GB Values Research Articles

Investigation of Deprotonation Reactions on Globular and Denatured Proteins at Atmospheric Pressure by ESSI-MS

Deprotonation reactions of multiply charged protein ions have been studied by introducing volatile reference bases at atmospheric pressure between an electrosonic spray ionization (ESSI) source and the inlet of a mass spectrometer. Apparent gas-phase basicities (GB(app)) of different charge states of protein ions were determined by a bracketing approach. The results obtained depend on the conformation of the protein ions in the gas phase, which is linked to the type of buffer used (denaturing or nondenaturing). In nondenaturing buffer, the GB(app) values are consistent with values predicted by the group of Kebarle using an electrostatic model (J. Mass Spectrom.2002, 38, 618) based on the crystal structures, but taking into account salt bridges between ionized basic and acidic sites on the protein surface. A new basicity order for the most basic sites was therefore obtained. An excellent agreement with the charge residue model (CRM) is obtained when comparing the observed and calculated maximum charge state. Decharging of the proteins in the electrosonic spray process could be also useful in the study on noncovalent complexes, by decreasing repulsive electrostatic interactions. A unified mechanism of the ESSI process is proposed.

Brønsted Basicities of Diamines in the Gas Phase, Acetonitrile, and Tetrahydrofuran

A comprehensive basicity study of alpha,omega-alkanediamines and related bases has been carried out. Basicities in acetonitrile (AN, pK(a) values), tetrahydrofuran (THF, pK(alpha) values), and gas phase (GP, GB values), were measured for 16, 14, and 9 diamine bases and for several related monoamines. In addition the gas-phase basicities and equilibrium geometries were computed for 19 diamino bases and several related monoamines at the DFT B3LYP 6-311+G** level. The effects of the different factors (intrinsic basicity of the amino groups, formation of intramolecular hydrogen bonds, and molecular strain) determining the diamine basicities were estimated by using the method of isodesmic reactions. The results are discussed in terms of molecular structure and solvation effects. The GP basicity is determined by the molecular size and polarizability, the extent of alkylation, and the energy effect of intramolecular hydrogen bond formation in the protonated base. The basicity trends in the solvents differ very much from those in GP: 1) The solvents severely compress the basicity range of the bases studied (3.5 times for the 1,3-propanediamine family in AN, and 7 times in THF), and 2) while stepwise alkylation of the basicity center leads to a steady basicity increase in the gas phase, the picture is complex in the solvents. Significant differences are also evident between THF and AN. The high hydrogen bond acceptor strength of THF leads to this solvent favoring the bases with "naked" protonation centers. In particular, the basicity order of N-methylated 1,3-propanediamines is practically inverse to that in the gas phase. The picture in AN is intermediate between that of GP and THF.

Experimental Gas-Phase Basicity Scale of Superbasic Phosphazenes

Seventeen superbasic phosphazenes and two Verkade's bases were used to supplement and extend the experimental gas-phase basicity scale in the superbasic region. For 19 strong bases the gas-phase basicity values (GB) were determined for the first time. Among them are such well-known bases as BEMP (1071.2 kJ/mol), Verkade's Me-substituted base (1083.8 kJ/mol), Et-N=P(NMe2)2-N=P(NMe2)3 (Et-P2 phosphazene, 1106.9 kJ/mol), and t-Bu-N=P(NMe2)3 (t-Bu-P1 phosphazene, 1058.0 kJ/mol). For the first time experimental GB values were determined for P2 phosphazenes. Together with our previous results self-consistent experimental gas-phase basicity scale between 1020 and 1107 kJ/mol is now established. This way an important region of the gas-phase basicity scale, which was earlier dominated by metal hydroxide bases, is now covered also with organic bases making it more accessible for further studies. The GB values for several superbases were calculated using density functional theory at the B3LYP/6-311+G** level. For the phosphazene family the standard deviation of the correlation between the experimental and theoretical values was 6.5 kJ/mol.

