Protonation Entropy Research Articles

Proton polytropic behavior of periodic density structures in the solar wind

Context. In recent years, mesoscales have gained scientific interest because they have been determined to be important in a broad range of phenomena throughout heliophysics. The solar wind mesoscale structures include periodic density structures (PDSs), which are quasi-periodic increases in the density of the solar wind that range from a few minutes to a few hours. These structures have been extensively observed in remote-sensing observations of the solar corona and in in situ observations out to 1 AU, where they manifest as radial length scales greater than or equal to the size of the Earth’s dayside magnetosphere, that is, from tens to hundreds of Earth radii (RE). While the precise mechanisms that form PDSs are still debated, recent studies confirmed that most PDSs are of solar origin and do not form through dynamics during their propagation in the interplanetary space. Aims. We further investigate the origin of PDSs by exploring the thermodynamic signature of these structures. To do this, we estimate the values of the effective polytropic index (Y) and the entropy of protons, which in turn are compared with the corresponding values found for the solar wind. Methods. We used an extensive list of PDS events spanning more than two solar cycles of Wind measurements (the entire Wind dataset from 1995 to 2022) to investigate the thermodynamic signatures of PDSs. With the use of wavelet methods, we classified these PDSs as coherent or incoherent, based on the shared periodic behavior between proton density and alpha-to-proton ratio, and we derive the proton polytropic index. Results. Our results indicate that the coherent PDSs exhibit lower Y values (Ῡ≈1.54) on average and a higher entropy than the values in the entire Wind dataset (Ῡ≈1.79), but also exhibit similarities with the magnetic cloud of an interplanetary coronal mass ejection. In contrast, incoherent PDSs exhibit the same Y values as those of the entire Wind dataset.

Slow solar wind modeling of the Metis/Solar Orbiter – Parker Solar Probe quadrature

In January 2021, Metis/SolO and PSP formed a quadrature from which the slow solar wind was able to be measured from the extended solar corona (3.5 – 6.3 R⊙) to the very inner heliosphere (23.2 R⊙). Metis/SolO remotely measured the coronal solar wind, finding a speed of 96 – 201 kms−1, and PSP measured the solar wind in situ, finding a speed of 219.34 kms−1. Similarly, the normalized cross-helicity and the normalized residual energy measured by PSP are 0.96 and -0.07. In this manuscript, we study the evolution of the proton entropy and the turbulence cascade rate of the outward Elsässer energy during this quadrature. We also study the relationship between solar wind speed, density and temperature, and their relationship with the turbulence energy, the turbulence cascade rate, and the solar wind proton entropy. We compare the theoretical results with the observed results measured by Metis/SolO and PSP.

Thermodynamic and spectroscopic investigations of Shewanella putrefaciens CN32 cell envelope

A quantitative understanding of prokaryotic cell envelope reactivity is necessary to accurately describe the surface complexation reactions known to impact metal transport in the subsurface. The gram-negative bacterium Shewanella putrefaciens CN32 (CN32) has been the focus of previous cell envelope studies in large part due to its ability to directly influence iron and uranium geochemistry; however, important gaps remain in our understanding of reactions occurring on its cell envelope. Combining surface complexation modeling of potentiometric titration data with isothermal titration calorimetry provides an additional method to evaluate how well surface complexation models (SCMs) represent cell envelope reactivity. It also gives us the ability to determine site-specific enthalpies and entropies of protonation, which can aid both in site structure identification and description of conformational changes to those sites, further improving our understanding of the CN32 cell envelope. We gathered potentiometric and isothermal calorimetric titration data of CN32 over a range of ionic strength (0.02 – 0.53 M) while monitoring cell viability and performing Fourier transform infrared spectroscopy (FTIR) investigations. FTIR analysis was performed over a range of pH (4 – 9.4) to confirm identification of proton-active sites. Surface complexation modeling revealed that models involving 3 and 4 sites could adequately describe the potentiometric titration data; however, a 3-site model best represented the calorimetric data. We inferred the following site identities based on characteristic enthalpies, entropies, and equilibrium constants of protonation: carboxyl (log(K): 4.79 – 4.95), phosphoryl (6.69 – 6.97), and amine groups (9.37 – 9.64). Spectral corroboration via FTIR indicated the presence of chemical moieties consistent with these functional group assignments. Ionic strength was shown to have a significant impact on the concentration of all three sites at 95% confidence. Entropies and equilibrium constants of protonation for carboxyl and phosphoryl sites did not significantly change with ionic strength. For the carboxyl group, enthalpies of protonation varied significantly between the lowest and highest ionic strengths examined, though enthalpies at intermediate concentrations were not distinguishable. Enthalpies of protonation for the amine group were statistically different at each ionic strength. Additionally, entropies and equilibrium constants of protonation for the amine group were significantly affected by all ionic strengths tested. The entropy of protonation for the amine site changed from −21.5 ± 6.1 to +109.0 ± 4.2 J/molK−1 as ionic strength increased from 0.02 to 0.53 M, respectively. A change in the entropy of protonation of this magnitude and sign is indicative of a substantial conformational change in this functional group. Cell viability was significantly impacted over the course the titrations; however, viability did not impact SCMs results. In sum, these data fill gaps in our understanding of CN32 and provide new insights into the intricacies of its cell envelope.

