Stabilization of an excess electron on molecular surfaces by a pair of HF molecules

This work presents the results of solvation of electrons on several hypothetical cyclooctane and cyclohexane molecular surfaces, using the hydrogen fluoride (HF) dimer. These complexes were constructed with extensive OH groups on one side of a hydrocarbon surface (i.e., cyclohexane sheets), which creates hydrogen‐bonded networks that can form, increasing the dipole moment of the system. Concurrently, the hydrogen atoms on the opposite side of the surface form a pocket of positive charge that can attract excess electrons. Two possible orientations for HF dimer solvation on eight molecular surfaces that have been demonstrated to be stable toward electron detachment are examined. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Gas-phase tautomers of protonated 1-methylcytosine. Preparation, energetics, and dissociation mechanisms

Tautomers of 1-methylcytosine that are protonated at N-3 (1+) and C-5 (2+) have been specifically synthesized in the gas phase and characterized by tandem mass spectrometry and quantum chemical calculations. Ion 1+ is the most stable tautomer in aqueous and methanol solution and is likely to be formed by electrospray ionization of 1-methylcytosine and transferred in the gas phase. Gas-phase protonation of 1-methylcytosine produces a mixture of 1+ and the O-2-protonated tautomer (3+), which are nearly isoenergetic. Dissociative ionization of 6-ethyl-5,6-dihydro-1-methylcytosine selectively forms isomer 2+. Upon collisional activation, ions 1+ and 3+ dissociate by loss of ammonia and [C,H,N,O], whose mechanisms have been established by deuterium labeling and ab initio calculations. The main dissociations of 2+ following collisional activation are losses of CH2=C=NH and HN=C=O. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations.     

Stabilization of NH tautomers of quinolines by osmium and ruthenium

Complexes OsH2Cl2(PiPr3)2 and RuH2Cl2(PiPr3)2 promote the tautomerization of quinoline and 8-methylquinoline to NH tautomers, which lie about 44 kcal.mol-1 above the usual CH tautomers. The NH tautomers are stabilized by coordination to the metal center and by means of a Cl...HN interaction. As a consequence, the six-coordinate elongated dihydrogen complexes OsCl2{kappa-C2-(HNC9H5R)}(eta2-H2)(PiPr3)2, the five-coordinate derivatives RuCl2{kappa-C2-(HNC9H5R)}(PiPr3)2, and the six-coordinate dihydrogen compounds RuCl2{kappa-C2-(HNC9H5R)}(eta2-H2)(PiPr3)2 (R = H, Me) have been isolated and characterized.     

The stabilization of arginine's zwitterion by dipole-binding of an excess electron

The arginine parent anion was generated by a newly developed, infrared desorption-electron photoemission hybrid anion source. The photoelectron spectrum of the arginine anion was recorded and interpreted as being due to dipole binding of the excess electron. The results are consistent with calculations by Rak, Skurski, Simons, and Gutowski, who predicted the near degeneracy of arginine's canonical and zwitterionic dipole bound anions. Since neutral arginine's zwitterion is slightly less stable than its canonical form, this work also demonstrates the ability of an excess electron to stabilize a zwitterion, just as ions and solvent molecules are already known to do.     

On the unusual stability of valence anions of thymine based on very rare tautomers: A computational study

We characterized anionic states of thymine using various electronic structure methods, with the most accurate results obtained at the CCSD(T)/aug-cc-pVDZ level of theory followed by extrapolations to complete basis set limits. We found that the most stable anion in the gas phase is related to an imino-oxo tautomer, in which the N1H proton is transferred to the C5 atom. This valence anion, aT(c5)(nl), is characterized by an electron vertical detachment energy (VDE) of 1251 meV and it is adiabatically stable with respect to the canonical neutral nT(can) by 2.4 kcal/mol. It is also more stable than the dipole-bound (aT(dbs)(can)), and valence anion aT(val)(can) of the canonical tautomer. The VDE values for aT(dbs)(can)and T(val)(can) are 55 and 457 meV, respectively. Another, anionic, low-lying imino-oxo tautomer with a VDE of 2458 meV has a proton transferred from N3H to C5 aT(c5)(n3). It is less stable than aT(val)(can) by 3.3 kcal/mol. The mechanism of formation of anionic tautomers with the carbons C5 or C6 protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to C5. The six-member ring structure of the anionic tautomers with carbon atoms protonated is unstable upon an excess electron detachment. Within the PCM hydration model, the low-lying valence anions become adiabatically bound with respect to the canonical neutral; becomes the most stable, being followed by aT(c5)(nl), aT(c5)(n3), aT(can), and aT(c5)(nl).     

