected and characterized in the rotational spectrum. In the homotrimer the weak N-H⋅⋅⋅N hydrogen bonds are assisted by subtle combinations of N-H⋅⋅⋅π and C-H⋅⋅⋅π interactions, producing several competing low-lying ring species in the gas phase. One of the aniline trimers is a symmetric top, topologically equivalent to the only observed phenol trimer. Conversely, addition of a water molecule to the aniline dimer introduces a leading O-H⋅⋅⋅N interaction, making water to behave as dominant hydrogen-bond pivot between the two aniline molecules. This combination of weak intermolecular interactions critically tests the performance of dispersion-corrected or parametrized density-functional methods. Evaluation of the B3LYP-D3(BJ) and M06-2X methods revealed deficiencies of the Truhlar functional to reproduce the experimental rotational data.

Sugar-peptidic bond interactions: spectroscopic characterization of a model system.

Sugars are small carbohydrates which play numerous roles in living organisms such as storage of energy or as structural components. Modifications of specific sites within the glycan chain can modulate a carbohydrate's overall biological function as it happens with nucleic acids and proteins. Hence, identifying discrete carbohydrate modifications and understanding their biological effects is essential. A study of such processes requires of a deep knowledge of the interaction mechanism at the molecular level. Here, we use a combination of laser spectroscopy in jets and quantum mechanical calculations to characterize the interaction between phenyl-ß-d-glucopyranoside and N-methylacetamide as a model to understand the interaction between a sugar and a peptide bond. The most stable structure of the molecular aggregate shows that the main interaction between the peptide fragment and the sugar proceeds via a C[double bond, length as m-dash]OH-O2 hydrogen bond. A second conformer was also found, in which the peptide establishes a C[double bond, length as m-dash]OH-O6 hydrogen bond with the hydroxymethyl substituent of the sugar unit. All the conformers present an additional interaction point with the aromatic ring. This particular preference of the peptide for the hydroxyl close to the aromatic ring could explain why glycogenin uses tyrosine in order to convert glucose into glycogen by exposing the O4H hydroxyl group for the other glucoses for the polymerization to take place.