Imino Tautomer Research Articles

Tautomeric equilibrium of creatinine and creatininium cation in aqueous solutions explored by Raman spectroscopy and density functional theory calculations

The Raman spectral studies of creatinine with pH dependence were performed to explore the effects of pH values on the Raman spectroscopy of creatinine. Firstly, we calculated vibrational spectra by DFT to derive the equilibrium geometries and protonated form of creatinine. Comparing simulated and observed Raman spectra of creatinine in aqueous solution at pH 2, it is found the theoretical predicted spectra agree well with those of the experiment while seven water molecules are aggregated around the creatinine. Additionally, the tautomeric equilibrium of creatinine in aqueous solutions was studied and two tautomers are found to coexist by comparing its experimental and calculated Raman spectra. A water dimer being used to solvate creatinine would make the thermodynamic energy favor convert from the imino tautomer to the amino tautomer. Besides, the molecular electrostatic potential (MEP) analysis of the creatinine further confirms their discrepancies of typical experimental Raman spectra at different pH values.

Spectroscopic study on the intermolecular double proton transfer in 4-(naphthalen-1-yl)-6-octyl-1,3,5-triazin-2-amine with acid

With 2,4,6-trichloro-1,3,5-triazine as starting material, a functionalized triazine derivative, 4-(naphthalen-1-yl)-6-octyl-1,3,5-triazin-2-amine (NOTA) was synthesized in 14% yield through three steps: Kumada cross-coupling, Suzuki coupling and amination. Intermolecular double proton transfer of NOTA with acetic acid (HOAc) and trifluoroacetic acid (TFA) in chloroform was investigated by UV–vis absorption and fluorescence emission. It is found that both NOTA/HOAc and NOTA/TFA undergo excited state double proton transfer, resulting in amino–imino tautomerization emission in excited state.

Synthesis, characterization and DFT study of 4H-benzo[h]chromene derivatives

Abstract Interaction of 4-methoxy-1-naphthol (1) with α-cyano-p-chlorocinnamonitrile (2a) and ethyl α-cyano-p-chlorocinnamate (2b) provided 2-amino-4-(4-chlorophenyl)-6-methoxy-4H-benzo[h]chromene-3-carbonitrile (3a) and ethyl 2-amino-4-(4-chlorophenyl)-6-methoxy-4H-benzo[h]chromene-3-carboxylate (3b), respectively. Structures of these compounds were established on the basis of IR, UV, 1H NMR, 13C NMR, 13C NMR-DEPT and MS data. Moreover, imino tautomer (3b′) has been designed. Using density functional theory geometries have been optimized at B3LYP/6-31G* level of theory, the absorption spectra has been computed by using time dependant density functional theory at TD-B3LYP/6-31G* level of theory. Moreover, the absorption spectra in solvents have been computed and compared with experimental data. The highest occupied molecular orbitals, lowest unoccupied molecular orbitals and HOMO–LUMO energy gap of studied systems have been discussed.

Two-photon excitation of the fluorescent nucleobase analogues 2-AP and tC

We present two-photon (2P) excitation measurements of two commonly-used and commercially available fluorescent nucleobase analogues in aqueous solution: the environment-sensitive adenine analogue 2-aminopurine (2-AP), which emits in the UV region, and the environment-insensitive cytosine analogue 1,3-diaza-2-oxophenothiazine (tC), which emits in the green region. We studied the 2P excitation of free 2-AP and found that the same emissive states are accessed as via one-photon (1P) and three-photon (3P) excitation. The 2P cross section of 2-AP was 0.2 GM, which is similar to that of related molecules such as NADH. We studied tC incorporated in three single-stranded DNA oligonucleotides by 1P and 2P spectroscopy and microscopy. We found that the same emissive states were populated via either the resonant or non-resonant pathway with a 2P cross section of 1.5 GM. Time-resolved fluorescence studies suggested that tC exists in solution as two species, which we assigned as the amino and imino tautomers. Using confocal microscopy, we found that similar emission rates could be achieved for 2P excitation, as compared with 1P excitation, but without the strong photobleaching observed in the latter case. Although we were unable to detect tC fluorescence at the single-molecule level, these results indicate that multiphoton excitation of nucleobase analogues is a promising approach for imaging applications.

Disentangling Intrinsic Ultrafast Excited-State Dynamics of Cytosine Tautomers

Gas-phase ultrafast excited-state dynamics of cytosine, 1-methylcytosine, and 5-fluorocytosine were investigated in molecular beams using femtosecond pump-probe photoionization spectroscopy to identify the intrinsic dynamics of the major cytosine tautomers. The results indicate that, upon photoexcitation in the first absorption band, the cytosine enol tautomer exhibits a significantly longer excited-state lifetime than its keto and imino counterparts. The initially excited states of the cytosine keto and imino tautomers decay with sub-picosecond dynamics for excitation wavelengths shorter than 300 nm, whereas that of the cytosine enol tautomer decays with time constants ranging from 3 to 45 ps for excitation between 260 and 285 nm.

Tautomerism in polyguanide

Tautomerism in polyguanide is theoretically discussed. For biguanide, triguanide (melamine), and linear polyguanides, stability of the most stable tautomer is investigated by semi-empirical and DFT calculations. Amino tautomers are more stable than the corresponding imino tautomers, and the energy gaps are ca. 6–7 kcal/mol per guanide unit. In polyguanides, amino tautomers with cis-type N C N C chains are preferred, and flat bands appear near the non-bonding level. Based on the cis-amino tautomers, applications to metal-complexing agents and magnetic materials are proposed.

Methylation Stabilizes the Imino Tautomer of dAMP and Amino Tautomer of dCMP in Solution

Alkylating agents cause methylation of adenosine and cytidine in DNA to generate 1-methyladenosine and 3-methylcytidine. These modified nucleosides can serve as regulators of cells or can act as agents of mutagenesis depending on the context and the partner enzymes. Solution structures and the chemical interactions with enzymes that lead to their recognition are of inherent interest. At physiological pH, 1-methyladenosine and 3-methylcytidine are presumed to be in the protonated amino forms in the literature. We report the structures, ionization states, and UV resonance Raman spectra of both substrates over a range of pH (2.5-11.0). The Raman excitation wavelength was tuned to selectively enhance Raman scattering from the nucleobase (260 nm) and further specifically from the imino form (210 nm) of 1-me-dAMP. We find that contrary to the general assumption, 1-me-dAMP is present in its neutral imino form at physiological pH and 3-me-dCMP is in the amino form.

2-Substituted agelasine analogs: Synthesis and biological activity, and structure and reactivity of synthetic intermediates

2-Substituted N-methoxy-9-methyl-9H-purin-6-amines were synthesized either from their corresponding 6-chloro-9-methyl-9H-purines or 2-chloro-N-methoxy-9-methyl-9H-purin-6-amine. Great diversity in the amino/imino tautomeric ratios was observed and calculated based on 1H NMR. The tautomers were identified by 1D and 2D 1H, 13C, and 15N NMR techniques, and showed significant variation both in 13C and 15N shift values. Comparison of the tautomeric ratios with Hammett F values revealed that as the field/inductive withdrawing abilities of the 2-substituent increased, the ratio of amino:imino tautomers was shifted toward the amino tautomer. Computational chemistry exposed the significance of hydrogen bonding between solvent and the compound in question to reach accurate predictions for tautomeric ratios. B3LYP/def2-TZVP density functional theory (DFT) calculations resulted in quantitatively more accurate predictions than when employing the less expensive BP86 functional. N-7-Alkylation of the 2-substituted N-methoxy-9-methyl-9H-purin-6-amines showed that when the field/inductive withdrawing ability of the 2-substituent reached a certain point the reactivity drastically dropped. This correlated with the atomic charges on N-7 calculated using a natural bond orbital (NBO) analysis. Biological screening of the final 2-substituted agelasine analogs indicated that the introduction of a methyl group in the 2-position is advantageous for antimycobacterial and antiprotozoal activity, and that an amino function may improve activity against several cancer cell lines.

Tautomers of cytosine and their excited electronic states: a matrix isolation spectroscopic and quantum chemical study

We have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated ab initio at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form.

