Exchangeable Hydrogen Research Articles

Hydrocarbon reactions on MoS 2 revisited, I: Activation of MoS 2 and interaction with hydrogen studied by transient kinetic experiments

MoS 2 was prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at 723–773 K and activated by procedures involving evacuation at 723 K and/or reduction at different temperatures, predominately 573 K. Bulk and surface properties of these samples were studied by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, oxygen chemisorption (273 K, pulse mode), and isotope exchange with D 2 to determine the quantity of exchangeable hydrogen. In addition, the quantity of hydrogenation-active surface hydrogen was determined by hydrogenation of ethylene in absence of gas-phase hydrogen. No obvious relationship was found between the S/Mo stoichiometry and the degree of Mo exposure; depending on the pretreatment, exposed Mo ions can be detected by oxygen chemisorption at widely varying quantities while the S/Mo ratio remains ⩾2. At more severe reduction treatments to remove additional sulfur, the oxygen chemisorption capacity decreases, likely due to migration of defects into the bulk. The reactivity of MoS 2, as expressed by the temperatures required to expose Mo ions and the quantity of exchangeable hydrogen, varies significantly between batches prepared by similar routes. Therefore, thorough surface characterization by a set of techniques including reactivity studies is an indispensable prerequisite for any meaningful study investigating the relationship between catalytic properties with preparation procedures. After activation, MoS 2 holds hydrogen, which is able to hydrogenate ethylene in absence of gas-phase H 2; the quantity of this hydrogenation-active surface hydrogen is not correlated with the hydrogenation activity, however.

Salicylate-spectrophotometric determination of inorganic monochloramine

On the basis of classical Berthelot reaction, a simple salicylate-spectrophotometric method was developed for quantitative determination of inorganic monochloramine in water samples. With the catalysis of disodium pentacyanonitrosylferrate(III), inorganic monochloramine reacts with salicylate in equimolar to produce indophenol compound which has an intense absorption at 703 nm. Parameters that influence method performance, such as pH, dosage of salicylate and nitroprussiate and reaction time, were modified to enhance the method performance. By using this method, inorganic monochloramine can be distinguished from organic chloramines and other inorganic chlorine species, such as free chlorine, dichloramine, and trichloramine. The molar absorptivities of the final products formed by these compounds are below ±3% of inorganic monochloramine, because of the α-N in them have only one exchangeable hydrogen atom, and cannot react with salicylate to produce the indophenol compound. The upper concentrations of typical ions that do not interfere with the inorganic monochloramine determination are also tested to be much higher than that mostly encountered in actual water treatment. Case study demonstrates that the results obtained from this method are lower than DPD-titrimetric method because the organic chloramines formed by chlorination of organic nitrogenous compounds give no response in the newly established method. And the result measured by salicylate-spectrophotometric method is coincident with theoretical calculation.

Hydrogen exchange of rhenium(VII) heptahydridobis(triphenylphosphine) with water, aniline, methanol, and itself

The hydride ligands of ReH 7(PPh 3) 2 undergo fast hydrogen exchange with water at room temperature while exchanging with the hydrogen atoms of aromatic solvent molecules at a slower rate. The amount of time required for isotopomer peaks to appear in the 1H NMR hydride resonance of ReH 7(PPh 3) 2 was used to distinguish fast hydrogen exchange from slow hydrogen exchange. The room temperature 1H NMR T 1 values of adventitious water resonances were used to distinguish two rhenium heptahydride compounds that participate in fast hydrogen exchange with water, ReH 7(PPh 3) 2 and ReH 7(AsPh 3) 2, from one compound that does not participate, ReH 7(PCy 3) 2. Fast hydrogen exchange between ReH 7(PPh 3) 2 and water is greatly slowed or stopped when the compound is dissolved in a solution that contains DMSO. The compound ReH 6D(PPh 3) 2, an isotopomer of ReH 7(PPh 3) 2 that was prepared in situ, exhibited an isotopomer shift in its 31P–{ 1H} NMR spectrum as well as D–P coupling when measured in the solvent anisole. In a solvent system containing deuteroaniline, nearly all of the hydride ligands were exchanged with deuterium, from aniline, in less than 2 h. Hydrogen exchange between the hydride ligands of ReH 7(PPh 3) 2 and deuteroaniline was found to be reversible upon the addition of normal aniline. The distribution of hydrogen isotopes in the rhenium coordination spheres reflected the overall composition of the exchangeable hydrogen isotopes present in the bulk sample. In a deuteromethanol-containing solvent system, the exchange of hydride ligands between separate ReH 7(PPh 3) 2 isotopomers was evident. The presumed hydrogen exchange intermediate, [ReH 6(H 2)(PPh 3) 2] +, may be responsible for the evident exchange of hydride ligands and may also be important to the thermal loss of dihydrogen from ReH 7(PPh 3) 2.

