Stabilization of very rare tautomers of 1-methylcytosine by an excess electron

We characterized valence anionic states of 1-methylcytosine using various electronic structure methods. We found that the most stable valence anion is related to neither the canonical amino-oxo nor a rare imino-oxo tautomer, in which a proton is transferred from the N4 to N3 atom. Instead, it is related to an imino-oxo tautomer, in which the C5 atom is protonated. This anion is characterized by an electron vertical detachment energy (VDE) of 2.12 eV and it is more stable than the anion based on the canonical tautomer by 1.0 kcal/mol. The latter is characterized by a VDE of 0.31 eV. Another unusual low-lying imino-oxo tautomer with a VDE of 3.60 eV has the C6 atom protonated and is 3.6 kcal/mol less stable than the anion of the canonical tautomer. All these anionic states are adiabatically unbound with respect to the canonical amino-oxo neutral, with the instability of 5.8 kcal/mol for the most stable valence anion. The mechanism of formation of anionic tautomers with carbon atoms 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 the C5 or C6 atom. The six-member ring structure of anionic tautomers with carbon atoms protonated is unstable upon an excess electron detachment. Indeed the neutral systems collapse without a barrier to a linear or a bicyclo structure, which might be viewed as lesions to DNA or RNA. Within the PCM hydration model, the anions become adiabatically bound with respect to the corresponding neutrals, and the two most stable tautomers have a carbon atom protonated.     

Quantum-chemical calculations of the relative stability of the keto-enol tautomers of 5-chlorouracyl

Relative energies (by MP4(SQTQ)/def2-TZVPP) for 13 tautomeric conformers of 5-chlorouracyl in the gas phase and in solution with consideration of nonspecific solvation in water are calculated. The geometrical parameters of the calculated conformers are analyzed. A stability series of tautomers with respect to the diketo form is obtained. It is shown that nonspecific solvation leads to changes in the stability series.     

The relative stability of the tautomers of α-Hydroxytetronic acid

The relative stability of the four tautomeric forms of α-hydroxytetronic acid was calculated with full geometry optimization at the STO -3G , 3–21G , 6–31G *, and 6–31G ** SCF levels. Correlation effects were estimated using the MP 2 method. Intramolecular hydrogen bonds are found to be of primary importance for the relative stability of the various tautomers, and the same tautomer that in the case of L -ascorbic acid is experimentally observed in the crystal as well as in solution is found to be the most stable one, if polarization functions on the hydrogen atoms are taken into account. It is concluded that even in the gas phase this tautomer predominates in α-hydroxytetronic acid as well as in L -ascorbic acid. © 1994 John Wiley & Sons, Inc.     

A theoretical investigation of the relative stability of hydrated glycine and methylcarbamic acid--from water clusters to interstellar ices

We have theoretically investigated how the low-energy conformers of the neutral and the zwitterionic forms of glycine as well as methylcarbamic acid are stabilized by the presence water. The MP2/6-311++G(d,p) method was utilized to conduct calculations on glycine and methylcarbamic acid in both isolated clusters and in clusters embedded in the conductor-like polarizable continuum model (C-PCM), where the clusters explicitly contain between one and ten water molecules. The neutral forms of glycine and methylcarbamic acid were found to have similar hydration energies, whereas the neutral methylcarbamic acid was determined to be approximately 32 kJ mol(-1) more stable than the neutral glycine in the isolated clusters and 30 kJ mol(-1) more stable in the C-PCM embedded clusters. Both the number and strength of the hydrogen bonding interactions between water and the zwitterions drive the stability. This lowers the relative energy of the glycine zwitterion from 50 kJ mol(-1) above neutral glycine, when there are two water molecules in the clusters to 11 kJ mol(-1) below for the clusters containing ten water molecules. For the methylcarbamic acid clusters with two water molecules, the zwitterion is 51 kJ mol(-1) higher in energy than the neutral form, but it remains 13 kJ mol(-1) above the neutral methylcarbamic acid in the clusters containing ten water molecules. When the bulk water environment is simulated by the C-PCM calculations, we find both the methylcarbamic acid and glycine zwitterionic forms have similar energies at 20 kJ mol(-1) above the neutral methylcarbamic acid energy and 10 kJ mol(-1) lower than the neutral glycine energy. Although neither methylcarbamic acid nor glycine have been detected in the interstellar medium yet, our findings indicate that methylcarbamic acid is the more stable product from methylamine and carbon dioxide reactions in a water ice. This suggests that methylcarbamic acid likely plays a role in the intermediate steps if glycine is formed in the interstellar medium.     

