Francisco J. Basterretxea

Free Fructose Is Conformationally Locked

Fructose has been examined under isolation conditions using a combination of UV ultrafast laser vaporization and Fourier-transform microwave (FT-MW) spectroscopy. The rotational spectra for the parent, all (six) monosubstituted (13)C species, and two single D species reveal unambiguously that the free hexoketose is conformationally locked in a single dominant β-pyranose structure. This six-membered-chair skeleton adopts a (2)C(5) configuration (equivalent to (1)C(4) in aldoses). The free-molecule structure sharply contrasts with the furanose form observed in biochemically relevant polysaccharides, like sucrose. The structure of free fructose has been determined experimentally using substitution and effective structures. The enhanced stability of the observed conformation is primarily attributed to a cooperative network of five intramolecular O-H···O hydrogen bonds and stabilization of both endo and exo anomeric effects. Breaking a single intramolecular hydrogen bond destabilizes the free molecule by more than 10 kJ mol(-1). The structural results are compared to ribose, recently examined with rotational resolution, where six different conformations coexist with similar conformational energies. In addition, several DFT and ab initio methods and basis sets are benchmarked with the experimental data.

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

Hygroscopic properties of internally mixed particles of ammonium sulfate and succinic acid studied by infrared spectroscopy.

Atmospheric aerosols typically consist of inorganic and organic material. Although the organic fraction can affect the behavior of mixed organic/inorganic particles, their physical properties are not well-understood. In this work, infrared spectra of internally mixed submicrometer particles of ammonium sulfate and succinic acid have been studied at ambient temperature in an aerosol flow cell. The spectra of dried particles show distinct features relative to the pure components, as a result of ion-molecule interactions between the inorganic and organic species. The hygroscopic behavior of the particles has been followed for several organic/inorganic mass ratios, showing that around equimolar composition, the mixed particles uptake water in a broad range of relative humidities (30-80%), substantially lower than the deliquescence relative humidity of the mixed system near 80%. Infrared spectra at predeliquescence relative humidities reveal that succinic acid is partially or completely in the liquid phase at much higher concentrations that those corresponding to a saturated solution of succinic acid. This behavior is proposed to arise from the ion-molecule interactions between the organic and inorganic components, which unstabilize the crystal structure relative to the pure solids and cause loss of translational order in the crystal, bringing about an increase in the Gibbs energy of the solid particles and allowing the uptake of water molecules prior to the deliquescence point. The obtained results show that water absorption prior to full deliquescence in this system has to be taken into account because it extends the range of relative humidities at which particles are partially or completely liquid.

Discriminating the structure of exo-2-aminonorbornane using nuclear quadrupole coupling interactions.

The intrinsic conformational and structural properties of the bicycle exo-2-aminonorbornane have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy and quantum chemical calculations. The rotational spectrum revealed two different conformers arising from the internal rotation of the amino group, exhibiting small (MHz) hyperfine patterns originated by the (14)N nuclear quadrupole coupling interaction. Complementary ab initio (MP2) and DFT (B3LYP and M05-2X) calculations provided comparative predictions for the structural properties, rotational and centrifugal distortion data, hyperfine parameters, and isomerization barriers. Due to the similarity of the rotational constants, the structural assignment of the observed rotamers and the calculation of the torsion angles of the amino group were based on the conformational dependence of the (14)N nuclear quadrupole coupling hyperfine tensor. In the most stable conformation (ss), the two amino N-H bonds are staggered with respect to the adjacent C-H bond. In the second conformer (st), only one of the N-H bonds is staggered and the other is trans. A third predicted conformer (ts) was not detected, consistent with a predicted conformational relaxation to conformer ss through a low barrier of 5.2 kJ mol(-1).

Infrared Spectroscopic Properties of Sodium Bromide Aerosols

The infrared extinction spectra of aqueous NaBr aerosols at ambient temperature have been measured as a function of relative humidity. Submicron-sized aerosol particles atomized from aqueous NaBr solutions at various concentrations are dried and/or mixed with nitrogen at different humidities and spectroscopically monitored as they flow through an infrared absorption cell. Estimated dry particle median diameters range from 0.24 to 0.15 microm, as calculated from Mie extinction theory. Measured deliquescence and efflorescence relative humidities (35-40% and 25-30%, respectively) are in accordance with previously reported ones. Our results show that NaBr particles take up water only moderately over the deliquescence point, with a significant increase at relative humidities above 70%. The effect of particle size onto water uptake properties has been studied, indicating that smaller particles take up lower amounts of water, and only increase their size significantly at relative humidities near saturation. Particle composition and diameter growth factors have been calculated from spectral data and are shown to be consistent with those predicted from thermodynamic data and Kohler theory. Band centers of liquid water in NaBr aerosols relative to pure water are blue-shifted up to 50 cm-1 at low humidities. Particle structure and phase, together with atmospheric implications, are also discussed.

The Distorted Tropane of Scopoline

The structural isomerization of scopine into scopoline (oscine) has been observed in a supersonic jet expansion using microwave spectroscopy. The rotational spectrum evidences a single structure in the gas phase, providing a first description of the (three-ring) structurally distorted tropane in scopoline. The absence of rotational signatures of any scopine conformation suggests a practically quantitative isomerization at the vaporization temperatures of the experiment (ca. 90 °C). The determined rotational parameters of scopoline reveal the structural consequences of the intramolecular cyclation of scopine, which breaks the original epoxy group and creates a new ether bridge and a 7β-hydroxytropane configuration. The hydroxy group further stabilizes the molecule by an O-H⋅⋅⋅N intramolecular hydrogen bond, which, in turn, forces the N-methyl group to the less stable axial form. Supporting ab initio (MP2) and DFT (B3LYP, M06-2X) calculations are included.