Acceleration of the Substrate Cα Deprotonation by an Analogue of the Second Substrate Palmitoyl-CoA in Serine Palmitoyltransferase

Serine palmitoyltransferase (SPT) is a key enzyme of sphingolipid biosynthesis and catalyzes the pyridoxal 5'-phosphate (PLP)-dependent decarboxylative condensation reaction of l-serine with palmitoyl-CoA to generate 3-ketodihydrosphingosine. The binding of l-serine alone to SPT leads to the formation of the external aldimine but does not produce a detectable amount of the quinonoid intermediate. However, the further addition of S-(2-oxoheptadecyl)-CoA, a nonreactive analogue of palmitoyl-CoA, caused the apparent accumulation of the quinonoid. NMR studies showed that the hydrogen-deuterium exchange at Calpha of l-serine is very slow in the SPT-l-serine external aldimine complex, but the rate is 100-fold increased by the addition of S-(2-oxoheptadecyl)-CoA, showing a remarkable substrate synergism in SPT. In addition, the observation that the nonreactive palmitoyl-CoA facilitated alpha-deprotonation indicates that the alpha-deprotonation takes place before the Claisen-type C-C bond formation, which is consistent with the accepted mechanism of the alpha-oxamine synthase subfamily enzymes. Structural modeling of both the SPT-l-serine external aldimine complex and SPT-l-serine-palmitoyl-CoA ternary complex suggests a mechanism in which the binding of palmitoyl-CoA to SPT induced a conformation change in the PLP-l-serine external aldimine so that the Calpha-H bond of l-serine becomes perpendicular to the plane of the PLP-pyridine ring and is favorable for the alpha-deprotonation. The model also proposed that the two alternative hydrogen bonding interactions of His(159) with l-serine and palmitoyl-CoA play an important role in the conformational change of the external aldimine. This is the unique mechanism of SPT that prevents the formation of the reactive intermediate before the binding of the second substrate.

Hydrogen and oxygen isotope ratios in human hair are related to geography

We develop and test a model to predict the geographic region-of-origin of humans based on the stable isotope composition of their scalp hair. This model incorporates exchangeable and nonexchangeable hydrogen and oxygen atoms in amino acids to predict the delta(2)H and delta(18)O values of scalp hair (primarily keratin). We evaluated model predictions with stable isotope analyses of human hair from 65 cities across the United States. The model, which predicts hair isotopic composition as a function of drinking water, bulk diet, and dietary protein isotope ratios, explains >85% of the observed variation and reproduces the observed slopes relating the isotopic composition of hair samples to that of local drinking water. Based on the geographical distributions of the isotope ratios of tap waters and the assumption of a "continental supermarket" dietary input, we constructed maps of the expected average H and O isotope ratios in human hair across the contiguous 48 states. Applications of this model and these observations are extensive and include detection of dietary information, reconstruction of historic movements of individuals, and provision of region-of-origin information for unidentified human remains.

