Resonance energies of the allyl cation and allyl anion: contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of propene

Density functional theory was employed to calculate the acidities and hydride abstraction enthalpies of propene (3) and propane (4), along with their vinylogues (5 and 6, respectively). The same reaction enthalpies were calculated for the propene vinylogues in which the terminal vinyl group was rotated perpendicular to the rest of the conjugated system (7). The contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of each vinylogue of 5 (n = 1-3) was computed and extrapolated to n = 0 (the parent propene system). The resonance energies of the allyl cation and anion were determined to be about 20-22 and 17-18 kcal/mol, respectively. Comparisons are made to resonance energies calculated using other methodologies.     

Vinyl hydrogen acidities of two stereoisomers

The gas-phase acidities of the vinyl hydrogens of cis- and trans-2-butene were measured by the silane kinetic method in a Fourier-transform ion cyclotron resonance spectrometer. The acidities of ethene and the secondary vinyl hydrogen of propene were measured by the same method. The method was calibrated using the known acidities of methane and benzene. The vinyl hydrogens of trans-2-butene are more acidic than the vinyl hydrogens of cis-2-butene by 4.5 kcal/mol; the acidities of ethene and the secondary vinyl hydrogen of propene are between those of the two butenes. The acidity of cis-2-butene is 409 +/- 2 kcal/mol, and the acidity of trans-2-butene is 405 +/- 2 kcal/mol. Density functional theory calculations are in good agreement with the experiments. The results are discussed in terms of steric interactions, polarizabilities, dipole-dipole interactions, and charge-dipole interactions.     

Determination of the gas-phase acidities of cysteine-polyalanine peptides using the extended kinetic method

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA3 and HSCA4, using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (Delta(acid)H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be Delta(acid)H(HSCA3) = 317.3 +/- 2.4 kcal/mol and Delta(acid)H (HSCA4) = 316.2 +/- 3.9 kcal/mol. Large entropy effects (Delta(DeltaS) = 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (Delta(acid)G(app)) of 322.1 +/- 2.4 kcal/mol (HSCA3) and 320.1 +/- 3.9 kcal/mol (HSCA4), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation.     

Acidities in cyclohexanediols enhanced by intramolecular hydrogen bonds

Equilibrium gas-phase acidities of the six isomeric cyclohexanediols were measured in a Fourier transform ion cyclotron resonance mass spectrometer. Although all six cyclohexanediols have the same functional groups and similar structures, the acidities vary over 11 kcal/mol. This large difference is due mostly to the balance between hydrogen bonding and geometric strain. To understand the origins of the acidity differences in more detail, the conformations and energetics of the cyclohexanediols were studied using density functional theory, which gave good agreement with the experimental acidities. Finally, methanol-methoxide and methanol-methanol interactions were used as a model for the hydrogen bonding.     

Determination of the electron affinities of alpha- and beta-naphthyl radicals using the kinetic method with full entropy analysis. The C-H bond dissociation energies of naphthalene.

The C - H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (Delta H (acid)) and electron affinity (EA) for both the alpha- and beta-positions. The gas-phase acidity of the naphthalene alpha- and beta-positions and the EAs of the alpha- and beta-naphthyl radicals were measured in the gas phase in a flowing after glow-triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding alpha- and beta-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both pi and sigma radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 +/- 1.4 and 31.4 +/- 1.0 kcal mol(-1) (1 kcal = 4.184 kJ) for the alpha- and beta-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed penta coordinate hydroxysiliconate intermediate. The measured acidities are 394.0 +/- 5.0 and 397.6 +/- 4.8 kcal mol(-1) for the alpha- and beta- positions, respectively. The C - H bond dissociation energies calculated from the thermochemical cycle are 113.4 +/- 5.2 and 115.4 +/- 4.9 kcal mol(-1) for the alpha- and beta-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene.     

Acidity of anilines: calculations vs experiment

The gas-phase acidities of ca. 60 monosubstituted anilines (with acidity span of almost 50 kcal mol(-1)) have been calculated using density functional theory (DFT) at the B3LYP/6-311+G** level. At this relatively simple level of theory the calculated (ΔG(calc)) and available experimental (ΔG(exp)) acidities are in reasonable quantitative correlation according to the following equation: ΔG(obs) = a + bΔG(calc), where a=20.79, b=0.942, n=27, R(2)=0.990, and s=0.78 kcal·mol(-1). The slope is not far from its ideal value. Substituent effects on the acidities were dissected separately into those operating in the neutral acid molecule and in its conjugated anion using the isodesmic homodesmotic reactions. All in all, both forms, neutral and anionic, are contributing in combination to make up the gross acidity of anilines. However, the contributions of the anions into the gross substituent effects are much larger than the substituent effects in the neutral anilines. Some of the systems were used in testing a relatively new theoretical model, COSMO-RS (conductor-like screening model for real solvents), using it for the prediction of pK(a) values in DMSO. The method proved to be rather accurate for showing pK(a) trends (R(2)=0.980 in DMSO). However, the predicted absolute pK(a) values were all somewhat lower (rmsd=2.49 kcal·mol(-1)) than the respective experimental values.     

