Laser ablation in liquids : Applications in the synthesis of nanocrystals

1.1. Historical Background Hydrogen bonds1,2 are one of the principal intermolecular forces. A special class are ionic hydrogen bonds (IHBs) that form between ions and molecules with bonds strengths of 5-35 kcal/mol, up to a third of the strength of covalent bonds. These strong interactions are critical, for example, in ionic clusters and nucleation, in electrolytes, ion solvation, and acid-base chemistry, in the structures of ionic crystals, surfaces, silicates, and clays, in surface adsorption, and in self-assembly in supramolecular chemistry and molecular crystals.3 IHBs are also important in bioenergetics including protein folding, enzyme active centers, formation of membranes and proton transport, and biomolecular recognition. With such wide-ranging roles, the fundamental properties of IHB interactions need to be understood. The energetics of IHB interactions cannot be isolated and quantified in the condensed phase. However, these interactions can be isolated and studied quantitatively in gas phase. These studies lead to a fundamental understanding of relations between IHB bond strengths and molecular structure, the solvation of ions, especially in the critical inner shells, and acid-based phenomena and bioenergetics. This review will present the basic insights that have been obtained in the past four decades. It will present a comprehensive review of the thermochemistry and its structural implications obtained from ab initio calculations. Relevant recent results from spectroscopy will be also illustrated. The advent of variable temperature high-pressure mass spectrometry (HPMS) introduced by Field and co-workers in the 1960s4 and pulsed high-pressure mass spectrometry (PHPMS) introduced by Kebarle in the late 1960s5 have been particularly significant. These workers realized that at pressures of several torrs and gas densities of 1016-1017 cm-3 in ion sources, ions can undergo 103-106 collisions with neutral molecules during typical residence times of 0.1-10 ms and establish thermal ion populations and * E-mail: m.mautner@solis1.com. 213 Chem. Rev. 2005, 105, 213−284

http://www.astro-ecology.com/PDFTheIonicHydrogenBondChemicalReviews2005Paper.pdf

The Ionic Hydrogen Bond

Photochemistry at adsorbate/metal interfaces

Current views of the role of beta-amyloid (Abeta) peptide fibrils range from regarding them as the cause of Alzheimer's pathology to having a protective function. In the last few years, it has also been suggested that soluble oligomers might be the most important toxic species. In all cases, the study of the conformational properties of Abeta peptides in soluble form constitutes a basic approach to the design of molecules with "antiamyloid" activity. We have experimentally investigated the conformational path that can lead the Abeta-(1-42) peptide from the native state, which is represented by an alpha helix embedded in the membrane, to the final state in the amyloid fibrils, which is characterized by beta-sheet structures. The conformational steps were monitored by using CD and NMR spectroscopy in media of varying polarities. This was achieved by changing the composition of water and hexafluoroisopropanol (HFIP). In the presence of HFIP, beta conformations can be observed in solutions that have very high water content (up to 99 % water; v/v). These can be turned back to alpha helices simply by adding the appropriate amount of HFIP. The transition of Abeta-(1-42) from alpha to beta conformations occurs when the amount of water is higher than 80 % (v/v). The NMR structure solved in HFIP/H2O with high water content showed that, on going from very apolar to polar environments, the long N-terminal helix is essentially retained, whereas the shorter C-terminal helix is lost. The complete conformational path was investigated in detail with the aid of molecular-dynamics simulations in explicit solvent, which led to the localization of residues that might seed beta conformations. The structures obtained might help to find regions that are more affected by environmental conditions in vivo. This could in turn aid the design of molecules able to inhibit fibril deposition or revert oligomerization processes.

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The α-to-β Conformational Transition of Alzheimer's Aβ-(1–42) Peptide in Aqueous Media is Reversible: A Step by Step Conformational Analysis Suggests the Location of β Conformation Seeding

Among various alcohols, those substituted with fluorine, such as 2,2,2-trifluoroethanol (TFE) or 3,3,3,3‘,3‘,3‘-hexafluoro-2-propanol (HFIP), have a marked potential to induce the formation of α-helical structures in peptides and to denature the native structures of proteins. However, the mechanism by which these alcohols exert their effects is unknown. Melittin, a bee venom peptide, is unfolded in the absence of alcohol, but is transformed to an α-helical structure upon addition of alcohols. On the other hand, addition of alcohols to β-lactoglobulin, a predominantly β-sheet protein, denatures the molecule and transforms it to an α-helical structure. We examined the role of several factors in these alcohol-induced transitions, i.e., relative dielectric constant, strength of hydrogen bond estimated by the pH titration of salicylic acid, and clustering of alcohol molecules measured by solution X-ray scattering. Although relative dielectric constant and hydrogen bond strength were confirmed to be important, ...