Conformationally Driven Gas-Phase H/D Exchange of Dinucleotide Negative Ions

Gas-phase hydrogen/deuterium exchange of six deprotonated dinucleotides with CD(3)OD was performed in the second hexapole of a Fourier transform ion-cyclotron resonance (FTICR) mass spectrometer. To complete these experiments, dynamic simulations were carried out to investigate the different conformations adopted by the dinucleotides. In the experimental conditions and in integrating the experimental and theoretical results, H/D exchange was shown to be controlled by hydrogen accessibility and not by the chemical nature of the heteroatom bearing the exchangeable hydrogen. A model including simultaneous H/D exchanges at the experimental time scale was used to reproduce the dinucleotide H/D exchange kinetic plots. The relay mechanism was not relevant for dinucleotides. This allowed the H/D exchange rates to be directly linked to conformations.

Characterization of thin water layers in pulp by tritium exchange. Part 1: Methods development

Abstract A new technique for measuring the monolayer water content of fiber is presented. Tritiated water is added to a pulp/water suspension, whereupon the tritium partitions between the bulk water and the pulp. In the pulp phase the tritium can exchange with free water, bound water, and with hydroxyl and other protons present in the pulp matrix. The free water in the pulp is then removed by displacement with acetone. The tritium remaining in the pulp is mostly associated with tightly bound water, with a small fraction being tied up with the exchangeable hydrogen in pulp. The procedure provides a value of 10% for the tightly bound water content of hardwood or softwood fiber, either bleached or unbleached. If this water is assumed to cover the fiber surface as a monolayer, then an estimate of the wet surface area of the fiber can be obtained. This estimate compares well with independent measurements of surface area.

Neutralization of soil acidity by animal manures: mechanism of reaction

The mechanism of reaction leading to neutralization of soil acidity by animal manures was studied in the laboratory. Five animal manures; rabbit manure (RBM), swine manure (SWM), goat manure (GTM), poultry manure (POM), and cow manure (COM) were respectively added at rates equivalent to 10, 20, 30, and 40 t ha-1 to an Ultisol having soil-water pH of 4.6 and KCl extractable acidity of 3.00 cmol kg-1. The amended soils were incubated at 70% water holding capacity for 3 weeks. Animal manures significantly increased the soil pH from 4.6 to values above 5.6 and also reduced exchangeable acidity from 3.00 cmol kg-1 to values below 0.35 cmol kg-1. The mechanism that best explained the neutralization reaction was found to be microbial decarboxylation of calcium-organic matter complex leading to the release and subsequent hydrolysis of calcium ions. The hydroxyl ions released in the hydrolytic reaction then reacts with both the exchangeable hydrogen and aluminum ions to form water and insoluble aluminum hydroxide (Al(OH)3) respectively.

Development of a new approach for microbial decontamination of water using modified Fenton's reaction

Microbial decontamination of water was carried out using a novel radical generating system consisting of ion exchange resin, copper and hydrogen peroxide. The system was successful in reducing the microbial load in water by more than 99% in 15 min and is effective against all the microorganisms tested. The method was also successful in decontaminating the flood water obtained from Industrial Canal and 17th Street Canal in New Orleans. Decontamination is due to the formation of hydroxyl radicals, formed during the decomposition of hydrogen peroxide by the metal-polymer complex.

Neutron diffraction and simulation studies of the exocyclic hydroxymethyl conformation of glucose

The techniques of neutron diffraction with isotopic substitution (NDIS) and molecular dynamics (MD) simulations have been used to examine the rotational conformation of the exocyclic hydroxymethyl group of D-glucopyranose. First order H/D NDIS experiments were performed on the H6 position in 3m aqueous glucose solutions where the average coherent scattering length of the exchangeable hydrogen atoms was zero (i.e., all correlations between exchangeable hydrogen atoms and other atoms cancel and thus are not present in the scattering data). This H6 experimental result suggests that no single conformation for the C4-C5-C6-O6 dihedral reproduces the observed scattering data well, but that a mixture of the gg and gt conformations, which has been suggested by NMR experiments, gives a reasonable agreement between the MD and experimental data.