The electron density of the water molecule

The electron density of the water molecule, as calculated by a standard program, is approximated by linear combinations of spherical Gaussians. The accuracy of the result is studied as a function of the numbers and positions of the Gaussians. Since this shows where the charge is located in the molecule it has immediate physical significance. The building-up of the density can be followed in more and more detail. From these expansions, point charge models of water are readily deduced. These are compared with models of similar kinds used by other authors. Some of the calculations have been repeated with a wavefunction of higher accuracy to investigate the stability of the results. Results show that the more accurate density requires more Gaussians to represent its greater complexity.     

Incremental solvation of nonionized and zwitterionic glycine

Microsolvation and combined microsolvation-continuum approaches are employed in order to examine the structures and relative energies of nonionized (N) and zwitterionic (Z) glycine clusters. Bridging structures are predicted to be the global minima after 3-5 discrete water molecules are included in the calculations. Calculations incorporating electron correlation stabilize the zwitterionic structures by about 7-9 kcal/mol relative to the N structures regardless of the number of discrete water molecules considered. Continuum calculations stabilize the Z structures relative to N structures; this effect decreases as the number of discrete water molecules is increased. Eight water molecules do not appear to fully solvate glycine.     

Effect of ether group on the electrochemical stability of zwitterionic imidazolium compounds

The cathodic stability of the zwitterionic imidazolium compounds was significantly enhanced by the introduction of an ether group at 1 or 2-position on the imidazolium ring. The cycle performance tests showed that the initial cell capacity was maintained almost unchanged up to 100 cycles at 0.5 and 1 C when 2.5 wt.% of 2-butoxymethyl-1-methylimidazolium-3-propylsulfonate or 2-butoxymethyl-1-butylimidazolium-3-propylsulfonate was added to the model electrolyte (1 M LiPF₆ in ethylene carbonate, dimethyl carbonate and ethylmethyl carbonate (1/1/1 v/v/v)).Structures of zwitterionic compounds and their interactions with lithium ions were theoretically investigated.     

MNDO study on relative stabilities of monosubstituted pyridine tautomers

Relative stabilities of monosubstituted hydroxy-, mercapto- and aminopyridine tautomers have been calculated using the MNDO procedure with a full geometry optimization. In all cases considered the “lactim” tautomers proved to be more stable, in full qualitative agreement with the gas-phase experimental data.     

Dipole-bound anions of beta-alanine: canonical and zwitterionic conformers

The remarkable stabilities of the dipole-bound anions of the canonical and zwitterionic conformers of beta-alanine are predicted at the high level of theories, in which the former is the global minimum and the latter, the anti zwitterionic anion, is the local minimum. In contrast to the dipole-bound anions of glycine, the gauche zwitterionic anion of beta-alanine is an unstable conformer. The vertical electron detachment energies for the canonical and anti zwitterionic anions are 58 and 1145 meV, respectively. The photodetachment electron spectrum of the canonical anion is theoretically simulated on the basis of the Franck-Condon factor calculations.     

Calculations of the pseudopotential for the excess electron in water and methane

The potentials for excess electrons in cavities of water and methane are analyzed with the use of the pseudopotential theory. The results are consistent with the previous discussions; the excess electron in water can probably be trapped in the cavity and that in methane will be quasifree. In the case of methane, the effect of the molecular coordination on the potential is further discussed by varying the cavity radius.     