Proton entropy excess and possible signature of pairing reentrance in hot nuclei

The entropy excess caused by one proton (proton entropy excess) has been extracted for several pairs of medium and heavy mass nuclei such as (237U and 238Np), (231Th and 232Pa), (211Po and 212At), (196Pt and 197Au), and (90Y and 91Zr) using the available nuclear level density data. The nature of entropy excess as a function of excitation energy E⁎ is found in a reasonable agreement with the microscopic calculations based on the exact pairing plus independent-particle model at finite temperature (EP+IPM). It is observed that the proton entropy excess is ∼0.1−0.5kB for the spherical systems and ∼1.0−1.2kB for the deformed ones, which is notably smaller than that of neutron entropy excess (∼1.3−2.0kB). This is due to the effect of Coulomb interaction as well as the single-particle level density of protons, which is less than that of neutrons. Moreover, a peak-like structure in the entropy excess observed at low E⁎<1 MeV is possibly associated with the pairing reentrance phenomenon caused by the weakening of blocking effect in odd nuclei at low temperature within the EP+IPM. However, this peak-like structure, which is more pronounced in the spherical nuclei than in the deformed isotopes, is not well supported by experimental observation due to strong fluctuations of the measured data. Hence, further experimental and theoretical studies are called in order to understand this effect.

Preoperative volumetric synthetic magnetic resonance imaging of the primary tumor for a more accurate prediction of lymph node metastasis in rectal cancer.

An accurate assessment of lymph node (LN) status in patients with rectal cancer is important for treatment planning and an essential factor for predicting local recurrence and overall survival. In this study, we explored the potential value of histogram parameters of synthetic magnetic resonance imaging (SyMRI) in predicting LN metastasis in rectal cancer and compared their predictive performance with traditional morphological characteristics and chemical shift effect (CSE). A total of 70 patients with pathologically proven rectal adenocarcinoma who received direct surgical resection were enrolled in this prospective study. Preoperative rectal MRI, including SyMRI, were performed, and morphological characteristics and CSE of LN were assessed. Histogram parameters were extracted on a T1 map, T2 map, and proton density (PD) map, including mean, variance, maximum, minimum, 10th percentile, median, 90th percentile, energy, kurtosis, entropy, and skewness. Receiver operating characteristic (ROC) curves were used to explore their predictive performance for assessing LN status. Significant differences in the energy of the T1, T2, and PD maps were observed between LN-negative and LN-positive groups [all P<0.001; the area under the ROC curve (AUC) was 0.838, 0.858, and 0.823, respectively]. The maximum and kurtosis of the T2 map, maximum, and variance of PD map could also predict LN metastasis with moderate diagnostic power (P=0.032, 0.045, 0.016, and 0.047, respectively). Energy of the T1 map [odds ratio (OR) =1.683, 95% confidence interval (CI): 1.207-2.346, P=0.002] and extramural venous invasion on MRI (mrEMVI) (OR =10.853, 95% CI: 2.339-50.364, P=0.002) were significant predictors of LN metastasis. Moreover, the T1 map energy significantly improved the predictive performance compared to morphological features and CSE (P=0.0002 and 0.0485). The histogram parameters derived from SyMRI of the primary tumor were associated with LN metastasis in rectal cancer and could significantly improve the predictive performance compared with morphological features and CSE.

Energy, Entropy and Quantum Tunneling of Protons and Electrons in Brain Mitochondria: Relation to Mitochondrial Impairment in Aging-Related Human Brain Diseases and Therapeutic Measures.

Adult human brains consume a disproportionate amount of energy substrates (2–3% of body weight; 20–25% of total glucose and oxygen). Adenosine triphosphate (ATP) is a universal energy currency in brains and is produced by oxidative phosphorylation (OXPHOS) using ATP synthase, a nano-rotor powered by the proton gradient generated from proton-coupled electron transfer (PCET) in the multi-complex electron transport chain (ETC). ETC catalysis rates are reduced in brains from humans with neurodegenerative diseases (NDDs). Declines of ETC function in NDDs may result from combinations of nitrative stress (NS)–oxidative stress (OS) damage; mitochondrial and/or nuclear genomic mutations of ETC/OXPHOS genes; epigenetic modifications of ETC/OXPHOS genes; or defects in importation or assembly of ETC/OXPHOS proteins or complexes, respectively; or alterations in mitochondrial dynamics (fusion, fission, mitophagy). Substantial free energy is gained by direct O2-mediated oxidation of NADH. Traditional ETC mechanisms require separation between O2 and electrons flowing from NADH/FADH2 through the ETC. Quantum tunneling of electrons and much larger protons may facilitate this separation. Neuronal death may be viewed as a local increase in entropy requiring constant energy input to avoid. The ATP requirement of the brain may partially be used for avoidance of local entropy increase. Mitochondrial therapeutics seeks to correct deficiencies in ETC and OXPHOS.

Proton Thermodynamics in a Protic Ionic Liquid, Ethylammonium Nitrate.

In order to investigate the proton solvation state in protic ionic liquids (PILs), ten acid dissociation enthalpies and entropies of eight compounds were determined in ethylammonium nitrate (EAN). Regardless of the nature of the compound, 24 kJ mol-1 larger enthalpy and 65 J mol-1 K-1 larger entropy than those in water, respectively, were observed. These values were reasonably explained by the differences in the proton solvation structure in EAN and water. Namely, protons in EAN exist as HNO3 , having a higher reaction energy than that of H3 O+ in water, undergo entropic stabilization as a result of the less-structured solvation. As such, the entropic effect of the proton solvation structure on the acid-base property is possibly applicable to all PILs. In addition, based on these proton thermodynamics, enthalpy and entropy windows were proposed as a novel perspective for the characterization of solvents. Use of this concept enabled the visualization of similarities and differences between EAN and water.

Acid-Promoted Crystallization of Amorphous Solid Water

In this study, the effect of externally added hydrogen chloride (HCl) on the crystallization of amorphous solid water (ASW) has been examined. ASW films containing small amounts of HCl, which dissociated into H+ (excess protons) and Cl– ions, and having a thickness of 90–360 monolayers (ML) were prepared on a Pt(111) substrate under an ultrahigh vacuum environment. The location of HCl in the samples was varied by controlling the stacking sequence of H2O and HCl during the film growth. Crystallization kinetics of these samples were examined by conducting temperature-programmed desorption experiments and isothermal kinetic measurements with reflection–absorption infrared spectroscopy for the temperature range of 137–145 K. Crystallization behaviors of pure and NaCl-doped ASW films were also examined for comparison. The results indicated that the excess protons accelerated the crystallization of the ASW films, in contrast to the retardation effect of Na+. In the presence of 0.1 ML HCl, the overall activation energy of ASW crystallization was reduced from 63.4 kJ·mol–1 in the absence of HCl to 48.5 kJ·mol–1, which is close to the activation energy of crystal growth. The crystallization started near the location of HCl injection in the samples, regardless of whether it was the vacuum/ASW interface or the film interior. These observations indicated that the excess protons facilitated the nucleation process and changed the rate-limiting step of ASW crystallization from nucleation to growth. An explanation based on thermodynamics has been proposed, that is, the configurational entropy of the excess protons in ice likely reduces the free-energy barrier of nucleation for the acid-doped ASW samples.

Entropy-Driven Spontaneous Reaction in Cryogenic Ice: Dissociation of Fluoroacetic Acids.

Chemical reactions are extremely difficult to occur in ice at low temperature, where atoms and molecules are frozen in position with minimal thermal energy and entropy. Contrary to this general behavior, certain weak acids including fluoroacetic acids dissociate spontaneously and more efficiently in cryogenic ice than in aqueous solution at room temperaure. The enhanced reactivity of weak acids is an unexpected consequence of proton-transfer equilibrium in ice. The configurational entropy of protons in ice shifts the acid dissociation equilibrium forward. This configurational entropy, although a solid-state property, is comparatively large in magnitude with the entropy of vaporization and can effectively drive proton-transfer reactions in ice.