Polar group enhanced gas-phase acidities of carboxylic acids: an investigation of intramolecular electrostatic interaction

We studied the effects of polar groups on the gas-phase acidities of carboxylic acids experimentally and computationally. In this connection, the gas-phase acidities (DeltaH(acid), the enthalpy of deprotonation, and DeltaG(acid), the deprotonation free energy) of borane-complexed methylaminoacetic acid ((CH(3))2N(BH(3))CH(2)CO(2)H) and methylthioacetic acid (CH(3)S(BH(3))CH(2)CO(2)H) were measured using the kinetic method in a flowing afterglow-triple quadrupole mass spectrometer. The values of DeltaH(acid) and DeltaG(acid) of (CH(3))2N(BH(3))CH(2)CO(2)H were determined to be 328.8 +/- 1.9 and 322.1 +/- 1.9 kcal/mol, and those of CH(3)S(BH(3))CH(2)CO(2)H were determined to be 325.8 +/- 1.9 and 319.2 +/- 1.9 kcal/mol, respectively. The theoretical enthalpies of deprotonation of (CH(3))2N(BH(3))CH(2)CO(2)H (329.2 kcal/mol) and CH(3)S(BH(3))CH(2)CO(2)H (325.5 kcal/mol) were calculated at the B3LYP/6-31+G(d) level of theory. The calculated enthalpies of deprotonation of N-oxide-acetic acid (CH(3)NOCH(2)CO(2)H, 329.4 kcal/mol) and S-oxide-acetic acid (CH(3)SOCH(2)CO(2)H, 328.6 kcal/mol) are comparable to the experimental results for borane-complexed methylamino- and methylthioacetic acids. The enthalpy of deprotonation of sulfone-acetic acid (CH(3)SO2CH(2)CO(2)H, 326.1 kcal/mol) is about 2 kcal/mol lower than the S-oxide-acetic acid. The calculated enthalpy of deprotonation of sulfoniumacetic acid, (CH(3))2S+CH(2)CO(2)H, is 243.0 kcal/mol. Compared to the corresponding reference molecules, CH(3)NHCH(2)CO(2)H and CH(3)SCH(2)CO(2)H, the dipolar group and the monopolar group substituted carboxylic acids are stronger acids by 11-14 and 97 kcal/mol, respectively. We correlated the changes of the acidity upon a polar group substitution to the electrostatic free energy within the carboxylate anion. The acidity enhancements in polar group substituted carboxylic acids are the results of the favorable electrostatic interactions between the polar group and the developing charge at the carboxyl group.     

Ab initioMO calculations on the acidities of water and methanol,and hydrogen bond energies of the conjugate ions with a water molecule

The acidities, deprotonation energies, of water and methanol were calculated by the use of the ab initio self-consistent-field (SCF ) molecular orbital (MO ) method with electron correlation computed by the thirdorder Møller–Plesset perturbation method and configuration interaction with double excitations. Zero-point vibrational energy correction translational energy change, and the PV work term were included to evaluate the accurate acidities. The calculated acidity difference including these corrections was 7 kcal/mol, which is somewhat smaller than the experimental ones (9.5–12.5 kcal/mol) recently determined. The hydrogen bond energies of the conjugate ions (OH^? and CH₃O^?) with a water molecule were calculated to be 2.3 kcal/mol near the Hartree–Fock limit; this energy only amounts to 25% of the (total) hydration energy difference between the two negative ions. The aqueous solvation effect on the acidity scale was discussed.     

