Pierre Çarçabal

Sugars in the gas phase. Spectroscopy, conformation, hydration, co-operativity and selectivity

The functional importance of carbohydrates in biological processes, particularly those involving specific molecular recognition, is immense. Characterizing the three-dimensional structures of carbohydrates and glycoconjugates and their interactions with other molecules, particularly the ubiquitous solvent, water, are key starting points on the road towards the understanding of these processes. The review introduces a new strategy, combining electronic and vibrational spectroscopy of mass-selected carbohydrate molecules and their hydrated (and also protonated) complexes, conducted under molecular beam conditions, with ab initio computation. Its early successes have revealed a uniquely powerful means of characterizing carbohydrate conformations and hydrated structures, the hydrogen-bonded networks they support (or which support them) and the specificity of their interactions with other molecules. The new information, obtained in the gas phase, complements that provided by more ‘traditional’ condensed phase methods such as NMR, X-ray diffraction, molecular mechanics and molecular dynamics calculations. The review concludes with a vision of the challenges and opportunities offered by applications of molecular beam spectroscopy and their relevance in a biological context. Contents PAGE 1.  Preamble 490 2. Sweetness and light: Sugars in the gas phase 492 3. Experimental and computational strategies 495 4. The conformational landscapes of some key monosaccharides: glucose, galactose, mannose, fucose and xylose 498 4.1. Notation 498 4.2. Glucose, galactose and mannose 499 4.3. Fucose and xylose 503 5. Probing the glycosidic linkage: lactose and glycan ‘building blocks’ 504 5.1. Notation 506 5.2. Lactose 506 5.3. Mannose disaccharides 508 6. Adding water to sugar: hydrogen-bonding, co-operativity and selectivity 512 6.1. Notation 512 6.2. Mono-hydrated complexes: glucose, galactose and mannose 512 6.3. Co-operativity and conformational selectivity 516 6.4. Mono-hydrated complexes: xylose and fucose 519 6.5. Some concluding remarks 521 7. Using sugars: imino sugars and peptide mimics 522 7.1. Sugar mimics: imino sugars 522 7.2. Mimicking peptide secondary structure: carbopeptoids 524 8. Challenges and opportunities 527 Acknowledgements 529 References 530

Sensing the anomeric effect in a solvent-free environment

The anomeric effect is a chemical phenomenon that refers to an observed stabilization of six-membered carbohydrate rings when they contain an electronegative substituent at the C1 position of the ring. This stereoelectronic effect influences the three-dimensional shapes of many biological molecules. It can be manifested not only in this classical manner involving interaction of the endocyclic oxygen atom (O5) found in such sugars with the C1 substituent (endo-anomeric effect) but also through a corresponding interaction of the electronegative exocyclic substituent with O5 (exo-anomeric effect). However, the underlying physical origin(s) of this phenomenon is still not clear. Here we show, using a combination of laser spectroscopy and computational analysis, that a truncated peptide motif can engage the two anomers of an isolated sugar in the gas phase, an environment lacking extraneous factors which could confound the analysis. (Anomers are isomers that differ in the orientation of the substituent at C1.) Complexes formed between the peptide and the α- or β-anomers of d-galactose are nearly identical structurally; however, the strength of the polarization of their interactions with the peptide differs greatly. Natural bond order calculations support this observation, and together they reveal the dominance of the exo- over the endo-anomeric effect. As interactions between oxygen atoms at positions C1 and C2 (O1 and O2, respectively) on the pyranose ring can alter the exo/endo ratio of a carbohydrate, our results suggest that it will be important to re-evaluate the influence, and biological effects, of substituents at position C2 in sugars.

Comprehensive characterization of the photodissociation pathways of protonated tryptophan

The photofragmentation of protonated tryptophan has been investigated in a unique experimental setup, in which ion and neutral issued from the photofragmentation are detected in coincidence, in time and in position. From these data are extracted the kinetic energy, the number of neutral fragments associated with an ion, their masses, and the order of the fragmentation steps. Moreover, the fragmentation time scale ranging from tens of nanoseconds to milliseconds is obtained. From all these data, a comprehensive fragmentation mechanism is proposed.

Spectral signatures and structural motifs in isolated and hydrated monosaccharides: phenyl α- and β-L-fucopyranoside

The conformation and structure of phenyl-α-L-fucopyranoside (α-PhFuc), phenyl-β-L-fucopyranoside (β-PhFuc) and their singly hydrated complexes (α,β-PhFuc·H2O) isolated in a molecular beam, have been investigated by means of resonant two photon ionization (R2PI) spectroscopy and ultraviolet and infrared ion-dip spectroscopy. Conformational and structural assignments have been based on comparisons between their experimental and computed near IR spectra, calculated using density functional theory (DFT) and their relative energies, determined from ab initio (MP2) calculations. The near IR spectra of ‘free’ and hydrated α- and β-PhFuc, and many other mono- and di-saccharides, provide extremely sensitive probes of hydrogen-bonded interactions which can be finely tuned by small (or large) changes in the molecular conformation. They provide characteristic ‘signatures’ which reflect anomeric, or axial vs. equatorial differences, both revealed through comparisons between α/β-PhFuc and α/β-PhXyl; or similarities, revealed through comparisons between fucose (6-deoxy galactose) and galactose; or binding motifs, for example, ‘insertion’ vs. ‘addition’ structures in hydrated complexes. At the monosaccharide level (the first step in the carbohydrate hierarchy), these trends appear to be general. In contrast to the monohydrates of galactose (β-PhGal) and glucose (β-PhGlc), the conformations of α- and β-PhFuc are unaffected by the binding of a single water molecule though changes in the R2PI spectra of multiply hydrated α-PhFucW(n) however, may reflect a conformational transformation when n ≥ 3.

