Aqueous solubilities and enthalpies of solution of adenine and guanine

A generator column—liquid chromatographic technique was used to determine the aqueous solubility of adenine from 20 to 30°C, and of guanine from 15 to 40°C. The 95% confidence limits of the solubilities and molar enthalpies of solution at 25°C are: 8.7±0.1×10^–3M and 33.5±0.5 kJ-mol^–1 for adenine; 3.9±0.1×10^–5M and 49.2±0.6 kJ-mol^–1 for guanine. the adenine enthalpy value includes a small correction for association in the saturated solutions. The previously undetermined molar enthalpy of the second ionization step of guanine (to form the doubly-charged guanine anion) is estimated from our data combined with other measurements to equal 33.8±2.9 kJ-mol^–1.     

An ab initio study of excited states of guanine in the gas phase and aqueous media: Electronic transitions and mechanism of spectral oscillations

Ground state geometries of the four tautomeric forms keto‐N9H, keto‐N7H, enol‐N9H, and enol‐N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4‐31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6‐311+G* basis set was used. These calculations were also extended to hydrogen‐bonded complexes of three water molecules with each of the keto‐N9H (G9‐3W) and keto‐N7H (G7‐3W) forms of guanine. Relative stabilities of the four above‐mentioned tautomers of guanine as well as those of G9‐3W and G7‐3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self‐consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6‐31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto‐N9H and keto‐N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto‐N7H form is most stable in the gas phase, while the keto‐N9H form is most stable in aqueous solution. It appears that both the keto‐N9H and keto‐N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto‐N9H form but the keto‐N7H form of the same would also make some contribution to it. The enol‐N9H and enol‐N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto‐N7H form of the molecule while the fluorescence of oxygen‐rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto‐N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000     

Calculation of Raman spectra and structure elucidation of guanine in the polycrystalline state and in aqueous solution

The frequencies and relative intensities of lines in the Raman spectra of the condensed states of guanine and its deutero substituted analog have been calculated. The effect of hydrogen bonding on the Raman spectrum has been studied. Guanine exists in different tautomeric forms (G-N₉H and G-N₇H) at different pH in aqueous solution (for acidic and alkaline media, respectively).     

QM/MM calculation of solvent effects on absorption spectra of guanine

Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited‐state calculations were extracted from ground‐state molecular dynamics (MD) simulations using the self‐consistent‐charge density functional tight binding (SCC‐DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited‐state calculations used time‐dependent density functional theory (TDDFT) and the DFT‐based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto‐N7H and keto‐N9H guanine, with particular focus on solvent effects in the low‐energy spectrum of the keto‐N9H tautomer. When compared with the vertical excitation energies of gas‐phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC‐DFTB‐based rather than B3LYP‐based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas‐phase and solution spectra, typically ca. 0.1–0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent‐induced structural changes and from electrostatic solute–solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Electronic absorption spectra of rhenium(VII) compounds: The effect of the coordination number

Changes in electronic absorption spectra in the following series: MeRe(L₂)O₃ McReO3 ReO4– ReO₆5–, have been studied in terms of the qualitative MO scheme.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1563–1566, June, 1996.     

Syntheses and crystal structures of new aurate salts of adenine or guanine nucleobases

Two new gold(III) complexes with adenine or guanine nitrogenous bases as counter‐cations were synthesized. These are 6‐amino‐7H‐purine‐1,9‐diium tetrachloridogold(III) chloride monohydrate, (C₅H₇N₅)[AuCl₄]Cl·H₂O, 1 , and 2‐amino‐6‐oxo‐6,7‐dihydro‐1H‐purin‐9‐ium tetrachloridogold(III) hemihydrate, (C₅H₆N₅O)[AuCl₄]·0.5H₂O, 2 . Their crystal structures were studied using single‐crystal X‐ray diffraction and FT–IR spectroscopic techniques. The arrangement of species in the studied crystal structures implies π‐stacking interactions, as well as concomitant C—H…π interactions, hydrogen bonds and other types of noncovalent interactions, which were studied qualitatively and quantitatively using the method of molecular Voronoi–Dirichlet polyhedra. The variation of the nitrogenous base from adenine to guanine results in evident differences in the packing of the species in the crystals of 1 and 2 . The splitting and shifting of bands in the FT–IR spectra of the title compounds reveals several features representative of noncovalent interactions in their crystal structures.     