Hydrolytic Deamination of 5,6-Dihydrocytosine in a Protic Medium: A Theoretical Study

The mechanism for the deamination reaction of 5,6-dihydrocytosine with H(2)O in a protic medium was investigated by DFT calculations at the B3LYP/6-311G(d,p) level of theory as a model reaction for the deamination reaction of 5,6-saturated cytosine derivatives. Two pathways were found. Pathway dhA, which can explain the deamination in a protic medium at acidic pH, and pathway dhBt, more representative of the reaction in a protic medium at neutral pH. Pathway dhA is a two-step mechanism initiated by the nucleophilic addition of a water molecule to carbon C4 of N3-protonated 5,6-dihydrocytosine with the assistance of a second water molecule, followed by elimination of an ammonium cation to form 5,6-dihydrouracil. The nucleophilic addition is rate-determining with an activation free energy of 116.0 kJ/mol in aqueous solution. Pathway dhBt is a four-step mechanism which starts with the water-assisted tautomerization of 5,6-dihydrocytosine to form the imino tautomer. This intermediate undergoes nucleophilic addition of water to carbon C4, which after protonation eliminates an ammonium cation, as in pathway dhA. The nucleophilic addition is again rate-determining, with an activation free energy of 113.3 kJ/mol in aqueous solution. The latter value is about 25 kJ/mol lower than its counterpart for cytosine, in agreement with the experimental observation that 5,6-saturated cytosine derivatives exhibit a much shorter lifetime in aqueous solution than their unsaturated counterparts. The evaluation of reactivity indices derived from conceptual DFT leads to the conclusion that this lower activation free energy can be attributed to a larger local electrophilic power of carbon C4 in 5,6-saturated derivatives.

Tautomerism, normal coordinate analysis, vibrational assignments, calculated IR, Raman and NMR spectra of adenine

The Raman (3700–100 cm −1), infrared (4000–200 cm −1), mass spectrum and 1H NMR temperature-dependent study of adenine have been recorded. Quantum chemical calculations were carried out for N(9)H- amino, N(7)H- amino, N(9)H- imino and N(7)H- imino adenine tautomers using RHF, B3LYP and MP2 with full electron correlation up to 6-311++G(d,p) basis set. The computational results reveal the non-planar N(9)H- amino conformer of adenine to be the most stable structure of adenine. The planar (C s) form of N(9)H- amino adenine is found to represent a transition state, lying 8 cm −1 above the non-planar conformer and with one imaginary frequency. Comparison between theoretical and experimental 1H NMR spectra favors the assignment of the signals to N9(H) and N7(H)- amino tautomers over the corresponding imino tautomers. On the other hand, the recorded IR, Raman and 13C NMR spectra were fully consistent with N9(H)- amino adenine tautomer, therefore it is the only tautomer in both the gas and solid phases and in solution. Moreover, the results of NH 2 potential surface scans utilizing B3LYP and MP2 = full methods at 6-31G(d) basis also support the non-planarity of N(9)H- amino adenine. The mass spectral measurements indicate the presence of 3% adenine dimmer which indicates weak inter-molecular hydrogen bonding interactions in adenine. Additionally, intramolecular hydrogen bonding is also predicted between N 1 and H 15 atoms. The theoretical infrared and Raman spectra have been successfully simulated by means of both DFT and MP2 calculations, allowing the interpretation of the complex bands observed. Aided by normal coordinate analysis, potential energy distributions and the calculated force constants, a revised and accurate vibrational assignment for all fundamentals has been provided for the non-planar N(9)H-adenine tautomer. The results are reported herein and compared with similar molecules whenever appropriate.

Optimum geometries and relative energies for guanine, the imino-enol tautomer of guanine, the enol tautomer of guanine, adenine, and the imino tautomer of adenine as found by the MINDO/2 SCF MO method

A MINDO/2 SCF MO geometry optimization of guanine (G), the imino-enol tautomer of guanine (G*), the enol tautomer of guanine (G**), adenine (A), and the imino tautomer of adenine (A*) was made. The optimized geometries for guanine and adenine agree well with crystallographic data. The optimized geometries for the tautomers show the correct trends in bond lengthening and bond angle adjustment. The energies of all five molecules were lowered relative to the initial assumed geometry by the optimization procedure. The energies of tautomerization were found to be 0.4, − 18.7, and 12.5 kcal for G ⇌ G*, G ⇌ G**, and A ⇌ A*, respectively. When solvent effects are included the energies of tautomerization are 5.2, − 10.0, and 8.7 kcal for G ⇌ G*, G ⇌ G**, and A ⇌ A*, respectively.