Effect of excess electron and one water molecule on relative stability of the canonical and zwitterionic tautomers of glycine

The anionic and neutral complexes of glycine with water were studied at at the coupled cluster level of theory with single, double, and perturbative triple excitations. The most stable neutral complex has a relatively small dipole moment (1.74 D) and does not bind an electron. Other neutral complexes involve a polar conformer of canonical glycine and support dipole-bound anionic states. The most stable anion is characterized by an electron vertical detachment energy of 1576 cm(-1), in excellent agreement with the experimental result of 1573 cm(-1). The (Gly.H(2)O)(-) complex supports local minima, in which the zwitterionic glycine is stabilized by one water and one excess electron. They are, however, neither thermodynamically nor kinetically stable with respect to the dipole-bound states based on the canonical tautomers of glycine. The electron correlation contributions to excess electron binding energies are important, in particular, for nonzwitterionic complexes. Our results indicate that the condensation energies for Gly((0,-))+H(2)O-->(Gly.H(2)O)((0,-)) are larger than the adiabatic electron affinity of Gly.H(2)O. The above results imply that collisions of Gly(-) with H(2)O might effectively remove Gly(-) from the ion distribution. This might explain why formation of Gly(-) and (Gly.H(2)O)(-) is very sensitive to source conditions. We analyzed shifts in stretching mode frequencies that develop upon formation of intra- and intermolecular hydrogen bonds and an excess electron attachment. The position of the main peak and a vibrational structure in the photoelectron spectroscopy spectrum of (Gly.H(2)O)(-) are well reproduced by our theoretical results.     

Theoretical study on the excitation energies of six tautomers of guanine: evidence for the assignment of the rare tautomers

The CASPT2//CASSCF method with the 6-31G basis set and an active space up to (16,12) was used to calculate the excitation energies for six tautomers of guanine. Our calculations provide further support on the recent reassignment of the near-UV resonant two-photon ionization (R2PI) spectrum, in which two rare tautomers of the 7H-oxo-imino form were proposed to replace the previously assigned 7H/9H-oxo-amino tautomers. The adiabatic excitation energies of the 7H-oxo-imino tautomers are calculated to be 0.3-0.5 eV higher than those of the 7H/9H-oxo-amino tautomers. Our calculations also indicate that the missing most stable tautomers (7H/9H-oxo-amino tautomers) in the R2PI experiment is possibly due to the existence of an ultrafast nonradiative deactivation process in the excited-state of these two tautomers.     

Charge transfer stabilization of an excess electron on a molecular surface

In a developed molecular surface model (Jalbout and Adamowicz, Mol Phys, 2006, 19, 3101), we suggested a series of cyclohexane and cyclooctane systems with a hydrogen‐bonded network of OH groups on one side of the surface and hydrogen atoms on the opposite side. The OH groups in this work increased the dipole moment of the complexes in order to form stable dipole bound anions. In this report we consider the dipole‐bound and solvated anions between a set of hypothetical molecular surfaces and NH₃BF₃. The resulting complexes are stable with respect to vertical electron detachment. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Effective mass of an excess electron in fluids with high polarizability

The effective mass of an excess electron injected into a dense non-polar fluid with high polarizability is considered. The pseudopotential theory of electron interaction with a cellular fluid is used. Phase shifts of the p-scattering of an electron in a fluid are calculated. The Bardeen formula is used for the effective mass calculation. The density dependence of the effective mass in fluid xenon is compared with experimental data.     

Ab-initio study of localization of an excess electron in sodium halide clusters

The equilibrium geometries of Na _n F _n and Na _n F _n?1 are optimized forn=2, 3, 4 at the SCF level. The Na _n F _n molecules appear as formed by Na⁺ and F^? ions. The paper studies the localization of the excess electron in the Na _n F _n?1 molecules. Na _n F _n?1 is obtained by removing from Na _n F _n a fluorine atom, and the excess electron takes the place of this lacking center, and allows a weak bond between the neighbouring Na atoms. Secondary minimum geometries, for which the excess electron has no defined place, are also studied.     

Origin of the magic numbers of water clusters with an excess electron

Electron-bound water clusters [e(-)(H(2)O)(n)] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H(2)O)(n=2,6,7, and (11)). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e(-)-water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics.     

Hydrogen detachment of the hydrated hydrohalogen acids upon attaching an excess electron

High level ab initio calculations are employed to investigate the excess electron attachment to the hydrated hydrohalogen acids. The excess electron leads to the dissociation of hydrogen halide acids, which results in the release of a hydrogen radical. Neutral HCl, HBr, and HI are dissociated by tetrahydration. Upon binding an excess electron, these hydrated hydrohalogen acids show that (i) the H-X bond strength weakens with redshifted H-X stretching frequencies, (ii) HX can have a bound-electron state, a dissociated structure, or a zwitter-ionic structure, and (iii) HClHBr is dissociated by tri/mono-hydration, while HI is dissociated even without hydration. This dissociation is in contrast to the case of electron attachment to hydrated hydrogen fluoric acids for which HF is not dissociated by more than ten water molecules.     

Stabilization of D5h and C2v valence tautomers of the croconate dianion

The non-benzenoid aromatic D(5h) and enediolate C(2v) form of C5O5(2-) have been stabilized by hydrogen bonding with urea and 1,3-dimethylurea, respectively, in the host lattices of two novel crystalline inclusion compounds.     

DFT studies on the favored and rare tautomers of neutral and redox cytosine

The complete tautomeric mixture consisting of nine prototropic tautomers has been studied in the gas phase at the DFT(B3LYP)/6-311+G(d,p) level for neutral, oxidized, and reduced cytosine. Rotational isomerism of the exo –OH group and geometrical isomerism of the exo =NH group have also been considered. Tautomeric conversions possible for cytosine have been compared with those for its structural models, 4-amino- and 2-hydroxypyrimidine. Effects of intramolecular interactions between neighboring groups for cytosine are analogous to those observed for model compounds. Although they are not very strong, they are sufficient to influence tautomeric equilibria and relative stabilities of individual tautomers. One-electron oxidation and one-electron reduction change tautomeric preferences. Tautomers that are rare forms for neutral cytosine become favored ones for oxidized and reduced cytosine. Aromaticity is not the main factor that dictates the tautomeric preferences. Stability of functional groups seems to be more important than full electron delocalization.     

UV-induced generation of rare tautomers of 2-thiouracils: a matrix isolation study

Unimolecular photoisomerization reactions were studied for 2-thiouracil, 6-aza-2-thiothymine, 1-methyl-2-thiouracil, and 3-methyl-2-thiouracil isolated in low-temperature Ar matrixes. The IR spectra have revealed that before UV irradiation all the matrix-isolated compounds adopted exclusively the oxo-thione tautomeric form. Upon UV (lambda > 320 nm) irradiation of the matrixes, two oxo-thiol photoproducts were generated for monomeric 2-thiouracil as well as for monomeric 6-aza-2-thiothymine. Generation of these products corresponds to transfer of a proton from either the N(1)-H or N(3)-H group to the sulfur atom of the C(2)=S thiocarbonyl moiety. The first of the above reactions was photoreversible. As a consequence, after prolonged UV irradiation most of the material was transformed into the oxo-thiol-N(1)H form. The hydroxy-thiol tautomers of 2-thiouracil and 6-aza-2-thiothymine were also photogenerated as minor products. For 1-methyl-2-thiouracil and 3-methyl-2-thiouracil, thione --> thiol phototautomeric reactions yielded the oxo-thiol isomers of the compounds. Since these reactions were photoreversible, the final stages of the photoinduced processes corresponded, for both methylated 2-thiouracils, to photostationary states. All the products of the investigated photoreactions were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(2d,p) level.     

