Hydrogen bond

About: Hydrogen bond is a research topic. Over the lifetime, 57701 publications have been published within this topic receiving 1306326 citations.

n → π * interactions in proteins

Abstract: Hydrogen bonds between backbone amides are common in folded proteins. Here, we show that an intimate interaction between backbone amides likewise arises from the delocalization of a lone pair of electrons (n) from an oxygen atom to the antibonding orbital (π*) of the subsequent carbonyl group. Natural bond orbital analysis predicted significant n→π* interactions in certain regions of the Ramachandran plot. These predictions were validated by a statistical analysis of a large, non-redundant subset of protein structures determined to high resolution. The correlation between these two independent studies is striking. Moreover, the n→π* interactions are abundant, and especially prevalent in common secondary structures such as α-, 310-, and polyproline II helices, and twisted β-sheets. In addition to their evident effects on protein structure and stability, n→π* interactions could play important roles in protein folding and function, and merit inclusion in computational force fields.

Refinement of the structure of beta-chitin.

Abstract: The structure of β-chitin has been refined by rigid-body least-squares methods, based on the intensity data for highly crystalline specimens from the pogonophore Oligobrachia ivanovi. The structure consists of an array of poly-N-acetyl-D-glucosamine chains all having the same sense, which are linked together in sheets by NH … OC hydrogen bonding of the amide groups. In addition to the O-3′H … O-5 intramolecular hydrogen bond, analogous to that in cellulose, the CH2OH side chain forms an intrasheet hydrogen bond to the carbonyl oxygen on the next chain. This structure shows considerably better agreement between observed and calculated intensities than that possessing an intersheet hydrogen bond, as had been proposed previously. The structure is consistent with the swelling properties of β-chitin and can also be seen to be analogous to that of native cellulose.

Nature and physical origin of CH/π interaction: significant difference from conventional hydrogen bonds

Abstract: Recently reported high-level ab initio calculations and gas phase spectroscopic measurements show that the nature of CH/π interactions is considerably different from conventional hydrogen bonds, although the CH/π interactions were often regarded as the weakest class of hydrogen bonds. The major source of attraction in the CH/π interaction is the dispersion interaction and the electrostatic contribution is small, while the electrostatic interaction is mainly responsible for the attraction in the conventional hydrogen bonds. The nature of the “typical” CH/π interactions is similar to that of van der Waals interactions, if some exceptional “activated” CH/π interactions of highly acidic C–H bonds are excluded. Shifts of C–H vibrational frequencies and electronic spectra also support the similarity. The hydrogen bond is important in controlling structures of molecular assemblies, since the hydrogen bond is sufficiently strong and directional due to the large electrostatic contribution. On the other hand, the directionality of the “typical” CH/π interaction is very weak. Although the “typical” CH/π interaction is often regarded as an important interaction in controlling the structures of molecular assemblies as in the cases of conventional hydrogen bonds, the importance of the “typical” CH/π interactions is questionable.

Crystal engineering: from weak hydrogen bonds to co-ordination bonds

Abstract: This article highlights the importance of crystal engineering in designing functional materials. Various rational design strategies will be discussed for controlling the molecular architectures of the materials using C–H⋯O, C–H⋯π, O–H⋯O, N–H⋯O and O–H⋯N hydrogen bonds and co-ordination bonds. The results described here show the role of guest molecules in templating the isomeric network structures with one set of molecular components. In particular, the materials that are designed based on strong hydrogen bonds and coordination bonds exhibited zeolite like and clay like properties.

Water catalysis of a radical-molecule gas-phase reaction.

Abstract: There has been considerable speculation about the role of water and water complexes in chemical gas-phase reactions, including the conjecture that water may act as a molecular catalyst through its ability to form hydrogen bonds. Here, we present kinetic studies in which the effect of water on the rate of the reaction between hydroxyl radicals and acetaldehyde has been measured directly in Laval nozzle expansions at low temperatures. An increasing enhancement of the reaction rate by added water was found with decreasing temperatures between 300 and 60 kelvin. Quantum chemical calculations and statistical rate theory support our conclusions that this observation is due to the reduction of an intrinsic reaction barrier caused by specific water aggregation. The results suggest that even single water molecules can act as catalysts in radical-molecule reactions.