Hydrogen bond

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

Intramolecular hydrogen bonding plays a crucial role in the photophysics and photochemistry of the GFP chromophore.

Abstract: In commonly studied GFP chromophore analogues such as 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (PHBDI), the dominant photoinduced processes are cis-trans isomerization and subsequent S-1 -> S-0 decay via a conical intersection characterized by a highly twisted double bond. The recently synthesized 2-hydroxy-substituted isomer (OHBDI) shows an entirely different photochemical behavior experimentally, since it mainly undergoes ultrafast intramolecular excited-state proton transfer, followed by S-1 -> S-0 decay and ground-state reverse hydrogen transfer. We have chosen 4-(2-hydroxybenzylidene)-1H-imidazol-5(4H)-one (OHBI) to model the gas-phase photodynamics of such 2-hydroxy-substituted chromophores. We first use various electronic structure methods (DFT, TDDFT, CC2, DFT/MRCI, OM2/MRCI) to explore the S-0 and S-1 potential energy surfaces of OHBI and to locate the relevant minima, transition state, and minimum-energy conical intersection. These static calculations suggest the following decay mechanism: upon photoexcitation to the S-1 state, an ultrafast adiabatic charge-transfer induced excited-state intramolecular proton transfer (ESIPT) occurs that leads to the S-1 minimum-energy structure. Nearby, there is a S-1/S-0 minimum-energy conical intersection that allows for an efficient nonadiabatic S-1 -> S-0 internal conversion, which is followed by a fast ground-state reverse hydrogen transfer (GSHT). This mechanism is verified by semiempirical OM2/MRCI surface-hopping dynamics simulations, in which the successive ESIPT-GSTH processes are observed, but without cis-trans isomerization (which is a minor path experimentally with less than 5% yield). These gas-phase simulations of OHM give an estimated first-order decay time of 476 Is for the S-1 state, which is larger but of the same order as the experimental values measured for OHBDI in solution: 270 Is in CH3CN and 230 fs in CH2Cl2. The differences between the photoinduced processes of the 2- and 4-hydroxy-substituted chromophores are attributed to the presence or absence of intramolecular hydrogen bonding between the two rings.

Multidimensional infrared spectroscopy of water. II. Hydrogen bond switching dynamics

Abstract: We use multidimensional infrared spectroscopy of the OH stretch of HOD in D2O to measure the interconversion of different hydrogen bonding environments The OH stretching frequency distinguishes hydrogen bonded (HB) and non-hydrogen-bonded (NHB) configurations by their absorption on the low (red) and high (blue) sides of the line shape Measured asymmetries in the two dimensional infrared OH line shapes are manifestations of the fundamentally different spectral relaxations of HB and NHB HB oscillators exhibit coherent oscillations within the hydrogen-bonded free energy well before undergoing activated barrier crossing, resulting in the exchange of hydrogen bonded partners Conversely, NHB oscillators rapidly return to HB frequencies within 150 fs These results support a picture where NHB configurations are only visited transiently during large fluctuations about a hydrogen bond or during the switching of hydrogen bonding partners The results are not consistent with the presence of entropically stabilized dangling hydrogen bonds or a conceptual picture of water as a mixture of environments with varying hydrogen bond strength separated by barriers >kT

V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose).

Abstract: The amylose fraction of starch occurs in double-helical A- and B-amyloses and the single-helical V-amylose. The latter contains a channel-like central cavity that is able to include molecules, “iodine’s blue” being the best-known representative. Molecular models of these amylose forms have been deduced by solid state 13C cross-polarization/magic angle spinning NMR and by x-ray fiber and electron diffraction combined with computer-aided modeling. They remain uncertain, however, as no structure at atomic resolution is available. We report here the crystal structure of a hydrated cycloamylose containing 26 glucose residues (cyclomaltohexaicosaose, CA26), which has been determined by real/reciprocal space recycling starting from randomly positioned atoms or from an oriented diglucose fragment. This structure provides conclusive evidence for the structure of V-amylose, as the macrocycle of CA26 is folded into two short left-handed V-amylose helices in antiparallel arrangement and related by twofold rotational pseudosymmetry. In the V-helices, all glucose residues are in syn orientation, forming systematic interglucose O(3)n⋅⋅⋅O(2)n+l and O(6)n⋅⋅⋅O(2)n+6/O(3)n+6 hydrogen bonds; the central cavities of the V-helices are filled by disordered water molecules. The folding of the CA26 macrocycle is characterized by typical “band-flips” in which diametrically opposed glucose residues are in anti rather than in the common syn orientation, this conformation being stabilized by interglucose three-center hydrogen bonds with O(3)n as donor and O(5)n+l, O(6)n+l as acceptors. The structure of CA26 permitted construction of an idealized V-amylose helix, and the band-flip motif explains why V-amylose crystallizes readily and may be packed tightly in seeds.