CHITOSAN-BASED FUEL CELL MEMBRANES

ABSTRACT Chitosan complex membranes are prepared and characterized at room temperature. They are expected to be used as proton exchange membranes. The studied membranes are cross-linked membranes with sulfuric acid; salt-complexed membranes with lithium nitrate; cross-linked and salt-complexed membranes; plasticized and salt-complexed membranes; cross-linked, plasticized, and salt-complexed membranes; and doped membranes with sulfuric acid. A fixed amount of ethylene carbonate is used as plasticizer. It is found that the ion exchange capacity and hydrogen gas permeability of all membranes is better than that of Nafion membranes. However, their proton conductivities are worse than Nafion membranes. It can be stated that ethylene carbonate does not improve conductivity. An optimum amount of lithium nitrate salt can enhance conductivity. The formation of a sulfate group in cross-linked membranes is necessary for proton conduction. The proton conductivities of 4%cross-linked and 50%LiNO3 membrane before and after acid doping are (3.11±0.40) × 10−2 and (6.64±0.11) × 10−2 S cm−1, respectively. That of Nafion is (8.02±1.19) × 10−2 S cm−1.

The relationship between the hydrogen and oxygen isotopes of freshwater bivalve shells and their home streams

Recent technical advances have facilitated the analysis of microgram-size samples of mollusk shell for the hydrogen isotope composition of the organic matrix. Our results suggest that these data are meaningful as the δ D shell values of samples of freshwater bivalve shells from different streams preserve the relative distribution of δ D values from the water they inhabited over their lifetime (δ D shell − δ D water = − 145‰) after correcting for the contribution of exchangeable hydrogen (∼ 36%) in organic matrix. When δ D shell values are compared to the δ 18O values of the mineral fraction of the shell (CaCO 3 as aragonite), they plot along a line that reflects the isotopic composition of the waters in which the bivalves lived. This relationship is potentially useful for paleoenvironmental reconstructions because the δ D value of the organic matrix may serve as an independent proxy for the δ 18O value of water in paleotemperature equations.

Determination of exchangeable hydrogen ions in boreal shield soils of Quebec

An unbuffered BaCl2 extraction for determining effective cation exchange capacity (ECEC) and exchangeable cations is often used for acidic forest soils. However, the contribution of exchangeable H+ to ECEC cannot be assessed using this method as H+ in the extract may be produced/consumed in reactions with free Al3+ and Al-OH complexes, or H+ may be added to the extract via non-exchangeable sources. Fundamentally, any valid measure of ECEC must include some estimate of exchangeable H+ concentration or a demonstration that it is negligible. Unfortunately, this procedure is often neglected in forest soil studies. In this paper, we assessed the significance of the contribution of exchangeable H+ to ECEC for upper soil horizons of three sites of Quebec’s Boreal Shield subject to various disturbance types (i.e., recent harvest, fire and mature forest). We also investigated whether the linear relationships between exchangeable H+ concentrations and soil pH are robust enough to develop regression models capable of predicting exchangeable H+. Exchangeable H+ in the FH samples was higher than that in the podzolic B samples, but the amount of adsorbed H+ relative to ECEC was nevertheless significant in the podzolic B horizons. The general linear relationship (i.e., FH and podzolic B samples as a single data set) developed from soil pH in water explained close to 70% of the variability of log (H+/ECEC). The relationships between log (exchangeable H+) and pH in water in the FH samples were however superior (R2 ≥ 80), either for all sites, disturbance types and sampling strategies (e.g., proportions of F and H horizons in sample). The relationships developed for podzolic B samples alone were not as strong, but ECEC was used efficiently in combination with soil pH to increase prediction capabili ties (R2 ≥ 0.61). Key words: Boreal Shield forest soils, effective cation exchange capacity, exchangeable H+, fire, FH and podzolic B horizons, harvesting, prediction models, soil pH.