Neurotransmitters in the gas phase: hydrated noradrenaline

The conformational and molecular structures of singly hydrated noradrenaline complexes have been explored through a combination of electronic structure computation (at the B3LYP/6-31+G*, MP2/6-31+G* and MP2/aug-cc-pVDZ levels of theory) and mass-selected ultraviolet and infrared ion-dip spectroscopy following laser ablation of the neurotransmitter into a freely expanding moist argon jet. Under these conditions, almost all the hydrated complexes are located in the global minimum energy configuration, associated with an extended, AG1a, ethanolamine side-chain conformation; the water molecule, which is located slightly above the plane of the catechol ring, is bound primarily as a proton acceptor to the m-OH substituent, and only weakly, as a proton donor, to the hydroxyl group on the side chain.

A computational and spectroscopic study of the gas-phase conformers of adrenaline

The conformational landscapes of the neurotransmitter l-adrenaline (l-epinephrine) and its diastereoisomer pseudo-adrenaline, isolated in the gas phase and un-protonated, have been investigated by using a combination of mass-selected ultraviolet and infrared holeburn spectroscopy, following laser desorption of the sample into a pulsed supersonic argon jet, and DFT and ab initio computation (at the B3LYP/6-31+G*, MP2/6-31+G* and MP2/aug-cc-pVDZ levels of theory). Both for adrenaline and its diastereoisomer, pseudo-adrenaline, one dominant molecular conformation, very similar to the one seen in noradrenaline, has been observed. It could be assigned to an extended side-chain structure (AG1a) stabilized by an OH → N intramolecular hydrogen bond. An intramolecular hydrogen bond is also formed between the neighbouring hydroxyl groups on the catechol ring. The presence of further conformers for both diastereoisomers could not be excluded, but overlapping electronic spectra and low ion signals prevented further assignments.

Biradicalic excited states of zwitterionic phenol-ammonia clusters.

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.

‘Naked’ and Hydrated Conformers of the Conserved Core Pentasaccharide of N-linked Glycoproteins and Its Building Blocks

N-glycosylation of eukaryotic proteins is widespread and vital to survival. The pentasaccharide unit −Man3GlcNAc2– lies at the protein-junction core of all oligosaccharides attached to asparagine side chains during this process. Although its absolute conservation implies an indispensable role, associated perhaps with its structure, its unbiased conformation and the potential modulating role of solvation are unknown; both have now been explored through a combination of synthesis, laser spectroscopy, and computation. The proximal −GlcNAc-GlcNAc– unit acts as a rigid rod, while the central, and unusual, −Man-β-1,4-GlcNAc– linkage is more flexible and is modulated by the distal Man-α-1,3– and Man-α-1,6– branching units. Solvation stiffens the ‘rod’ but leaves the distal residues flexible, through a β-Man pivot, ensuring anchored projection from the protein shell while allowing flexible interaction of the distal portion of N-glycosylation with bulk water and biomolecular assemblies.

Infrared spectra of the C2H2–HCl complexes: An experimental and ab initio study

By means of a pulsed slit jet and an infrared tunable diode laser spectrometer, the vibration–rotation absorption spectra of the complexes C2H2–H35Cl and C2H2–H37Cl have been observed for the first time in the 3.6 μm region of the ν1 band correlated with the HCl stretch. All the lines of the spectrum have been assigned for J=0 to 18 and Ka=0, 1, 2, 3. To determine the band origin and the rotational and centrifugal constants, the observed line frequencies have been fitted to those determined by the Watson Hamiltonian in the A reduction. A force constant model has been used to derive the binding energy De of the complex and the intermolecular stretching harmonic frequency from the experimental spectroscopic constants. The available experimental results concerning these complexes and other isotopic forms D35Cl and D37Cl were compared with ab initio calculations performed at the coupled-cluster single double triple [CCSD(T)] level of theory. The comparison turned out to be very good for all the properties con...

Binding energies of micro-hydrated carbohydrates: measurements and interpretation

The strength of the interaction between three monosaccharides (O-phenyl-β-D-gluco-, β-D-galacto- and α-D-mannopyranoside) and a single water molecule has been investigated experimentally in the gas phase by means of 2-colour UV-UV ionisation and dissociation threshold measurements. Their binding energies have also been calculated using dispersion corrected DFT methods and the resolution of identity approximation. The calculated and experimental relative binding energies are in good correspondence at all considered levels of theory, and the RI-B97D+disp/TZVPP level of theory in particular, provides very good agreement with a considerable reduction in computational time. Although these systems experience some conformational changes upon photo-ionisation, the experimental measurements lead to reliable estimates of the binding energies of the different conformers of the monosaccharide–water complexes and their relative values reflect their structural differences.