One‐electron redox properties of DNA nucleobases and common tautomers

The redox properties of DNA play an influential role in several important processes such as DNA mutation and the interaction of DNA with drugs. Structural changes in DNA nucleobases from its canonical form to its tautomeric forms can alter these properties and may lead to DNA mutation due to altered base‐pairing properties. Experimental results for the standard value of DNA redox properties vary due to choice of methodology and solvent. Theoretical determination of these properties is helpful in pinpointing standard values but still vary depending on methodology and chosen experimental benchmark. However, it is of importance to identify the overall trend of electron mobility within DNA while providing reliable standard values for redox reactions. In this work, we present the results of theoretical calculations for redox properties. Using the thermodynamic cycle, we can approximate reliable values. We report the electron affinities, ionization potentials, and redox potential for the canonical DNA nucleobases and their rare tautomers. For each of these properties, we evaluate its overall trend to gain a greater understanding of the role that electron attachment and electron mobility have within the DNA strand. All calculations are computed at the M06‐2X/6–31++G(d,p) level of theory. © 2014 Wiley Periodicals, Inc.     

Shape resonance states of the low-energy electron attachments to DNA base tautomers

Two different types of shape resonance states, π* and σ*, formed in the low-energy electron attachments to the low-lying tautomers of DNA bases (adenine, guanine, thymine, and cytosine) in the gas phase are investigated using the quantum scattering method with the non-empirical model potentials in a symmetry-adapted, single-center expansion. Four and three π* states are predicted for purines and pyrimidines, respectively. Comparing the different tautomers of a certain DNA base, we find distinct differences both in the resonance energies and the resonant wave function patterns of π* states. As for the lowest three π* states, the energetic values predicted in this work are also compared with the theoretical and experimental results available in the literature.     

Thermal Stabilization of an Endoglucanase by Cyclization

An intein-driven protein splicing approach allowed for the covalent linkage between the N- and C-termini of a polypeptide chain to create circular variants of the endo-??-1,3-1,4-glucanase, LicA, from Bacillus licheniformis. Two circular variants, LicA-C1 and LicA-C2, which have connecting loops of 20 and 14 amino acids, respectively, showed catalytic activities that are approximately two and three times higher, respectively, compared to that of the linear LicA (LicA-L1). The thermal stability of the circular variants was significantly increased compared to the linear form. Whereas the linear glucanase lost half of its activity after 3?min at 65?°C, the two circular variants have 6-fold (LicA-C1) and 16-fold (LicA-C2) increased half-life time of inactivation. In agreement with this, fluorescence spectroscopy and differential scanning calorimetry studies revealed that circular enzymes undergo structural changes at higher temperatures compared to that of the linear form. The effect of calcium on the conformational stability and function of the circular LicAs was also investigated, and we observed that the presence of calcium ions results in increased thermal stability. The impact of the length of the designed loops on thermal stability of the circular proteins is discussed, and it is suggested that cyclization may be an efficient strategy for the increased stability of proteins.     

Photoelectron spectroscopy of adiabatically bound valence anions of rare tautomers of the nucleic acid bases

Anionic states of nucleic acid bases (NABs) are involved in DNA damage by low-energy electrons and in charge transfer through DNA. Previous gas phase studies of free, unsolvated NAB parent anions probed mostly dipole-bound states, which are not present in condensed phase environments. Recently, we demonstrated that very rare tautomers of uracil (U), cytosine (C), adenine (A), and guanine (G), which are obtained from canonical tautomers through N-to-C proton transfers, support valence anionic states. Here we report the photoelectron spectrum of the final member of the NABs series: the valence state of the thymine (T) anion. Additionally, we summarized the work of all five NABs. All of the newfound anionic tautomers of the NABs may be formed via dissociative electron attachment followed by hydrogen atom reattachment to a carbon atom. Furthermore, these unusual tautomers may affect the structure and properties of DNA and RNA exposed to low-energy electrons. The new valence states observed here, unlike dipole bound states, could exist in condensed phases and may be relevant to radiobiological damage.