Secondary isotope effects in liquid chromatography behaviour of 2H and 3H labelled solutes and solvents

The separation of solutes that differ only in the extent of isotopic substitution of their hydrogen atoms, using either mixtures of isotopically non-modified or perdeuterated solvents as mobile phases, is described. The occurrence of a secondary isotope effect is demonstrated in reversed-phase liquid chromatography, which is independent of the nature of the stationary phase (different octadecyl-bonded silicas, an embedded alkylamide-bonded silica, as well as one polymeric stationary phase were tested), and the water content and the nature of organic modifier of the mobile phase. The separation of 24 structurally different isotopologue pairs (apolar compounds and polar compounds with exchangeable or non-exchangeable hydrogen atoms) is examined using reversed-phase liquid chromatography. It is found that the greater the number of isotopically substituted hydrogen atoms in a given organic solute, the better is the separation of a particular isotopologue pair. The single secondary isotope effect is shown to be dependent on the number of isotopic substitutions. The greater the number of these substitutions, the smaller is the single isotope effect. The single secondary isotope effect is higher for aromatic hydrocarbons than for aliphatic hydrocarbons. A secondary isotope effect is also observed in chiral chromatography and normal-phase liquid chromatography, as well as on changing the nature of the substituting isotope, i.e.: tritium instead of deuterium. Thus, we have demonstrated that the total secondary isotopic effect for hydrogen/tritium is higher than for hydrogen/deuterium. This isotope effect involves only the consequences of changes in interactions due to nuclear motions. Overall this study confirms the predominance of hydrophobic effects in retention processes in reversed-phase liquid chromatography. In reversed-phase liquid chromatography, a secondary isotope effect related to mobile phase composition is also observed. The behaviour of deuterium oxide and water in mobile phases of the same composition (%, w/w) is compared. Independent of the nature of the organic modifier (methanol, acetonitrile or ethanol), the effect of replacing H 2O with 2H 2O in the mobile phase, is an increase in the retention factors and an improvement in the chromatographic resolution of isotopologue pairs. This increase in the resolution is not accompanied by a change in the chromatographic selectivity. The measurement of liquid–liquid extraction coefficients proves that the effect is mainly due to the modification of the phase ratio. In general the effect of 2H-labelled solvents ( 2H 2O and C 2H 3CN) as mobile phase components, compared to their isotopically non-modified isomers, can be rationalized on the basis of their lower polarisabilities. Overall the use of perdeuterated rather than isotopically non-modified solvents as mobile phase components leads to the most efficient separation systems.

Bonding between polyacrylamide and smectite

In various applications of polyacrylamide (PAM) in soil erosion control, soil conditioning, water treatment, and oil recovery, the polymer mainly reacts with clay minerals. The objective of this study was to investigate the probable bonding mechanisms between PAM and smectite, a common clay mineral that occurs in soils and sediments. Three polyacrylamides – nonionic PAM 903N, cationic PAM 494C, and anionic PAM 836A – were investigated. Additionally, the impact of exchangeable cation, Na +, K +, Mg 2+, Al 3+, Cu 2+, and Ni 2+, on bonding strength was investigated. The PAM-smectite complexes were analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. After adsorption on Ca 2+-smectite, the asymmetric and symmetric stretching vibrations of the NH 2 groups of nonionic PAM blue-shifted 140 and 186 cm −1, respectively, suggesting that the amino group was less H-bonded in the adsorbed PAM than in the bulk polymer. The C O band showed only a 3 cm −1 blue-shift, suggesting that the carbonyl oxygen was still bonded by other atoms or ions. The IR-band shifts were affected by the exchangeable cation. For example, the stretching vibrations of NH 2 and C O bonds of adsorbed PAM in complexes containing transition-metal cations (Cu 2+ or Ni 2+) occurred at frequencies 20 and 9 cm −1 lower than those containing exchangeable alkali cations (Na + or K +). The IR bands of amide groups of the complexes containing exchangeable Ca 2+, Mg 2+, and Al 3+ fell between those containing transition-metal cations and those containing alkali cations. The IR band shifts suggest that carbonyl (C O) oxygens contributed more than the amino group (NH 2) to the bonding between PAMs and smectite. The IR-band shifts associated with the exchangeable cation suggest that: (1) an ion-dipole interaction or coordination occurs between the amide groups of the PAMs and the exchangeable cations and (2) hydrogen bonding exists between the amide groups and water molecules in the hydration shells of exchangeable cations. Similar IR band shifts were observed in the cationic PAM 494C- and anionic PAM 836A-smectite complexes, suggesting that the nonionic acrylamide units in these copolymers reacted similarly with smectite as the nonionic PAM 903N.