Substituent effects on the acidity of weak acids. 1. Bicyclo[2.2.2]octane-1-carboxylic acids and bicyclo[1.1.1]pentane-1-carboxylic acids

The acidities of 3- and 4-substituted bicyclooctane-1-carboxylic acids and 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids have been calculated at the MP2/6-311++G** theoretical level. There is good agreement between the calculated and observed gas-phase acidities. The acidities of the 4-substituted bicyclooctane acids were found to be linearly dependent on the C-X bond dipoles, as expected from a field effect. The substituents had a negligible effect on the electron density at C1. The difference in acidity between 4-chlorobicyclo[2.2.2]octane-1-carboxylic acid and the parent acid (6.2 kcal/mol) is reproduced by the Kirkwood-Westheimer treatment of substituent effects on acidity, but only if the bicyclooctane ring is given an effective dielectric constant of unity. The acidities of the 3-substituted bicyclooctane acids are linearly related to the corresponding 4-substituted acids with a slope of 0.9. The acidities of the 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids are linearly related to the C-X bond dipoles for this ring system (which are different than those for the bicyclooctanes), and they are also linearly related to the acidity of the 4-substituted bicyclo[2.2.2]octanecarboxylic acids with a slope of 1.34. The larger slope is due to the smaller bridgehead-bridgehead distance in the bicyclopentane ring than in bicyclo[2.2.2]octane.     

Background of the Hammett equation as observed for isolated molecules: meta- and para-substituted benzoic acids

Fundamental model compounds for the Hammett equation, meta- and para-substituted benzoic acids, were investigated by the density functional theory at the B3LYP/6-311+G(d,p) level. Energies of 25 acids and of their anions were calculated in all possible conformations and from them the energies of the assumed mixture of conformers. Relative acidities correlated with the experimental gas-phase acidities almost within the experimental uncertainty, much more precisely than in the case of previous calculations at lower levels. Dissection of the substituent effects into those operating in the acid molecule and in the anion was carried out by means of isodesmic reactions starting from monosubstituted benzenes. Both effects are cooperating in the resulting effect on the acidity; those in the acid molecule are smaller but not negligible. They are also responsible for some deviations from the Hammett equation (through-resonance of para donor substituents) and for the weaker resonance in the acid molecule in meta derivatives; in the anions the inductive and resonance effects are almost equal. On the other hand, the cooperation of effects in the acid and in the anion makes the relative acidity more sensitive to electron withdrawing and is probably one of the reasons why the Hammett equation is so generally valid.     

Thermochemical studies of N-methylpyrazole and N-methylimidazole

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.     

New insight on the origin of the unusual acidity of Meldrum's acid from ab initio and combined QM/MM simulation study.

Ab initio molecular orbital and combined QM/MM Monte Carlo simulations have been carried out to investigate the origin of the unusually high acidity of Meldrum's acid. Traditionally, the high acidity of Meldrum's acid relative to that of methyl malonate has been attributed to an additive effect due to the presence of two E esters in the dilactone system. However, the present study reveals that there is significant nonadditive effect that also makes major contributions. This results from preferential stabilization of the enolate anion over that of Meldrum's acid due to anomeric stereoelectronic interactions. To investigate solvent effects on the acidity in aqueous solution, the relative acidities of Z and E conformers of methyl acetate have been determined in combined ab initio QM/MM simulations. There is significant solvent effect on the conformational equilibria for both the neutral ester and its enolate anion in water, leading to stabilization of the E stereoisomer. However, the computed solvent effect of 4.4 kcal/mol in favor of the E isomer of methyl acetate is largely offset by the favorable solvation of 3.4 kcal/mol for the E conformer of the enolate anion. This leads to an enhanced acidity of 3.4 kcal/mol for the (E)-methyl acetate in water over the Z conformer. In Meldrum's acid, it is the preferential stabilization of the enolate anion due to anomeric effects coupled with the intrinsically higher acidity of the E conformation of ester that is responsible for its high acidity.     

Interpretation of Brønsted acidity by triadic paradigm: a G3 study of mineral acids

Deprotonation enthalpies and the gas-phase acidities of 24 inorganic acids are calculated by using composite G3 and G2 methodologies. The computed values are in very good accordance with available measured data. It is found that the experimental DeltaH(acid) values of the FSO(3)H and CF(3)SO(3)H are too high by some 6 and 7 kcal mol(-1), respectively. Furthermore, a new DeltaH(acid) value for HClO(4) of 300 kcal mol(-1) is recommended and suggested as a threshold of superacidicity in the gas phase. The calculated deprotonation enthalpies are interpreted by employing the trichotomy paradigm. Taking into account that the deprotonation enthalpy is a measure of acidity, it can be safely stated that the pronounced acidities of mineral acids are to a very large extent determined by Koopmans' term with very few exceptions, one of them being H(2)S. To put it in another way, acidities are predominantly a consequence of the ability of the conjugate bases to accommodate the excess electron charge, since Koopmans' term in trichotomy analysis is related to conjugate base anion. The final state is decisive in particular for superacids like ClSO(3)H, CF(3)SO(3)H, HClO(4), HBF(4), HPF(6), HAlCl(4), and HAlBr(4). However, in the latter two molecules the bond dissociation energy of the halogen-H bond substantially contributes to their high acidity too. Therefore, acidity of these two most powerful superacids studied here is determined by cooperative influence of both initial and final state effects. It should be emphasized that acidity of hydrogen halides HCl and HBr is a result of concerted action of all three terms included in triadic analysis. A byproduct of the triadic analysis are the first adiabatic ionization energies of the anionic conjugate bases. They are in fair to good agreement with the experimental data, which are unfortunately sparse. A fairly good qualitative correlation is found between the gas-phase deprotonation enthalpies of six mineral O-H acids and available Hammett-Taft sigma(p)- constants of the corresponding substituent groups.     