Clustering of Fluorine-Substituted Alcohols as a Factor Responsible for Their Marked Effects on Proteins and Peptides

Hydrogen-Bonded Cluster Formation and Hydrophobic Solute Association in Aqueous Solutions of Ethanol

Molecular association in ethanol-water mixtures studied by mass spectrometric analysis of clusters generated through adiabatic expansion of liquid jets

The structure of clusters in ethanol–water binary solutions at xE (ethanol mole fraction) ≥ 0.2 was investigated by the mass-spectrometric analysis of clusters isolated from liquid droplets and X-ray diffraction measurements of the intact solution. The average number of water molecules (Na) of the ethanol m-mer hydrates (5 < m < 15) in the mixtures at 0.2 ≤ xE ≤ 0.8 was found to be expressed by a function of the water mole fraction (xw), Na = {(m/4.2) − 1}2 (xw + 0.85), suggesting that the fundamental structure of ethanol polymer-hydrates is invariable in this region. An inflection point of Na was found at xE ≈ 0.2. The radial distribution functions (RDFs) from X-ray measurements demonstrated a decrease of linear hydrogen bonds at 2.8 A with increasing xE, while keeping the strongest peak at 4.8 A for 0.4 ≤ xE ≤ 1.0. On the basis of a composition analysis of the mass spectra and the concentration dependence of RDFs, we propose the most likely model of ethanol–water binary clusters formed in the region 0.2...

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Structure of Clusters in Ethanol–Water Binary Solutions Studied by Mass Spectrometry and X-Ray Diffraction

Unprotonated cluster ions (H2O)+n (2≤n≤10) and (Ar)m(H2O)+n (1≤m≤3; 2≤n≤7) are detected in the mass spectra, for the first time, in addition to normally observed protonated ions (H2O)n−1H+ when supersonic cluster beams of water–argon mixtures (P≳2 atm) are photoionized with the vacuum‐UV resonance lines at 11.83 and 11.62 eV. The protonated water cluster ions (H2O)n−1H+ are produced by ionization of neutral water clusters (H2O)n followed by intracluster proton‐transfer reactions, whereas the unprotonated cluster ions (H2O)+n are generated via the photoionization of water–argon binary clusters: (Ar)m(H2O)n+hν →(H2O)+n+mAr+e−. The excess energies on ionization can be randomized within the [(Ar)m(H2O)+n]*vip clusters (‘‘intracluster excess energy dissipation’’) and finally converted to the decompositions of argon atoms, giving rise to the stable (H2O)+n and various (Ar)k(H2O)+n ions. The (H2O)+n ions have the distinct potential energy minima along the proton transfer reaction coordinates.

Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O)+n (2≤n≤10)

Photodetachment and photodissociation of surface molecules of pure and mixed ices containing NH3 and H2O at 90∼130 K have been investigated by using a mass spectrometer situated in a ultrahigh vacuum apparatus. NH3 molecules were excited to the lowest singlet state by an ArF laser at 193 nm (hν=6.4 eV). The detachment of NH3 molecules from pure NH3 solids was observed at a low laser fluence of 0.1 mJ/cm2. Two photons of 248 nm laser (2hν=10 eV) also detached H2O molecules from pure H2O ices. Mixed solids of NH3 and H2O released both molecules from the surface upon irradiation with 193 nm laser. The translational energies of the detached molecules were of the order of the intermolecular interactions of the constituent molecules (0.03∼0.2 eV) and the energy distributions were not changed by the increase of laser fluence. A detachment mechanism explaining all of the observations is proposed. The force acting on the detachment is ascribed to ‘‘electronic exchange repulsion between an excited molecule and a gr...

Nobuyuki Nishi

Papers

Hydrogen-Bonded Cluster Formation and Hydrophobic Solute Association in Aqueous Solutions of Ethanol

Molecular association in ethanol-water mixtures studied by mass spectrometric analysis of clusters generated through adiabatic expansion of liquid jets

Structure of Clusters in Ethanol–Water Binary Solutions Studied by Mass Spectrometry and X-Ray Diffraction

Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O)+n (2≤n≤10)

Photodetachment, photodissociation, and photochemistry of surface molecules of icy solids containing NH3 and pure H2O ices