Protein Folding Mechanisms Studied by Time-Resolved Electrospray Mass Spectrometry

Mass spectrometry (MS) owes its remarkable rise as a bioanalytical tool to the development of two “soft” ionization techniques: matrix-assisted laser desorption/ ionization (MALDI) (1,2) and electrospray ionization (ESI) (3). The salient feature of soft ionization is that a wide range of analytes, from small metabolites to very large biomolecular systems, can be transferred intact into the gas phase, thus making them amenable to mass spectrometric analysis. For MALDI-MS, the sample is embedded into a crystalline matrix prior to analysis. For ESI, in contrast, ionization is initiated directly from the liquid phase by spraying analyte solution from the tip of an electrically charged capillary. The small droplets formed by the ESI emitter undergo rapid evaporation and fi ssion, ultimately releasing the analyte as multiply protonated (or deprotonated) species (4).

Effect of gender, body composition, and equilibration time on the 2H-to-18O dilution space ratio

Physiological sources of variation in the 2H-to-18O dilution space ratio (DSR) were examined in 34 males and 20 females (4-78 yr; 14.7-143.2 kg; 1.8-61.0% body fat). Dilution spaces were obtained by time 0 extrapolation of isotope washout over 10-14 days, and body composition was obtained by underwater weight (adults) or bioelectrical impedance (children). The mean DSR was 1.050 +/- 0.015 (range 1.029-1.111), significantly higher (P < 0.001) than the traditionally assumed value of 1.029 based on exchange over 4 h. Use of the value 1.029 causes a systematic 8% overestimate of energy expenditure from doubly labeled water, relative to use of the value 1.05. The DSR was not related to body composition or age but was significantly higher (P < 0.05) in males (1.052 +/- 0.016) than in females (1.044 +/- 0.012). This gender effect was not explained by differences in the number of exchangeable hydrogens in the body. We conclude 1) variation in the 2H-to-18O DSR is not explained by body composition but is influenced by the chemical availability of exchangeable hydrogens to undergo exchange; 2) because the DSR is not easily predicted, use of the observed dilution spaces are recommended; 3) if a fixed DSR is used, values of 1.044 and 1.052 are recommended in females and males, respectively.

Hydrogen—Deuterium Exchange in Bovine Serum Albumin Protein Monitored by Fourier Transform Infrared Spectroscopy, Part I: Structural Studies

The structure of the amide I band of bovine serum albumin (BSA) was determined using an H/D exchange experiment. The difference between the dry and hydrated exchange spectrum revealed the fine structure of the amide I band. The band at 1717 +/- 2 cm(-1) is due to the vibration of the COOH moieties from the protein side chains. Band components at 1682 +/- 2 cm(-1), 1655 +/- 2 cm(-1), and 1637 +/- 2 cm(-1) are assigned to the vibrations of the backbone C=O. These three bands belong to vibrations of three different populations of amide groups differing in the number of established H-bonds. The connectivity between the frequencies of various amide vibrations was determined by two-dimensional generalized correlation spectroscopy and spectral decomposition. About 7% of the whole exchangeable hydrogen atom population (NH, NH2, and OH groups from backbone and side chains) remains unexchanged, and these hydrogen atoms belong mainly to the NH groups, which are H-bonded to specific C=O groups. Moreover, this study concerns the approximately 10% of hydrogen atoms belonging to a particular HN...O=C population with a characteristic amide A frequency at 3290 cm(-1) and an amide I band at 1655 +/- 2 cm(-1), usually attributed to the alpha-helical structure that remains unexchanged. At higher temperature the exchange is more efficient. Upon heating, a further 4% of these NH groups are deuterated. The comparison of the exchange spectrum at higher temperature with the structural changes of the protein at the same temperature implies that the change in overall dynamics of the protein improve the level of exchange.