Simultaneous determination of guanine and adenine in DNA using an electrochemically pretreated glassy carbon electrode

A simple and reliable method based on adsorptive stripping at an electrochemically pretreated glassy carbon electrode (GCE) was proposed for simultaneous or individual determination of guanine and adenine in DNA. The detection sensitivity of guanine and adenine was improved greatly by activating the GCE electrochemically. After accumulation on pretreated GCE at open circuit for 5 min or at the potential of +0.3 V for 120 s, guanine and adenine produced well-defined oxidation peaks at about +0.8 and +1.1 V, respectively in pH 5 phosphate buffer. The detection limit for individual measurement of guanine and adenine was 4.5 ng ml⁻¹ (3×10⁻⁸ mol l⁻¹) and 4 ng ml⁻¹ (3×10⁻⁸ mol l⁻¹), respectively. Acid-denatured DNA showed two oxidation peaks corresponding to guanine and adenine residues in the same buffer. The proposed method can be used to estimate the guanine and adenine contents in DNA with good selectivity in a linear range of 0.25-5 μg ml⁻¹.     

氧化石墨烯修饰碳离子液体电极同时测定鸟嘌呤和腺嘌呤

以离子液体1-丁基-3-甲基咪唑六氟磷酸盐为粘合剂制备了碳糊电极,然后将氧化石墨烯滴涂到碳糊电极表面制成了一种新型的氧化石墨烯修饰碳离子液体电极。研究了鸟嘌呤和腺嘌呤在修饰电极上的电化学行为。实验结果表明,在0.1 mol/L醋酸盐缓冲溶液中(pH4.5),鸟嘌呤和腺嘌呤在该修饰电极上具有良好的电化学行为,在2.0×10-7～1.5×10-5mol/L浓度范围内鸟嘌呤和腺嘌呤的浓度在该电极上与电化学响应信号呈良好的线性关系,相关系数分别为为0.992和0.996。信噪比为3时,检出限为1.0×10-8mol/L。     

Vibrational spectra of 2-methylpropanal: Assignments,self-associations and solvent effects

Raman and infrared spectra of 2-methylpropanal (CH₃)₂CHCHO, (CH₃)₂CDCHO and (CH₃)₂CHCDO in different physical states (liquid, solid and solution) have been investigated between 4000 and 40 cm⁻¹. A complete assignment is carried out on the basis of one predominant conformer (anticlinal) in equilibrium with another less stable one (synperiplanar). Some vibrational modes of the synperiplanar form are identified. The CO stretching region is carefully examined. The complex structure of this band as a function of the physical state of the pure aldehyde and the polarity of the solvent for solutions is discussed. The structure can be explained by Fermi resonances between combinations and the fundamental νCO mode. In the pure liquid, as well as in solution, the broadness of the band can be due to random dipolar interactions. In polar basic solvents (CH₃CN and DMSO), these interactions implicate both aldehyde and solvent. When the solvent is slightly acidic (HCCl₃), weak hydrogen bonds between the aldehydic group and the solvent may occur. Moving from the liquid to the solid state, in addition to the fact that the less stable conformer disappears, molecular association between the CO groups by charge transfer may account for the observed changes.     

多璧碳管/聚中性红修饰电极的制备及对鸟嘌呤和腺嘌呤的应用研究

结合纳米材料的电催化特性和中性红聚合物薄膜的分子识别能力, 以玻碳电极为基体制备了多壁碳管/聚中性红(MWNT/PNR)修饰电极, 并用表面扫描电镜和循环伏安法进行了表征. 实验表明, 该修饰电极对腺嘌呤(A)和鸟嘌呤(G)都表现出了良好的电催化性能. 在最佳条件下, 用示差脉冲伏安法对A和G进行了测定, 其氧化峰电流于A和G的浓度分别在0.01～4 μmol/L和0.01～8 μmol/L范围内呈良好的线性关系, 检测限均为5×10-9 mol/L (S/N=3). 该修饰电极可以用来同时测定DNA中的A和G.     

Electronic absorption spectra of selenocarbonyl and thiocarbonyl compounds

Electronic absorption spectra of a variety of thiocarbonyl and selenocarbonyl compounds have been compared to point out their similarities. Interesting correlations between the absorption maxima and electronegativities of substituents in both the seleno- and thio-carbonyl compounds have been reported.     

Hierarchy of equations to calculate the linear spectra of molecular aggregates: Time‐dependent and frequency domain formulation

In a recent publication (Ritschel et al., J. Chem. Phys. 2015, 142, 034115) we have derived a hierarchy of coupled differential equations in time domain to calculate the linear optical properties of molecular aggregates. Here, we provide details about issues concerning the numerical implementation. In addition we present the corresponding hierarchy in frequency domain.     