Base Strengths of Substituted Tritylamines, N‐Alkylanilines, and Tribenzylamine in Aqueous Solution and the Gas Phase: Steric Effects Upon Solvation and Resonance Interactions

AbstractThe dissociation constants of the conjugate acids of N‐tritylacetamide (1h; pK = 3.81) and N‐benzyl‐N‐methyl‐4,4′,4′′‐trimethoxytritylamine (4i; pK = 9.86) have been measured in aqueous acetonitrile at 25 °C and at other temperatures to determine the enthalpies and entropies of reaction. For 1h, ΔH⊖ = 40.7 kJ·mol−1 and ΔS⊖ = 64 J·K−1·mol−1, and for 4i ΔH⊖ = 9.1 kJ·mol−1 and ΔS⊖ = −159 J·K−1·mol−1. In addition, gas‐phase base strengths at 25 °C (GB values in kJ·mol−1) of TrNH2 (1a; 902.1), TrNHPh (1c; 926.3), TrNHAc (1h; 929.7), TrNHC6H4(o‐NO2) (1i; 895.0), DMTrNH2 (3a; 921.3), DMTrNHCH2CO2Me (3b; 879.1), and DMTrNH(p‐NO2Bn) (3d; 886.6) have been determined by ICR measurements. The GB of TrNHAc corresponds to protonation at oxygen and B3LYP/6‐31G* calculations indicate that the N‐protonated isomer is 46.4 kJ·mol−1 less stable, i.e. the GB value for N‐protonation is 883.3 kJ·mol−1. Correspondingly, the literature GB value of 857.6 5 kJ·mol−1 for N‐methylacetamide corresponds to protonation at oxygen, and B3LYP/6‐31G* calculations indicate that the N‐protonated isomer is 58.1 kJ·mol−1 less stable, i.e. the GB value for N‐protonation of MeNHAc is 799.5 kJ·mol−1. The GB of PhNH(tBu) (5; 920.1 kJ·mol−1) has been measured and compared with values for other N‐alkylanilines, PhNHR, including PhNHTr; the results indicate that the increasing GB values as R increases in size are due solely to the increasing polarisability of R. This indicates that the increasing solution base strength of PhNHR as R increases in size is a solvation effect and is not due to decreasing resonance interactions between the nitrogen lone‐pair and the phenyl ring. Similarly, the base‐strengthening effect in solution of the (substituted) trityl in TrNHZ, where Z is an alkyl with an electron‐withdrawing group, is shown to be due to solvation phenomena as it is absent in the gas phase; for one such compound, TMTrNHCH2CO2Me (4b; pK = 9.30), ΔH⊖ = 17.9 kJ·mol−1 and ΔS⊖ = −118 J·K−1·mol−1. In contrast, the difference in solution base strengths between MeNHAc (pK = −0.56) and TrNHAc (pK = 3.81) is attributed, at least in part, to a reduced base‐weakening resonance interaction between the lone pair on N and the acetyl group in TrNHAc, as the effect is also evident in the gas phase. The GB value for tribenzylamine (6) has also been measured (965.2 kJ·mol−1) and is unexceptional; this indicates that the low base strength of 6 in aqueous solution (pK = 4.90 at 25 °C) is a solvation effect which is expressed mainly through an abnormally large positive entropy of reaction (ΔS⊖ = 76 J·K−1·mol−1), the value of ΔH⊖ (50.5 kJ·mol−1) being only slightly larger than normal for tertiary amines. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Brillouin-gain coefficients of chalcogenide glasses

We derived the relation between a figure of merit, M1, for the diffraction efficiency of acousto-optical devices and the Brillouin-gain coefficient, gB. We then estimated the values of gB and the phonon lifetime for typical As2S3 and As2Se3 chalcogenide glasses from known acousto-optical parameters. We found that the magnitude of gB for these two glasses is greater than 20 times that of fused silica.

Application of the kinetic method to bifunctional bases: MIKE and CID-MIKE test cases

An assessment of the kinetic method and its applicability to the determination of the basicity of bidentate molecules is done by considering several examples previously studied by equilibrium methods. Selected examples are: 1,2-ethanediol, 1,3-propanediol, glycerol, 1,4-butanediol, 1,2-dimethoxyethane, 2-methoxyethanol methoxyacetone, 1,2-diamino ethane, 1,3-diamino propane and 1,4-diaminobutane. It is generally observed that the orthodox use of the method leads to GB(M), PA(M) and protonation entropy values different from that obtained by the equilibrium method.