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides.

This paper gathers structural and thermochemical informations related to the gas-phase basicity of molecules containing cyanides (nitriles) and isocyanides (isonitriles) functional groups. It constitutes the sixth part of a general review devoted to gas-phase basicities of polyfunctional compounds. A large corpus of cyanides and isocyanides molecules is examined under seven major chapters. In the first one, a rapid overview of the definitions and methods leading to gas-phase basicity, GB, proton affinity, PA, and protonation entropy, Δp S°, is given. In the same chapter, several aspects of the gas phase chemistry of protonated cyanides and isocyanides are also presented. Chapters II-VI detail the protonation energetics of aliphatic, unsaturated, and heteroatom substituted (halogens, O, S, N, P) cyanides. A seventh chapter is devoted to isocyanides. Experimental data available in the literature (120 references) were reevaluated according to the presently adopted basicity scale that is the NIST database anchored to PA(NH3 ) = 853.6 kJ/mol and GB (NH3 ) = 819 kJ/mol. In this latter source, however, several erroneous values have been identified which were corrected in the present review. Structural and energetic information given by G4MP2 quantum chemistry computations on ca. 60 typical systems are presented. The present review includes the GB, PA, and Δp S° values of ca. 110 cyanides and isocyanides, and, for selected examples, is completed by a set of computed heats of formation (Δf H°) at 0 and 298 K.

Vitamin C: an experimental and theoretical study on the gas-phase structure and ion energetics of protonated ascorbic acid.

In order to investigate the gas-phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan, we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6-31 + G(d,p) level of theory. The gaseous reactant species, the protonated AA, [C6 H8 O6 ]H+ , were generated by electrospray ionization of a 10-3 M H2 O/CH3 OH (1 : 1) AA solution. In order to structurally characterize the gaseous [C6 H8 O6 ]H+ ionic reactants, we estimated the proton affinity and the gas-phase basicity of AA by the extended Cooks's kinetic method and by computational methods at the B3LYP/6-31 + G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity of 875.0 ± 12 kJ mol-1 and protonation entropy ΔSp 108.9 ± 2 J mol-1 K-1 , a gas-phase basicity value of AA of 842.5 ± 12 kJ mol-1 at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations. Copyright © 2016 John Wiley & Sons, Ltd.

The trailing edges of high‐speed streams at 1 AU

AbstractThe trailing‐edge rarefactions of 54 high‐speed streams at 1 AU are analyzed. The temporal durations of the trailing‐edge rarefactions agree with ballistic calculations based on the observed speeds of the fast and slow wind bounding the rarefactions. A methodology is developed to measure solar‐wind compression and rarefaction using the orientations of solar‐wind current sheets. One focus is to determine the signature that best describes the location of the trailing‐edge stream interface between coronal‐hole‐origin plasma and streamer‐belt‐origin plasma; based on the current‐sheet orientations, on the magnetic‐field strength, on the intensity of the electron strahl, and on the intensity of the negative vorticity, an inflection point in the temporal profile of the solar‐wind velocity is taken as the best indicator of the trailing‐edge stream interface. Computer simulations support this choice. Using superposed‐epoch analysis, the plasma properties and turbulence properties of trailing‐edge rarefactions are surveyed. Whereas the signatures of the coronal‐hole/streamer‐belt (slow‐wind/fast‐wind) boundary in the leading edge (corotating interaction region) stream interface are simultaneous, they are not simultaneous in the trailing edge, with ion‐charge‐state signatures occurring on average 13.7 h prior to the proton entropy signature. It is suggested that differences in the leading and trailing edges of coronal holes on the Sun might account for the differences in the leading and trailing edges of high‐speed streams at 1 AU: the formation timescales, heating timescales, and charge‐state‐equilibration timescales of closed flux loops in the corona might be involved.

Triplex-forming PNA modified with unnatural nucleobases: the role of protonation entropy in RNA binding

Peptide nucleic acid (PNA) modified with unnatural nucleobases enables the formation of a highly stable triplex with a double-stranded RNA at physiological pH. In this communication, we evaluated kinetics and thermodynamics of PNA/RNA triplex formation as a function of both pH and temperature. Protonation entropy was found to be the major factor responsible for the destabilization of the triplex and for the progressive decrease in the association rate at more basic pHs.