Gaseous acidity and basicity scales as guides to chemical ionization in reversed-phase liquid chromatography-mass spectrometry with direct liquid introduction

During direct liquid introduction (DLI) liquid chromatography-mass spectrometry (LC-MS), the detectability of acetone is shown to be predictable, based on consideration of the gas-phase acidities and basicities of solvents methanol and acetonitrile and modifiers formic acid and ammonium formate. Consequently, ion formation in DLI LC-MS resembles gas-phase chemical ionization processes; since the order of acidities is altered in solution, solution ionization must be much less important than gas-phase. Ion populations could be predicted on the basis of gas-phase proton affinities and acidities: acetone could be detected in all solvent mixture by positive ions, but it was not readily detected by negative ions when acid was also present. In solvents without additives it was always detected.     

The gas-phase acidity of 2(3H)-oxepinone: a step toward an experimental heat of formation for the 2-oxepinoxy radical

In an effort to gain further insight into the oxidation of the phenyl radical, this contribution details the first of three experiments designed to establish the heat of formation of the 2-oxepinoxy radical. We report here the synthesis of the previously unknown 2(7H)-oxepinone (12a) and 2(3H)-oxepinone (12b). We have determined the gas-phase acidity (Delta(acid)H(298)) of 12b by means of a bracketing study employing a flowing afterglow apparatus with quadrupole mass spectrometric detection. In this experiment, compound 12b was reacted in the gas phase with a series of bases of varying strength. A proton-transfer reaction was observed when 12b was reacted with t-BuS(-), but not when 12b was reacted with HS(-). We conclude that the gas-phase acidity of 12b lies between those of t-BuSH and H(2)S, and it is thereby assigned a value of Delta(acid)H(298) = 352 +/- 2 kcal/mol. Additional support for this value was found by performing the reverse reactions (i.e. the 2-oxepinoxy anion (15a) was reacted with proton sources of differing acidities). Anion 15a underwent a proton-transfer reaction with H(2)S but not with t-BuSH, in agreement with the results from the forward reactions. The experimental value of the gas-phase acidity agrees well with those from DFT calculations, which predicted Delta(acid)H(298) = 348.9 kcal/mol at the B3LYP/6-31+G(d) level and 349.2 kcal/mol at the B3LYP/aug-cc-pVTZ level.     

Acidity of adenine and adenine derivatives and biological implications. A computational and experimental gas-phase study

The gas-phase acidities of adenine, 9-ethyladenine, and 3-methyladenine have been investigated for the first time, using computational and experimental methods to provide an understanding of the intrinsic reactivity of adenine. Adenine is found to have two acidic sites, with the N9 site being 19 kcal mol(-1) more acidic than the N10 site; the bracketed acidities are 333 +/- 2 and 352 +/- 4 kcal mol(-1), respectively. Because measurement of the less acidic site can be problematic, we benchmarked the adenine N10 measurement by bracketing the acidity of 9-ethyladenine, which has the N9 site blocked and allows for exclusive measurement of the N10 site. The acidity of 9-ethyladenine brackets to 352 +/- 4 kcal mol(-1), comparable to that of the N10 site of the parent adenine. Calculations and experiments with 3-methyladenine, a harmful mutagenic nucleobase, uncovered the surprising result that the most commonly written tautomer of 3-methyladenine is not the most stable in the gas phase. We have found that the most stable tautomer is the "N10 tautomer" 10, as opposed to the imine tautomer 3. The bracketed acidity of 10 is 347 +/- 4 kcal mol(-1). Since 10 is not a viable species in DNA, 3 is a likely tautomer; calculations indicate that this form has an extremely high acidity (320-323 kcal mol(-1)). The biological implications of these results, particularly with respect to enzymes that cleave alkylated bases from DNA, are discussed.     