Single Hydration of the Peptide Bond: The Case of the Vince Lactam

2-Azabicyclo[2.2.1]hept-5-en-3-one (ABH or Vince lactam) and its monohydrated complex (ABH···H(2)O) have been observed in a supersonic jet by Fourier transform microwave spectroscopy. ABH is broadly used in the synthesis of therapeutic drugs, whereas the ABH···H(2)O system offers a simple model to explain the conformational preferences of water linked to a constrained peptidic bond. A single predominant form of the Vince lactam and its singly hydrated complex have been detected, determining the rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling tensor. The monohydrated complex is stabilized by two hydrogen bonds (C═O···H-O and N-H···O) closing a six-membered ring. The complexation energy has been estimated to be ∼10 kJ mol(-1) from experimental results. In addition, the observed structure in the gas phase has been compared with solid-phase diffraction data. The structural parameters and binding energies of ABH···H(2)O have also been compared with similar molecules containing peptide bonds. Ab initio (MP2) and density functional (M06-2X and B3LYP) methods have supported the experimental work, describing the rotational parameters and conformational landscape of the title compound and its singly hydrated complex.

Infrared laser absorption spectroscopy of the v19a band of jet-cooled fluorobenzene

The diode laser absorption spectrum of the v19a band of fluorobenzene molecule near 1500 cm−1 has been measured in a pulsed supersonic jet and transitions from low J, Ka, and Kc. values (2–24) are reported. The band centre has been found at and rotational constants for the upper state as determined from a least-squares fitting to experimental data have been found to be A19a, = 0.188 77(5), B19a = 0.08569(5) and C19a = 0.058 94(1) cm−1. In addition, a number of weak transitions assigned to δKa = ±2, δKc = ±1. ±3 have been observed, with higher intensities than those expected in the rigid rotor model. A discussion of their origin is presented.

THE CM-, MM-, AND SUB-MM-WAVE SPECTRUM OF ALLYL ISOCYANIDE AND RADIOASTRONOMICAL OBSERVATIONS IN ORION KL AND THE SgrB2 LINE SURVEYS

Organic isocyanides have an interesting astrochemistry and some of these molecules have been detected in the interstellar medium (ISM). However, rotational spectral data for this class of compounds are still scarce. We provide laboratory spectra of the four-carbon allyl isocyanide covering the full microwave region, thus allowing a potential astrophysical identification in the ISM. We assigned the rotational spectrum of the two cis (synperiplanar) and gauche (anticlinal) conformations of allyl isocyanide in the centimeter-wave region (4-18 GHz), resolved its 14N nuclear quadrupole coupling (NQC) hyperfine structure, and extended the measurements into the millimeter and submillimeter-wave (150-900 GHz) ranges for the title compound. Rotational constants for all the monosubstituted 13C and 15N isotopologues are additionally provided. Laboratory observations are supplemented with initial radioastronomical observations. Following analysis of an extensive dataset (>11000 rotational transitions), accurate ground-state molecular parameters are reported for the cis and gauche conformations of the molecule, including rotational constants, NQC parameters, and centrifugal distortion terms up to octic contributions. Molecular parameters have also been obtained for the two first excited states of the cis conformation, with a dataset of more than 3300 lines. The isotopic data allowed determining substitution and effective structures for the title compound. We did not detect allyl isocyanide either in the IRAM 30 m line survey of Orion KL or in the PRIMOS survey toward SgrB2. Nevertheless, we provided an upper limit to its column density in Orion KL.

In Vitro Surfactant and Perfluorocarbon Aerosol Deposition in a Neonatal Physical Model of the Upper Conducting Airways

Objective Aerosol delivery holds potential to release surfactant or perfluorocarbon (PFC) to the lungs of neonates with respiratory distress syndrome with minimal airway manipulation. Nevertheless, lung deposition in neonates tends to be very low due to extremely low lung volumes, narrow airways and high respiratory rates. In the present study, the feasibility of enhancing lung deposition by intracorporeal delivery of aerosols was investigated using a physical model of neonatal conducting airways. Methods The main characteristics of the surfactant and PFC aerosols produced by a nebulization system, including the distal air pressure and air flow rate, liquid flow rate and mass median aerodynamic diameter (MMAD), were measured at different driving pressures (4–7 bar). Then, a three-dimensional model of the upper conducting airways of a neonate was manufactured by rapid prototyping and a deposition study was conducted. Results The nebulization system produced relatively large amounts of aerosol ranging between 0.3±0.0 ml/min for surfactant at a driving pressure of 4 bar, and 2.0±0.1 ml/min for distilled water (H2Od) at 6 bar, with MMADs between 2.61±0.1 µm for PFD at 7 bar and 10.18±0.4 µm for FC-75 at 6 bar. The deposition study showed that for surfactant and H2Od aerosols, the highest percentage of the aerosolized mass (∼65%) was collected beyond the third generation of branching in the airway model. The use of this delivery system in combination with continuous positive airway pressure set at 5 cmH2O only increased total airway pressure by 1.59 cmH2O at the highest driving pressure (7 bar). Conclusion This aerosol generating system has the potential to deliver relatively large amounts of surfactant and PFC beyond the third generation of branching in a neonatal airway model with minimal alteration of pre-set respiratory support.