Proton affinity of dipeptides containing alanine and diaminobutyric acid

Thermochemical properties, including the proton affinity, the gas-phase basicity, and the protonation entropy, were determined for the acetylated unnatural amino acid, 2,4-diaminobutyric acid, AcDab and two isomeric dipeptides containing alanine (Ala) and Dab, AcAlaDab and AcDabAla. The experiments were carried out using a triple quadrupole mass spectrometer. The extended Cooks kinetic method was applied to determine the values of the proton affinity. The proton affinities for the three compounds were determined to be 235.1±2.0kcal/mol (AcDab), 244.7±2.0kcal/mol (AcAlaDab), and 242.0±2.0kcal/mol (AcDabAla). The gas-phase basicities (GB) and the protonation entropies (ΔpS) for these peptides were determined accordingly. The results suggested that the dipeptides were more basic than the single amino acid by about 7kcal/mol, and the C-terminal Dab peptide, AcAlaDab, was more basic than the N-terminal Dab peptide, AcDabAla, by about 2.5kcal/mol. Computational studies were carried out via a series of steps including conformational search using the MMFF force field, followed by geometry optimization and energy calculations at the B3LYP/6-311++G(2d,p)//B3LYP/6-311+G(d) level of theory. Theoretically predicted proton affinities were in reasonably good agreement with the experiments. The higher basicity of the C-Dab dipeptide was likely due conformational effect that stabilized the charged C-Dab peptide more efficiently than the N-Dab peptide.

Thermodynamic characterization of proton‐ionizable functional groups on the cell surfaces of ammonia‐oxidizing bacteria and archaea

The ammonia-oxidizing archaeon Nitrosopumilus maritimus strain SCM1 (strain SCM1), a representative of the Thaumarchaeota archaeal phylum, can sustain high specific rates of ammonia oxidation at ammonia concentrations too low to sustain metabolism by ammonia-oxidizing bacteria (AOB). One structural and biochemical difference between N.maritimus and AOB that might be related to the oligotrophic adaptation of strain SCM1 is the cell surface. A proteinaceous surface layer (S-layer) comprises the outermost boundary of the strain SCM1 cell envelope, as opposed to the lipopolysaccharide coat of Gram-negative AOB. In this work, we compared the surface reactivities of two archaea having an S-layer (strain SCM1 and Sulfolobus acidocaldarius) with those of four representative AOB (Nitrosospira briensis, Nitrosomonas europaea, Nitrosolobus multiformis, and Nitrosococcus oceani) using potentiometric and calorimetric titrations to evaluate differences in proton-ionizable surface sites. Strain SCM1 and S.acidocaldarius have a wider range of proton buffering (approximately pH 10-3.5) than the AOB (approximately pH 10-4), under the conditions investigated. Thermodynamic parameters describing proton-ionizable sites (acidity constants, enthalpies, and entropies of protonation) are consistent with these archaea having proton-ionizable amino acid side chains containing carboxyl, imidazole, thiol, hydroxyl, and amine functional groups. Phosphorous-bearing acidic functional groups, which might also be present, could be masked by imidazole and thiol functional groups. Parameters for the AOB are consistent with surface structures containing anionic oxygen ligands (carboxyl- and phosphorous-bearing acidic functional groups), thiols, and amines. In addition, our results showed that strain SCM1 has more reactive surface sites than the AOB and a high concentration of sites consistent with aspartic and/or glutamic acid. Because these alternative boundary layers mediate interaction with the local external environment, these data provide the basis for further comparisons of the thermodynamic behavior of surface reactivity toward essential nutrients.

Explicit Solvation Shell Model and Continuum Solvation Models for Solvation Energy and pKa Determination of Amino Acids.

The study of the Explicit Solvation Shell Model (ESS) presented recently [da Silva, E. F.; Svendsen, H. F.; Merz, K. M. J. Phys. Chem. A 2009, 113, 6404.] for calculation of solvation free energy of ions is extended for the study for amino acids. Solvation free energies and pKa of a data set of 10 amino acids is calculated using ESS. The data set of amino acids is selected based on their potential to be regarded as solvents for postcombustion CO2 capture processes. Calculated results are compared against experimental pKa and pKa calculated from PCM, SM8T, and DivCon continuum solvation models. Error estimates of pKa from different models vs experimental pKa data are also given to evaluate the results calculated by different solvation models. This study also involves a comprehensive study of gas phase basicity, proton affinity,ΔGacid0, ΔHacid0, protonation entropy with density functional methods (B3LYP/6-311++G(d,p)) and composite methods (G3MP2B3, G3MP2, CBS-QB3, G4MP2) and their comparison with experimen...

Thorough theoretical search of conformations of neutral, protonated and deprotonated glutamine in gas phase

There are large discrepancies among the theoretical and experimental results of the proton affinity (PA) of glutamine (Gln). To provide a reliable basis for the theoretical investigation, extensive conformational searches have been performed for neutral, protonated and deprotonated Gln in gas phase by optimizing the trial structures generated by allowing for all combinations of internal single-bond rotamers. The structures and hydrogen bonding features, relative electronic energies, zero point vibrational energies, rotational constants, dipole moments, vertical ionization energies of the low energy conformers and equilibrium conformational distributions are presented. PA, GB (gas phase basicity), PDE (proton dissociation enthalpy) and GA (gas phase acidity) of Gln were computed by the theoretical approaches of BHandHLYP, B3LYP, B97D, MP2, G3MP2B3, M062X and CCSD. The computed relative conformational energies and PA, GB, PDE and GA are dependent on the theoretical approach and the basis set. Analysis of the computational results shows that the extended kinetic method provides an accurate estimate of PA and overestimate of the entropic effect, while all other experiments underestimate PA of Gln. The best theoretical estimates of PA, GB, PDE, GA and the protonation and deprotanation entropies for Gln are 987.2±4.0, 945.1±5.8, 1385.3±9.0, 1362.9±9.1kJ/mol and −32.4±6.2 and 33.9±5.1J/mol/K, respectively.

The Seebeck coefficient and the Peltier effect in a polymer electrolyte membrane cell with two hydrogen electrodes

We report that the Seebeck coefficient of a Nafion membrane cell with hydrogen electrodes saturated with water vapour, at 1bar hydrogen pressure and 340K, is equal to 670±50μV/K, meaning that the entropy change of the anode reaction at reversible conditions (67J/(Kmol)) corresponds to a reversible heat release of 22kJ/mol. The transported entropy of protons across the membrane at Soret equilibrium was estimated from this value to 1±5J/(Kmol). The results were supported by the expected variation in the Seebeck coefficient with the hydrogen pressure. We report also the temperature difference of the electrodes, when passing electric current through the cell, and find that the anode is heated (a Peltier heat effect), giving qualitative support to the result for the Seebeck coefficient. The Seebeck and Peltier effects are related by non-equilibrium thermodynamics theory, and the Peltier heat of the cathode in the fuel cell is calculated for steady state conditions to 6±2kJ/mol at 340K. The division of the reversible heat release between the anode and the cathode, can be expected to vary with the current density, as the magnitude of the current density can have a big impact on water transport and water concentration profile.

Gas-phase structures and thermochemistry of neutral histidine and its conjugated acid and base

Extensive exploration of the conformational space of neutral, protonated and deprotonated histidine has been conducted at the G4MP2 level. Theoretical protonation and deprotonation thermochemistry as well as heats of formation of gaseous histidine and its ionized forms have been calculated at the G4 level considering either the most stable conformers or an equilibrium population of conformers at 298 K. These theoretical results were compared to evaluated experimental determinations. Recommended proton affinity and protonation entropy deduced from these comparisons are PA(His) = 980 kJ mol(-1) and ΔpS(His) ∼ 0 J mol(-1) K(-1), thus leading to a gas-phase basicity value of GB(His) = 947.5 kJ mol(-1). Similarly, gas phase acidity parameters are ΔacidH(o)(His) = 1373 kJ mol(-1), ΔacidS(His) ∼ 10 J mol(-1) K(-1) and ΔacidG(o)(His) = 1343 kJ mol(-1). Computed G4 heats of formation values are equal to -290, 265 and -451 kJ mol(-1) for gaseous neutral histidine and its protonated and deprotonated forms, respectively. The present computational data correct, and complete, previous thermochemical parameter estimates proposed for gas-phase histidine and its acido-basic properties.

ChemInform Abstract: Gas Phase Basicities of Polyfunctional Molecules. Part 3. Amino Acids

AbstractReview: 217 refs.