Reaction mechanisms of thiamin diphosphate enzymes

Thiamin diphosphate (ThDP), the biologically active form of vitamin B1, is an essential cofactor that is involved in numerous metabolic pathways, such as the oxidative and non-oxidative decarboxylation of 2-oxo acids, interketol transfer between sugar phosphates, electron transfer reactions and the formation of amino acid precursors. ThDP consists of a thiazolium nucleus, an aminopyrimidine ring, and a diphosphate moiety that is required to provide tight binding to the target enzyme. The electrophilic nature of the positivelycharged thiazolium ring is key to the stabilization of carbanion states of the cofactor (i.e. the ThDP ylide) and of covalent substrate-ThDP conjugates in terms of a ‘Umpolung – or polarity inversion’ mechanism predestining ThDP as an effective participant in the reversible catalytic cleavage of carbon-carbon bonds of vicinal dicarbonyl or a-hydroxyketone groups, exhibiting a chemical ability not possessed by protein functional groups. The first committed step in all ThDP-dependent enzymes is the deprotonation of the thiazolium C2 and attack of the resulting carbanion on a substrate carbonyl. X-ray crystallographic analysis of ThDP enzymes has demonstrated the cofactor to be bound in the canonical V conformation, with the amino group of the aminopyrimidine and C2 of the thiazolium moiety juxtaposed with respect to hydrogen-bonding distance. A strictly conserved glutamate residue interacts with nitrogen 1 of the aminopyrimidine. The conserved structure of the cofactor binding site, and the observed loss of activity accompanyingmutagenesis of the conserved glutamate or uponuse of ThDP analogs lacking nitrogen 1 or the amino group of the aminopyrimidine, all support a mechanism in which proton removal fromC2 of ThDP is assisted by a catalytic triad consisting of the conserved glutamate, the aminopyrimidine and thiazolium moieties of the cofactor, presumably involving the imino tautomer of the aminopyrimidineas thegeneralbase forprotonabstraction. This minireview series, based on lectures given at an international conference on thiamin held in Wittenberg, Germany, in May 2008, highlights recent advancements in the mechanistic understanding of ThDP-promoted biochemical reactions. Jordan et al. have accumulated spectroscopic evidence for the occurrence and interconversion of the different tautomeric and ionization states of enzymebound ThDP along the pathways of various ThDP enzymes. The data clearly implicate the delicately balanced equilibria of the different cofactor forms on the enzymes’ active sites, such that all of these states are thermodynamically accessible at the pH optimum of a given ThDP enzyme. As outlined above, a glutamate residue interacting with the cofactor aminopyrimidine has hitherto been believed to be vital for tautomerization of the aminopyrimidine and thus chemical activation of ThDP. However, structural and mechanistic studies on the ThDP-dependent enzyme glyoxylate carboligase (GCL), as reviewed by Shaanan and Chipman, have revealed valine as having replaced the expected glutamate. Mechanistic analysis suggested that GCL sacrifices the otherwise strictly conserved mechanism of protein-cofactor activation as a consequence of evolutionary adaption to avoid formation of overlystabilized intermediates as thermodynamic traps. The oxidative decarboxylation of 2-oxo acids such as pyruvate is a key reaction of intermediary metabolism in virtually all organisms and requires ThDP to act in tandem with additional cofactors (e.g. flavins, iron-sulfur cluster, lipoic acid) as redox partners. The review by Tittmann, devoted to the mechanistic analysis of redox reactions of ThDP enzymes, shows how some of these enzymes make use of elegant radical chemistry in an intimate coupling mechanism of oxidation–reduction and the formation of energy-rich metabolites such as acetyl-CoA or acetyl phosphate.