Charged states of proteins. Reactions of doubly protonated alkyldiamines with NH(3): solvation or deprotonation. Extension of two proton cases to multiply protonated globular proteins observed in the gas phase.

The apparent gas-phase basicities (GB(app)'s) of basic sites in multiply protonated molecules, such as proteins, can be approximately predicted. An approach used by Williams and co-workers was to develop an equation for a diprotonated system, NH(3)(CH(2))(7)NH(3)(2+), and then extend it with a summation of pairwise interactions to multiply protonated systems. Experimental determinations of the rates of deprotonation of NH(3)(CH(2))(7)NH(3)(2+) by a variety of bases B, in the present work, showed that GB(app) = GB(NH(3)) = 196 kcal/mol. This result is supported also by determinations of the equilibria: NH(3)(CH(2))(p)NH(3)(2+) + NH(3) = NH(3)(CH(2))(p)NH(3) x NH(3)(2+), for p = 7, 8, 10, 12. The described experimental GB(app) is 14 kcal/mol higher than the value predicted by the equation used by Williams and co-workers but in agreement with an ab initio result by Gronert. Equations based on electrostatics are developed for the two proton and multiproton systems which allow the evaluation of GB(app) of the basic sites on proteins. These are applied for the evaluation of GB(app) of the basic sites and of N(SB), the maximum number of protons that the nondenatured proteins, carbonic anhydrase (CAII), cytochrome c (CYC), and pepsin, can hold. The N(SB) values are compared with the observed charges, Z(obs)'s, when the nondenatured proteins are produced by electrospray and found in agreement with the proposal by de la Mora that Z(obs) is determined by the number of charges provided by the droplet that contains the protein, according to the charge residue model (CRM). The GB(app) values of proteins have many other applications. They can be compared with experimental measurements and are also needed for the understanding of the thermal denaturing of charged proteins and the thermal dissociation of charged protein complexes.

Extraction of the oxide charge density at front and back interfaces of SOI nMOSFETs devices

This work presents a new method to extract the oxide charge densities at front ( Q ox1) and back ( Q ox2) interfaces of fully depleted SOI nMOSFETs. The proposed method is based on the influence of the front and back gate voltages on the back and front channel current regime, respectively. To extract Q ox2, the drain current curve is measured as a function of the back gate voltage V GB with the front interface inverted. When the back interface condition changes due to the back gate voltage, kinks occur in the front drain current for specific V GB values which are used by the method. Similarly, the back drain current as a function of the front gate voltage V GB with the back interface inverted shows some kinks for specific V GF values which are used by the method to extract Q ox1. MEDICI simulations were used to support the analysis and the method was applied experimentally.

Gas-phase basicities for ions from bradykinin and its des-arginine analogues.

Apparent gas-phase basicities (GB(app)s) for [M + H]+ of bradykinin, des-Arg1-bradykinin and des-Arg9-bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GB(app) of 225.8 +/- 4.2 kcal x mol(-1), bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1-bradykinin and des-Arg9-bradykinin have GB(app) values of 222.8 +/- 4.3 kcal x mol(-1) and 214.9 +/- 2.3 kcal x mol(-1), respectively. One purpose of this work was to determine a suitable reaction efficiency 'break point' for assigning GB(app) values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GB(app)s. Support for this criterion is provided by the fact that our GB(app) values for des-Arg1-bradykinin and des-Arg9-bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GB(app)s for bradykinin ions from the two studies differ by 10.3 kcal x mol(-1). The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GB(app), while the basicity of des-Arg1-bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position.

Experimental and Theoretical Studies of the Possible and Preferred Site of Protonation in Polyfunctional N1,N1-Dimethyl-N2-benzoylformamidine in the Gas Phase

Abstract Proton-transfer reactions for N1,N1-dimethyl-N2-benzoylformamidine containing three basic sites, the carbonyl-O, imino-N, and amino-N atoms, have been studied by experimental (FT-ICR) and theoretical (ab initio) methods. The ICR study gave a GB of 225.5 kcal mol−1 for this compound. The empirical structure-basicity relationships predicted GB values of 224.8 and 212.8 kcal mol−1 for protonation at the imino-N and carbonyl-O atoms, respectively. The observed GB values is close to the value for the imino-N atom. Theoretical calculations (B3LYP/6-311+G(2d,p)//HF/6-31G(d)) also showed that protonation at the imino-N atom is preferred by 7 kcal mol−1 compared to protonation at the carbonyl-O atom.

Gas-Phase Basicities and Acidities of Ethyl-, Vinyl-, and Ethynylarsine. An Experimental and Theoretical Study

The gas-phase basicities and acidities of ethynyl-, vinyl- (or ethenyl-), and ethylarsine have been measured by means of FT-ICR techniques. The structures of the different neutral, protonated, and deprotonated species were elucidated by means of ab initio molecular orbital calculations at the MP2 level. Their final energies were calculated in the framework of the G2 theory. The agreement between calculated and experimental values for both basicities and acidities was fairly good. Protonation of vinylarsine and ethynylarsine at the Cα carbon leads to the breaking of the C−As bond, such that the corresponding protonated species can be viewed as complexes between ethylene and acetylene, respectively, with a AsH2+ cation. More importantly, these complexes are predicted to be as stable as the cations formed by direct protonation at the arsenic atom. For the particular case of ethynylarsine, this result is consistent with the experimental evidence. The gas-phase basicity decreases (GB values decrease) and acidi...

The intrinsic basicity of urea. An experimental (FT ICR) and theoretical (G2) study

The gas phase basicity of urea ( 1), GB( 1), [i.e. the standard Gibbs energy change for the reaction 1H +(g) ⇄ 1(g) + H +(g) ] has been determined experimentally for the first time. This was achieved by means of Fourier Transform Ion Cyclotron Resonance Spectroscopy (FT ICR). 1 and 1H + have been treated at the G2 level. This study suggests that 1(g) is very nearly a 1:1 mixture of two conformers of C 2 and C s symmetries. 1(g) is an oxygen base. The calculated and experimental values of GB( 1) are in excellent agreement. An equally excellent agreement also exists between the experimental and calculated standard enthalpies of formation of 1(g). The vibrational frequencies of the conformers of 1 and of 1H + have been calculated. This information, as well as the structures optimized at the MP2(FULL)/6-31G(d) level have been examined and compared to the available experimental data. The results thus obtained shed light on the mechanism of the stabilizing effect of 1H +(g) provided by the amino groups.

Evaluating electrostatic contributions to binding with the use of protein charge ladders.

Electrostatic interactions between charges on ligands and charges on proteins that are remote from the binding interface can influence the free energy of binding (delta Gb). The binding affinities between charged ligands and the members of a charge ladder of bovine carbonic anhydrase (CAII) constructed by random acetylation of the amino groups on its surface were measured by affinity capillary electrophoresis (ACE). The values of delta Gb derived from this analysis correlated approximately linearly with the charge. Opposite charges on the ligand and the members of the charge ladder of CAII were stabilizing; like charges were destabilizing. The combination of ACE and protein charge ladders provides a tool for quantitatively examining the contributions of electrostatics to free energies of molecular recognition in biology.

Gas Phase Basicities of Phenylacetylenes. Intrinsic Resonance Demand of the 1-Phenylvinyl Cation

Abstract Gas-phase basicities (GB) of a series of substituted phenylacetylenes were determined by measuring the standard free energy changes of proton transfer equilibria using a pulsed ion cyclotron resonance mass spectrometer. The LArSR analysis of the substituent effect on the GB values gave an r+ value of 1.18 and a ρ value of −14.0. The r+ value is significantly larger than that obtained for the GB of α-methylstyrenes but is smaller than that for chloride ion affinities of benzyl cations, while the ρ value is similar in the three systems. This means that the 1-phenylvinyl cation has an intermediate resonance demand between those of the α-cumyl cation and the benzyl cation. Further, the r+ value of 1.18 for 1-phenylvinyl cation obeys the same “resonance demand vs. carbocation stability" relationship observed for a series of ordinary sp2-hybridized benzylic carbocations. This reveals that the π-delocalization mechanism in the vinyl cation system has no unique feature but it is a part of a continuous spectrum of resonance demands of varying benzylic carbocations characterized by r+ = 1.18. A comparison of the present GB values with the substituent effect for the acid-catalyzed hydration of phenylacetylenes in solution has shown that the r+ value is perceptibly smaller in the latter case. This suggests that the π-delocalization in the transition state differs considerably from that of the stable phenylvinyl cation intermediate. This is in contrast to the situation in the SN1 solvolysis where the r+ value is identical to the value characteristic for the formation of the fully developed carbocation.