The isomerization of methoxy radical: intramolecular hydrogen atom transfer mediated through acid catalysis

The catalytic ability of water, formic acid, and sulfuric acid to facilitate the isomerization of the CH(3)O radical to CH(2)OH has been studied. It is shown that the activation energies for isomerization are 30.2, 25.7, 4.2, and 2.3 kcal mol(-1), respectively, when the reaction is carried out in isolation and with water, formic acid, or sulfuric acid as a catalyst. The formation of a doubly hydrogen bonded transition state is central to lowering the activation energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. The changes in the rate constant for the CH(3)O-to-CH(2)OH isomerization with acid catalysis have also been calculated at 298 K. The largest enhancement in the rate, by over 12 orders of magnitude, is with sulfuric acid. The results of the present study demonstrate the feasibility of acid catalysis of a gas-phase radical isomerization reaction that would otherwise be forbidden.     

Toward the prediction of the activity of antioxidants: Experimental and theoretical study of the gas-phase acidities of flavonoids

The relative gas-phase acidities were determined for eight flavonoids, applying the kinetic method, by means of electrospray-ion trap mass spectrometry. The experimental acidity order, myricetin > luteolin > quercetin > (+/-)-taxifolin > kaempferol > apigenin > (+)-catechin > (+/-)-naringenin shows good agreement with the order obtained by theoretical calculations at the B3LYP/6-311 + G(2d,2p)//HF/6-31G(d) level. Moreover, these calculations provide the gas-phase acidities of the different OH groups for each flavonoid. The calculated acidity values (Delta(ac)H), corresponding to the most favorable deprotonation, cover a narrow range, 314.8-330.1 kcal/mol, but the experimental method is sensitive enough to differentiate the acidity of the various flavonoids. For all the flavones and the flavanol, catechin, the 4'-hydroxyl group is the most favored deprotonation site whereas for the flavanones studied, taxifolin and naringenin, the most acidic site is the 7-hydroxyl group. On the other hand, the 5-hydroxyl, in flavones and naringenin, and the 3-hydroxyl, in taxifolin and catechin, are always the less acidic positions. The acidity pattern observed for this family of compounds mainly depends on the following structural features: The ortho-catechol group, the 2,3 double bond and the 4-keto group.     

The RS-.HSR Hydrogen Bond: acidities of alpha,omega-dithiols and electron affinities of their monoradicals

Gas-phase acidities (deltaGo(acid)) have been measured for 1,2-ethanedithiol, 1,3-propanedithiol, and 1,4-butanedithiol, using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Adiabatic electron affinities (EAs) of the thiolate monoradicals of these compounds were assigned from electron photodetachment spectra of their corresponding thiolate monoanions, acquired using a cw-ICR. The dithiols exhibit enhanced acidities (up to 8.7 kcal/mol in deltaGo(acid)) and greater EAs (up to 6.7 kcal/mol) than analogous monothiol species. These differences are attributed to an intramolecular RS-.HSR hydrogen bond in the thiolate anion. Considerations of the RO-.HOR hydrogen bond in monoanions of alpha,omega-diols and in the [CH(3)O-.HOCH(3)] complex anion suggest that the RS-.HSR hydrogen bond provides up to 9 kcal/mol extra stabilization.     

The acidity and proton affinity of the damaged base 1,N6-ethenoadenine in the gas phase versus in solution: intrinsic reactivity and biological implications

1,N(6)-ethenoadenine (epsilonA) is a highly mutagenic lesion that is excised from human DNA by the enzyme alkyladenine DNA glycosylase (AAG). In an effort to understand the intrinsic properties of 1,N(6)-ethenoadenine, we examined its gas phase acidity and proton affinity using quantum mechanical calculations and mass spectrometric experimental methods. We measure two acidities for epsilonA: a more acidic site (DeltaH(acid) = 332 kcal mol(-1); DeltaG(acid) = 325 kcal mol(-1)) and a less acidic site (DeltaH(acid) = 367 kcal mol(-1); DeltaG(acid) = 360 kcal mol(-1)). We also find that the proton affinity of the most basic site of 1,N(6)-ethenoadenine is 232-233 kcal mol(-1) (GB = 224 kcal mol(-1)). These measurements, when compared to calculations, establish that, under our experimental conditions, we have only the canonical tautomer of 1,N(6)-ethenoadenine present. We also compare the gas phase acidic properties of epsilonA with that of the normal bases adenine and guanine and find that epsilonA is the most acidic. This supports the theory that AAG and other related enzymes may cleave damaged bases as anions. Furthermore, comparison of the gas phase and aqueous acidities indicates that the nonpolar environment of the enzyme enhances the acidity differences of epsilonA versus adenine and guanine.