Tautomerism of guanidines studied by 15N NMR: 2-hydrazono-3-phenylquinazolin-4(3H)-ones and related compounds

2-Hydrazono-3-phenylquinazolin-4(3H)-ones 11a-i are shown by (15)N NMR to exist in DMSO solution predominantly as the imino tautomers B and not the amino tautomers A. 2-Hydrazino-benzimidazole derivative 12 and 2-hydrazino-4,6-dimethylpyrimidine derivative 13 were found to exist predominantly as the amino tautomers.

Ionization Energy Thresholds of Microhydrated Adenine and Its Tautomers

In the present work the vertical and adiabatic ionization energy thresholds (IET) of adenine, and its amino and imino tautomers complexed with 1-3 water molecules are presented. The vertical and adiabatic IETs have been calculated at the B3LYP and P3 levels of theory, using the standard 6-31++G(d,p) basis set. The results show that there is hardly any effect of microhydration on the vertical DeltaIET of adenine, which is at odds with the experimental values determined by Kim et al. (J. Phys. Chem. 1996, 100, 7933). In an attempt to assign the experimental DeltaIET values, calculations have been performed on the microhydrated amino and imino tautomers of adenine. Vertical DeltaIET calculations and adiabatic DeltaIET calculations on adenine N7H tautomers complexed with water are in better agreement with the experimental results than are calculations involving the canonical (N9H) form of adenine.

Head‐to‐Head Right‐Handed Cross‐Links of the Antitumor‐Active Bis(μ‐N,N′‐di‐p‐tolylformamidinato)dirhodium(II,II) Unit with the Dinucleotides d(GpA) and d(ApG)

Reactions of cis-[Rh(2)(DTolF)(2)(NCCH(3))(6)](BF(4))(2) with the dinucleotides d(GpA) and d(ApG) proceed to form [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}], respectively, with bridging purine bases spanning the Rh-Rh unit in the equatorial positions. Both dirhodium adducts exhibit head-to-head (HH) arrangement of the bases, as indicated by the presence of H8/H8 NOE cross-peaks in the 2D ROESY NMR spectra. The guanine bases bind to the dirhodium core at positions N7 and O6, a conclusion that is supported by the absence of N7 protonation at low pH values and the notable increase in the acidity of the guanine N1H sites (pK(a) approximately 7.4 in 4:1 CD(3)CN/D(2)O), inferred from the pH-dependence titrations of the guanine H8 proton resonances. In both dirhodium adducts, the adenine bases coordinate to the metal atoms through N6 and N7, which induces stabilization of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H adenine sites (pK(a) approximately 7.0-7.1 in 4:1 CD(3)CN/D(2)O), as compared to the imino form of free adenosine. The presence of the adenine bases in the rare imino form is further corroborated by the observation of DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] in CD(3)CN at -38 degrees C. The 2D NMR spectroscopic data and the molecular modeling results suggest the presence of right-handed variants, HH1R, in solution for both adducts (HH1R refers to the relative base canting and the direction of propagation of the phosphodiester backbone with respect to the 5' base). Complete characterization of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] by 2D NMR spectroscopy and molecular modeling supports anti-orientation of the sugar residues for both adducts about the glycosyl bonds as well as N- and S-type conformations for the 5'- and 3'-deoxyribose residues, respectively.

A DFT study of molecular structures and tautomerizations of 2-benzoylpyridine semicarbazone and picolinaldehyde N-oxide thiosemicarbazone and their complexations with Ni(II), Cu(II), and Zn(II)

The structure optimizations of picolinaldehyde N-oxide thiosemicarbazone (Hpiotsc), 2-benzoylpyridine semicarbazone (H2BzPS), their imino tautomers and their complexes with Ni(II), Cu(II), and Zn(II) were carried out using DFT calculations. The structures of Hpiotsc and H2BzPS ligands, transition states of their tautomerizations were obtained at the B3LYP/6-31+G(d,p) level and their thermodynamic properties were derived from the frequency calculations at the same level of theory. The B3LYP/LANL2DZ-optimized structures of Hpiotsc and H2BzPS complexes with Ni(II), Cu(II), and Zn(II), and the thermodynamic properties of their complexations derived from the B3LYP/LANL2DZ-frequency calculations were obtained. The B3LYP/LANL2DZ-optimized geometrical parameters for the [Ni(Hpiotsc)2]2+, [Cu(Hpiotsc).Cl2], and [Zn(Hpiotsc).Cl2] complexes show good agreement with their corresponding X-ray crystallographic data.

Substituent effect on the amino–imino tautomerism of aminothiazoles

The effect of type and substituent position on the conformational, structure and amino–imino tautomerism of aminothiazoles in the gas, water and carbon tetrachloride phases has been studied using the B3LYP/6-311++G(d,p) and MP2(full)/6-311++G(d,p) levels. They showed that the amino form is more stable than the imino in the three phases. This was in a good agreement with the experimental data. The electron donating group (–CH 3); positioned at four and five, increased the amount of imino form. Despite the greater substituent stability of the imino, the amino tautomer was found to be the predominant species. The electron-withdrawing group attached to the exocyclic nitrogen increased the acidity of exocyclic –NH group, thus facilitating the hydrogen transfer to the endocyclic nitrogen atom. The total energy of the imino form was lower than that of the amino, exhibiting the coexistence of the two structures in the gas phase. The tautomerization became spontaneous in solutions at room temperature and the imino was the predominate.

Unprecedented Head-to-Head Right-Handed Cross-Links between the Antitumor Bis(μ-N,N‘-di-p-tolylformamidinate) Dirhodium(II,II) Core and the Dinucleotide d(ApA) with the Adenine Bases in the Rare Imino Form

Reactions of the anticancer active compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2 with 9-ethyladenine (9-EtAdeH) or the dinucleotide d(ApA) proceed with bridging adenine bases in the rare imino form (A*), spanning the Rh-Rh bond at equatorial positions via N7/N6. The inflection points for the pH-dependent H2 and H8 NMR resonance curves of cis-[Rh2(DTolF)2(9-EtAdeH)2](BF4)2 correspond to N1H deprotonation of the metal-stabilized rare imino tautomer, which takes place at pKa approximately 7.5 in CD3CN-d3, a considerably reduced value as compared to that of the imino form of 9-EtAdeH. Similarly, coordination of the metal atoms to the N7/N6 adenine sites in Rh2(DTolF)2{d(ApA)} induces formation of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H sites (pKa approximately 7.0 in CD3CN-d3), as compared to the imino form of the free dinucleotide. The presence of the adenine bases in the rare imino form, due to bidentate metalation of the N6/N7 sites, is further corroborated by DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of Rh2(DTolF)2{d(ApA)} in CD3CN-d3 at -38 degrees C. Due to the N7/N6 bridging mode of the adenine bases in Rh2(DTolF)2{d(ApA)}, only the anti orientation of the imino tautomer is possible. The imino form A* of adenine in DNA may result in AT-->CG transversions or AT-->GC transitions, which can eventually lead to lethal mutations. The HH arrangement of the bases in Rh2(DTolF)2{d(ApA)} is indicated by the H8/H8 NOE cross-peaks in the 2D ROESY NMR spectrum, whereas the formamidinate bridging groups dictate the presence of one right-handed conformer HH1R in solution. Complete characterization of Rh2(DTolF)2{d(ApA)} by 2D NMR spectroscopy and molecular modeling supports the presence of the HH1R conformer, anti orientation of both sugar residues about the glycosyl bonds, and N-type conformation for the 5'-A base.

Imino Tautomers of Gas-Phase Guanine from Mid-Infrared Laser Spectroscopy

We reinvestigated the assignment of the three major guanine conformers detected via resonance enhanced two-photon ionization (R2PI) in supersonic expansions and present IR/UV double resonance spectra in the spectral region between 1500 and 1800 cm(-1). Comparison with B3LYP/TZVPP and RI-MP2/cc-pVQZ calculations shows that both conformers B and C are 7H-keto tautomers with an imine group in the 2-position. They differ only in the local conformation of the imine group but are otherwise identical. Conformer A is an amino-enol form with the OH group in the trans position.