Background inward current in ventricular and atrial cells of the guinea-pig.

Atrial and ventricular myocytes were exposed to Ca(2+)- and K(+)-free solutions containing blockers of gated channel and exchange currents. Replacement of external sodium by large organic cations revealed a background sodium current ib,Na. In atrial cells, the average conductance was 5.0 pS pF-1. In ventricular cells the conductance was 2.3 pS pF-1. Together with previous results, these figures reveal a strong gradient of background current density: sinus > atrium > ventricle. Replacement of sodium with inorganic cations showed that the channel selectivity behaves like an Eisenman group III/IV sequence, in agreement with previous results. The permeability of the channel to TMA was found to be pH dependent, suggesting that protonation of the channel is a factor determining permeation in addition to ionic size. The values of gb,Na obtained from these experiments are very similar to those assumed in computer modelling of cardiac cell electrical activity.

Studies in the Grazing of Heather Moorland in North-East Scotland. III. Floristics

By the method of SPE-GC/ECD, this paper studied the distribution and composition of organochlorine pesticides (OCPs) in the surface water body of Chaohu Lake. At the 13 sampling sites of study area, a total of 16 kinds of OCPs, including HCHs, DDTs, beta-Endosulfan, Aldrin, hexachlorobenzene, and Heptachlor, were detected. The total OCPs concentration was 0.54-64.01 ng x L(-1), and the main components were HCHs and DDTs, whose average concentrations were 5.56 ng x L(-1) and 16.57 ng x L(-1), respectively, accounting for about 84.75% of the total. The origins of OCPs in study area were historical residues and the recent discharge of pesticides. The concentrations of all existences accorded with the target values of GB 3838-2002. The concentrations of OCPs in the surface water body of Chaohu Lake were much lower, and the risk was low in general.

An evaluation of the roles of intercrystalline and interphase boundary sliding in two-phase superplastic alloys

Experiments were conducted to measure the contribution of boundary sliding in the superplastic Zn-22% Al eutectoid alloy at an elongation of 100%. The results show that the sliding contribution is a maximum in the superplastic region II and decreases in the less superplastic regions I and III. The values obtained for the sliding contribution are very low due to two physical constraints in the experimental procedure at high specimen elongations: 1. (i) a failure to account for the increase in surface area with elongation and 2. (ii) a limitation on the maximum feasible offset in surface marker lines. In Zn-22% Al, maximum sliding tends to occur at the Zn-Zn intercrystalline boundaries, there is slightly less sliding at the Zn-Al interphase boundaries and sliding is a minimum at the Al-Al intercrystalline boundaries. It is demonstrated that the results on Zn-22% Al and earlier experimental data on the Pb-62% Sn eutectic alloy are consistent with the occurrence of maximum sliding on the interfaces having the highest values of δD gb , where δ is the width of the boundary and D gb is the grain boundary diffusion coefficient.

Grain boundary contribution to the electrical conductivity of polycrystalline Cu films

Cu films of 1000 AA were grown at various substrate temperatures with increasing grain sizes at 10-5 Torr. The grain boundary resistivity ( rho gb=3.58*10-12 Omega cm2) and reflection coefficient (R=0.2635) at room temperature (22 degrees C) remained almost the same for different grain sizes, suggesting that the potential barrier between adjoining grains remains the same. Impurities and other defects have negligible effect on these values of rho gb and R. Results support the Mayadas-Schatzkes (M-S) grain boundary model (1970) of electrical conductivity for polycrystalline films.