Oxygen effects in aqueous solutions of laccases: Absorption spectra and reactivity

The optical absorption spectra of laccases in aqueous solutions were found to undergo reversible changes in the presence of O₂. It was suggested that dioxygen is coordinated in the active center of the completely oxidized native enzyme. Abnormal behavior of superoxide radical anions upon variation of the laccase concentration was found by pulse radiolysis. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1471–1474, August, 2000.     

Fabrication of poly(2,6-pyridinedicarboxylic acid)/MWNTs modified electrode for simultaneous determination of guanine and adenine in DNA

The fabrication of poly(2,6-pyridinedicarboxylic acid)/MWNTs modified glass electrode(PPDA/MWNTs/GCE) was proposed and used for individual or simultaneous determination of guanine and adenine.The performances of the PPDA/MWNTs/GCE were characterized with cyclic voltammetry(CV).The modified electrode exhibited enhanced electrocatalytic behavior and good stability for the detection of guanine and adenine.Differential pulse voltammetry(DPV) was used to determine the concentration of guanine,adenine.The detection limit(S/N = 3) for guanine and adenine was 0.045μmol/L and 0.05μmol/L,respectively.The electrochemical method for the measurement of guanine and adenine in calf thymus DNA was also developed with this modified electrode and the result was satisfactory.     

Solvatochromic effects in the absorption and fluorescence spectra of indazole and its amino derivatives

Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Berg's equation indicate different geometries of the amines in the S₀ and S₁ states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S₁ state in comparison to that in the S₀ state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S₀ and S₁ states. Results so obtained are compared and discussed.     

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.     

Simultaneous determination of adenine guanine and thymine at multi-walled carbon nanotubes incorporated with poly(new fuchsin) composite film

A composite film (MWCNTs-PNF) which contains multi-walled carbon nanotubes (MWCNTs) along with the incorporation of poly(new fuchsin) (PNF) has been synthesized on glassy carbon electrode (GCE), gold (Au) and indium tin oxide (ITO) by potentiostatic methods. The presence of MWCNTs in the composite film enhances surface coverage concentration (Γ) of PNF to ≈176.5%, and increases the electron transfer rate constant (k_s) to ≈346%. The composite film also exhibits promising enhanced electrocatalytic activity towards the mixture of biochemical compounds such as adenine (AD), guanine (GU) and thymine (THY). The surface morphology of the composite film deposited on ITO has been studied using scanning electron microscopy and atomic force microscopy. These two techniques reveal that the PNF incorporated on MWCNTs. Electrochemical quartz crystal microbalance study reveals the enhancement in the functional properties of MWCNTs and PNF. The electrocatalytic responses of analytes at MWCNTs and MWCNTs-PNF films were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). From electrocatalysis studies, well separated voltammetric peaks have been obtained at the composite film for AD, GU and THY, with the peak separation of 320.3 and 132.7 mV between GU-AD and AD-THY respectively. The sensitivity of the composite film towards AD, GU and THY in DPV technique is 218.18, 12.62 and 78.22 mA M⁻¹ cm⁻² respectively, which are higher than MWCNTs film. Further, electroanalytical studies of AD, GU and THY present in single-strand deoxyribonucleic acid (ssDNA) have been carried out using semi-derivative CV and DPV.     

A theoretical study of solvent effects on tautomerism and electronic absorption spectra of 3-hydroxy-2-mercaptopyridine and 2,3-dihydroxypyridine

The tautomerization equilibria of 3-hydroxy-2-mercaptopyridine (HMP) and 2,3-dihydroxypyridine (DHP) in vacuo and in ethanol solution have been studied using the density functional theory (DFT) at B3LYP/6-31Gd level. The results indicate that the thione form of HMP and the keto form of DHP are the most stable tautomers in the equilibrium, and the energy barrier for the thiol-thione and enol-keto proton transfer decreases significantly when the tautomerism is mediated by a specific ethanol molecule in solution. The time-dependent density functional theory--polarizable continuum model (TDDFT-PCM) calculations on all tautomers of HMP and DHP in vacuo and in ethanol have assigned the lowest pi --> pi* excitations of thione and keto tautomers to the observed absorption bands of HMP and DHP in solutions. The solvation is predicted to have relatively small effect on these pi --> pi* excitations in ethanol.