Conformational search for zwitterionic leucine and hydrated conformers of both the canonical and zwitterionic leucine using the DFT-CPCM model

The stable conformations for zwitterionic leucine have been searched for in solution as well as in gas phase. A total of 54 trial structures were generated by considering possible combinations of single bond rotamers. It is observed that zwitterions are not stable in gas phase. In order to investigate the zwitterions of leucine in solution, the calculations for all trial structures of zwitterions were performed initially at the PM3 level and 14 the lowest energy structures were reoptimized at the B3LYP/6-311G(d) level using the CPCM model. Seven of these conformers of zwitterionic leucine were found to be stable in solution. The five most stable conformers were then reoptimized at the B3LYP/6-311++G(d, p) level. The energy ordering of the canonical leucine(neutral) conformers were also considered on the basis of single point energy calculations at the B3LYP/6-311++G(d, p) level using the CPCM model. The chemical hardness, chemical potential, vertical ionization energy and vertical electron affinity were calculated for a few of the most stable canonical leucine and its zwitterions in solution. The effects of explicit addition of water molecules (microsolvation) on the structure and the energy of both canonical and zwitterionic conformers of leucine were investigated. It is noted that in gas phase, the singly and doubly hydrated canonical (neutral) forms are more stable than their zwitterionic counterparts. The solvated zwitterions and canonical structures of leucine were further investigated using the discrete/SCRF model with zero, one and two water molecules. In solution, the continuum solvent model shows that the bare zwitterionic form is more stable than the bare canonical form by 1.6 kcal/mol. This energy separation is increased to 3.8 and 4.8 kcal/mol with inclusion of one and two water molecules, respectively. The optimized structural parameters for the most stable zwitterionic leucine with zero, one and two water molecules in solution were compared with those reported for l-leucine crystal, which shows a close agreement between the optimized geometrical parameters of the zwitterionic leucine with two water molecules in solution with the experimental geometrical parameters for l-leucine crystal. It is also observed that when the structures of zwitterions with one and two explicit water molecules are optimized in solution, the geometrical parameters and their relative energies are found to be appreciably modified. We have also calculated the vibrational spectra of the most stable solvated zwitterionic leucine as well as for the most stable structure of zwitterionic leucine with one and two water molecules in solution.     

The effect of N-substituent on the relative thermodynamic stability of unionized and zwitterionic forms of α-diphenylphosphino-α-amino acids

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A non-empirical molecular orbital study on the relative stabilities of adenine and guanine tautomers

The relative stabilities of a series of adenine and guanine tautomers have been calculated using anab initio Hartree-Fock-Roothaan SCF MO method. The calculated relative stabilities agree in general with the results of earlier semiempirical studies. According to the present study, tautomeric forms with regular Kekulé structure for the six-membered purine ring are the most stable. The amine-imine tautomerization of purine bases is not likely to be responsible for spontaneous mutations in DNA.     

Effect of polydispersity on the relative stability of hard-sphere crystals

By extending the nonequilibrium potential refinement algorithm and lattice switch method to the semigrand ensemble, the semigrand potentials of the fcc and hcp structures of polydisperse hard-sphere crystals are calculated with the bias sampling scheme. The result shows that the fcc structure is more stable than the hcp structure for polydisperse hard-sphere crystals below the terminal polydispersity.     

Effect of substituents on the relative stability of furoxan isomers

The overall energy of isomers of substituted furoxans has been calculated using the MINDO/3 method. Comparison with experimental results indicated that the calculation correctly predicted the structure of the more stable isomer. Correlation of the difference of the overall energy of the isomers (E) with the induction and resonance constants of the substituents, and comparison of the values of E calculated from the correlation equation with the experimental values have also been carried out.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 264–266, February, 1986.