Surface‐enhanced Raman scattering (SERS) of random silver or gold particle arrays on aminodextran‐coated polystyrene beads

AbstractCitrate and/or chemically modified aminodextran (Amdex) SERS bands were detected from silver particle‐Amdex‐polystyrene (PS) beads either as singlets or small clusters, prepared by reaction between aqueous silver nitrate, Amdex‐coated PS beads, and aqueous sodium citrate solution. However, citrate on individual silver or gold particles of the same size was not detected with 633‐nm excitation. SERS of dyes such as rhodamine 6 G was readily observed on individual silver particles or on the gold beads. Three distinct modes of citrate/modified‐Amdex binding to the silver particle‐Amdex‐PS beads were characterized: (1) exchangeable citrate bound through one or more carboxylate groups—citrate could be washed away with H2O or D2O; displaced by successively greater concentrations of R6G; or squeezed out mechanically by applying the pressure of a second coverslip on top of a droplet of bead suspension; (2) nonexchangeable Amdex bound to silver via a primary alcoholate ion of oxidized, glutaraldehyde‐cross‐linked Amdex; and (3) nonexchangeable oxidized Amdex bound to silver via carboxylate group(s) but with H/D exchangeable alcohol hydrogen atom. That oxidation of the glucopyranose residues of Amdex on PS beads had taken place by Ag(I) as silver nitrate in aqueous solution to split out carboxylate groups was first identified from their characteristic vibrational modes in the SERS spectra of colloidal silver suspensions. Minimum amplification factors for (R6G on Ag)/(R6G on Au) with 633‐nm excitation were estimated as 1 × 1015/1 × 1011. An extra SERS enhancement of about 1000‐fold due to multiplicity of metal particles per bead was included. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Gas-Phase Structure of Protonated Histidine and Histidine Methyl Ester: Combined Experimental Mass Spectrometry and Theoretical ab Initio Study

Gas-phase H/D exchange experiments with CD3OD and D2O and quantum chemical ab initio G3(MP2) calculations were carried out on protonated histidine and protonated histidine methyl ester in order to elucidate their bonding and structure. The H/D exchange experiments show that both ions have three equivalent fast hydrogens and one appreciably slower exchangeable hydrogen assigned to the protonated amino group participating in a strong intramolecular hydrogen bond (IHB) with the nearest N(sp2) nitrogen of the imidazole fragment and to the distal ring NH-group, respectively. It is taken for granted that the proton exchange in the IHB is much faster than the H/D exchange. Unlike in other protonated amino acids (glycine, proline, phenylalanine, tyrosine, and tryptophan) studied earlier, the exchange rate of the carboxyl group in protonated histidine is slower than that of the amino group. The most stable conformers and the enthalpies of neutral and protonated histidine and its methyl ester are calculated at the G3(MP2) level of theory. It is shown that strong intramolecular hydrogen bonding between the amino group and the imidazole ring nitrogen sites is responsible for the stability and specific properties of the protonated histidine. It is found that the proton fluctuates between the amino and imidazole groups in the protonated form across an almost vanishing barrier. Proton affinity (PA) of histidine calculated by the G3(MP2) method is 233.2 and 232.4 kcal mol(-1) for protonation at the imidazole ring and at the amino group nitrogens, respectively, which is about 3-5 kcal mol(-1) lower than the reported experimental value.