Barrier-free intermolecular proton transfer in the uracil-glycine complex induced by excess electron attachment

The photoelectron spectra (PES) of anions of uracil-glycine and uracil-phenylalanine complexes reveal broad features with maxima at 1.8 and 2.0 eV. The results of ab initio density functional B3LYP and second order M?ller-Plesset theory calculations indicate that the excess electron occupies a π^* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of glycine to the O8 atom of uracil. As a result, the four most stable structures of the anion of uracil-glycine complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated glycine. The similarity between the PES spectra for the uracil complexes with glycine and phenylalanine suggests that the BFPT is also operative in the case of the latter anionic species. The BFPT to the O8 atom of uracil may be related to the damage of nucleic acid bases by low energy electrons because the O8 atom is involved in a hydrogen bond with adenine in the standard Watson-Crick pairing scheme. Received 6 April 2002 Published online 13 September 2002     

Dendrimeric Gd(III) complex of a monophosphinated DOTA analogue: optimizing relaxivity by reducing internal motion

A marked increase of relaxivity has been observed upon rigidifying the internal frame of Gd-containing PAMAM dendrimers: the effect has been attained by either protonation of the dendrimer or by forming supramolecular adducts with cationic polyaminoacids.     

Theoretical methods in thermodynamics of condensed phases

Theoretical possibilities of determining energetic and thermodynamic characteristics of chemical entities in gaseous and condensed (solid and liquid) phases are briefly reviewed. The considerations include quantum chemistry methods which enable evaluation of energetic quantities and statistical thermodynamics dependencies necessary for determining other thermodynamic characteristics. The possible applications of these methods are also discussed in brief.     

Light- and temperature-assisted spin state annealing: accessing the hidden multistability

Among responsive multistable materials, spin crossover (SCO) systems are of particular interest for stabilizing multiple spin states with various stimulus inputs and physical outputs. Here, in a 2D Hofmann-type coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂] (isoq = isoquinoline), a medium-temperature annealing process is introduced after light/temperature stimulation, which accesses the hidden multistability of the spin state. With the combined effort of magnetic, crystallographic and Mössbauer spectral investigation, these distinct spin states are identified and the light- and temperature-assisted transition pathways are clarified. Such excitation-relaxation and trapping-relaxation joint mechanisms, as ingenious interplays between the kinetic and thermodynamic effects, uncover hidden possibilities for the discovery of multistable materials and the development of multistate intelligent devices.

Two new two-stage manipulation protocols, namely light- and temperature-assisted spin state annealing (LASSA/TASSA), are applied to a spin crossover coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂], revealing the hidden multistability of spin states.     

X‐ray investigations of bicyclic α‐methylene‐δ‐valerolactones. V. (4aS,7R,8aR)‐7‐Isopropenyl‐4a‐methyl‐3‐methyleneperhydrochromen‐2‐one

The title compound, C₁₄H₂₀O₂, adopts a conformation in which the δ‐valerolactone and cyclohexane rings are almost coplanar with one another. The γ‐methyl substituent occupies an axial position with respect to the cyclohexane ring. The δ‐valerolactone moiety adopts an envelope arrangement, while the cyclohexane ring exists in a chair conformation.     

Propagating transverse electric and transverse magnetic modes in liquid crystal-clad planar waveguides

ABSTRACT

In a planar dielectric waveguide, weak confinement of a propagating mode in a high index core leads to a measurable evanescent interaction with the cladding. In this work, we study the effect of a reorientable anisotropic cladding on the behaviour of Transverse Electric (TE) and Transverse Magnetic (TM) mode polarisations using a liquid crystal (LC)-clad waveguide architecture. The polarised evanescent field of a guided mode interacts with a voltage-tunable birefringent LC cladding to deflect an out-coupled beam. Experimental measurements are coupled with a theoretical framework and show good consistency with simulation results. We isolate the effect of mode confinement by changing the thickness of the high index core. Interactions between the LC index ellipsoid and the mode polarisation are probed by changing the initial alignment of the LC. Finally, we examine the difference in deflection between TE and TM modes, which incorporates both a change in mode confinement and a difference in LC index components.     

Interaction with glycine increases stability of a mutagenic tautomer of uracil. A density functional theory study

The most stable structures for the gas-phase complexes of minor tautomers of uracil (U) with glycine (G) were characterized at the density functional B3LYP/6-31++G level of theory. These are cyclic structures stabilized by two hydrogen bonds. The relative stability of isolated tautomers of uracil was rationalized by using thermodynamic and structural arguments. The stabilization energies for complexes between the tautomers of U and G result from interplay between the stabilizing two-body interaction energies and destabilizing one-body terms. The latter are related to the energies of (i) tautomerization of the unperturbed moieties and (ii) distortions of the resulting rare tautomers in the complex. The two-body term describes the interaction energy between distorted tautomers. The two-body interaction energy term correlates with perturbations of length of the proton-donor bonds as well as with deprotonation enthalpies and proton affinities of the appropriate monomer sites. It was demonstrated that the relative instability of rare tautomers of uracil is diminished due to their interactions with glycine. In particular, the instability of the third most stable tautomer (U(III)) is decreased from 11.9 kcal/mol for non-interacting uracil to 6.7 kcal/mol for uracil in a complex with the zwitterionic tautomer of glycine. A decrease of instability by 5.2 kcal/mol could result in an increase of concentration of U(III) by almost 5 orders of magnitude. This is the tautomer with proton donor and acceptor sites matching guanine rather than adenine. Moreover, kinetic characteristics obtained for the glycine-assisted conversion of the most stable tautomer of uracil (U(I)) to U(III) indicate that the U(I)<-->U(III) thermodynamic equilibrium could be easily attained at room temperature. The resulting concentration of this tautomer falls in a mutationally significant range.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

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.     

Barrier-free intermolecular proton transfer induced by excess electron attachment to the complex of alanine with uracil

The photoelectron spectrum of the uracil-alanine anionic complex (UA)(-) has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6 and 2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)(-) anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second-order M?ller-Plesset level of theory with 6-31++G(*) (*) basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61, and 0.57 eV. The electron hole in complexes of uracil with alanine is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)(-) occupies a pi(*) orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as a neutral radical of hydrogenated uracil solvated by a deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine, and together they indicate that the BFPT process is not very sensitive to the nature of the amino acid's hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons.