Intermolecular Clamping by Hydrogen Bonds: 2-Pyridone center dot NH3

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH3 has been performed. The S-1 <- S-0 spectrum extends up to approximate to 1200 cm(-1) above the 0(0)(0) band, close to twice the range observed for 2PY. The S-1 state nonradiative decay for vibrations above approximate to 300 cm(-1) in the NH3 complex is dramatically slowed down relative to bare 2PY. Also, the Delta v=2,4,... overtone bands of the v(1)' and v(2)' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY center dot NH3, which implies that the bridging (2PY)NH center dot center dot center dot NH3 and H2NH center dot center dot center dot O=C H-bonds clamp the 2PY at a planar geometry in the S-1 state. The mass-resolved UV vibronic spectra of jet-cooled 2PY center dot NH3 and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S-0 and S-1 equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S-1 <- S-0 vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C-S symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H2NH center dot center dot center dot O=C H-bond distance is approximate to 0.7 angstrom shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H2NH center dot center dot center dot O distance to increase by 0.2 angstrom upon S-1 <- S-0 electronic excitation, while the (2PY)NH center dot center dot center dot NH3 H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

N-H center dot center dot center dot pi hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole-benzene

The N-H center dot center dot center dot pi hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr center dot Bz, Pyr center dot Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H center dot center dot center dot pi hydrogen bond to the benzene ring. The pyrrole is tipped by omega(S(0)) = +/- 13 degrees relative to the surface normal of Bz. The N center dot center dot center dot ring distance is 3.13 angstrom. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle omega(S(1)) = +/- 21 degrees. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H center dot center dot center dot pi interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1) <- S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr center dot Bz in the same region exhibits a weak 0(0)(0) band that is red-shifted by 58 cm(-1) relative to that of Bz (38 086 cm(-1)). The origin appears due to symmetry-breaking of the p-electron system of Bz by the asymmetric pyrrole NH center dot center dot center dot pi hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0(0)(0) band. The Bz moiety in Pyr center dot Bz exhibits a 6a(0)(1) band at 0(0)(0) + 518 cm(-1) that is about 20x more intense than the origin band. The symmetry breaking by the NH center dot center dot center dot pi hydrogen bond splits the degeneracy of the v(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by similar to 6 cm(-1). Both the 0(0)(0) and 6(0)(1) bands of Pyr center dot Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration omega', in agreement with the change of the omega tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.

Rifaximin treatment for the irritable bowel syndrome with a positive lactulose hydrogen breath test improves symptoms for at least 3 months

While rifaximin was able to improve symptoms in patients with irritable bowel syndrome (IBS) in phase III trials, these results are yet to be repeated in phase IV studies.

DEVELOPMENT OF POROUS CONCRETE GEOPOLYMER USING HYDROGEN PEROXIDE AS BLOWING AGENT.

Metakaolin and volcanic ashes respectively called MK and ZG were used as aluminosilicate raw materials for the synthesis of porous geopolymers. The hydrogen peroxide was used as a blowing agent. The geopolymer prepared were characterized by Spectroscopy Fourier Transform Infrared (SFTI), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and the determination of some physical properties such as bulk density, total porosity, volume shrinkage, compressive strength and thermal conductivity was also carried out. The presence of pores in the geopolymer obtained is function of the percentage of hydrogen peroxide added, causing the concomitantly decrease in compressive strength, apparent density and thermal conductivity. The use of hydrogen peroxide as a blowing agent gave some thermal insulation properties to these geopolymers which could be close to the properties of some commercial insulation materials.

Self-adjoint extensions for confined electrons: From a particle in a spherical cavity to the hydrogen atom in a sphere and on a cone

In a recent study of the self-adjoint extensions of the Hamiltonian of a particle confined to a finite region of space, in which we generalized the Heisenberg uncertainty relation to a finite volume, we encountered bound states localized at the wall of the cavity. In this paper, we study this situation in detail both for a free particle and for a hydrogen atom centered in a spherical cavity. For appropriate values of the self-adjoint extension parameter, the bound states localized at the wall resonate with the standard hydrogen bound states. We also examine the accidental symmetry generated by the Runge–Lenz vector, which is explicitly broken in a spherical cavity with general Robin boundary conditions. However, for specific radii of the confining sphere, a remnant of the accidental symmetry persists. The same is true for an electron moving on the surface of a finite circular cone, bound to its tip by a 1/r1/r potential.

Sodium/hydrogen exchanger NHA2 is critical for insulin secretion in β-cells

NHA2 is a sodium/hydrogen exchanger with unknown physiological function. Here we show that NHA2 is present in rodent and human β-cells, as well as β-cell lines. In vivo, two different strains of NHA2-deficient mice displayed a pathological glucose tolerance with impaired insulin secretion but normal peripheral insulin sensitivity. In vitro, islets of NHA2-deficient and heterozygous mice, NHA2-depleted Min6 cells, or islets treated with an NHA2 inhibitor exhibited reduced sulfonylurea- and secretagogue-induced insulin secretion. The secretory deficit could be rescued by overexpression of a wild-type, but not a functionally dead, NHA2 transporter. NHA2 deficiency did not affect insulin synthesis or maturation and had no impact on basal or glucose-induced intracellular Ca(2+) homeostasis in islets. Subcellular fractionation and imaging studies demonstrated that NHA2 resides in transferrin-positive endosomes and synaptic-like microvesicles but not in insulin-containing large dense core vesicles in β-cells. Loss of NHA2 inhibited clathrin-dependent, but not clathrin-independent, endocytosis in Min6 and primary β-cells, suggesting defective endo-exocytosis coupling as the underlying mechanism for the secretory deficit. Collectively, our in vitro and in vivo studies reveal the sodium/proton exchanger NHA2 as a critical player for insulin secretion in the β-cell. In addition, our study sheds light on the biological function of a member of this recently cloned family of transporters.