Stereochemical, structural, and thermodynamic origins of stability differences between stereoisomeric benzo[a]pyrene diol epoxide deoxyadenosine adducts in a DNA mutational hot spot sequence.

Benzo[a]pyrene (BP), a prototype polycyclic aromatic hydrocarbon (PAH), can be metabolically activated to the enantiomeric benzo[a]pyrene diol epoxides (BPDEs), (+)-(7R,8S,9S,10R)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and the (-)-(7S,8R,9R,10S) enantiomer. These can react with adenine residues in DNA, to produce the stereoisomeric 10S (+)- and 10R (-)-trans-anti-[BP]-N(6)-dA adducts. High-resolution NMR solution studies indicate that in DNA duplexes the 10R (-) adduct is intercalated on the 5'-side of the modified adenine, while the 10S (+) adduct is disordered, exhibits multiple adduct conformations, and is positioned on the 3'-side of the modified adenine. Duplexes containing the 10S (+) adduct positioned at A within codon 61 of the human N-ras sequence CAA are thermodynamically less stable and more easily excised by human DNA repair enzymes than those containing the 10R (-) adduct. However, the molecular origins of these differences are not understood and represent a fascinating opportunity for elucidating structure-function relationships. We have carried out a computational investigation to uncover the structural and thermodynamic origins of these effects in the 11-mer duplex sequence d(CGGACAAGAAG).d(CTTCTTGTCCG) by performing a 2-ns molecular dynamics simulation using NMR solution structures as the basis for the starting models. Then, we applied the MM-PBSA (molecular mechanics Poisson-Boltzmann surface area) method to compute free energy differences between the stereoisomeric adducts. The 10R (-) isomer is more stable by approximately 13 kcal/mol, of which approximately 10 kcal/mol is enthalpic, which agrees quite well with their observed differences in thermodynamic stability. The lower stability of the 10S (+) adduct is due to diminished stacking by the BP moiety in the intercalation pocket, more helix unwinding, and a diminished quality of Watson-Crick base pairing. The latter stems from conformational heterogeneity involving a syn-anti equilibrium of the glycosidic bond in the modified adenine residue. The lower stability and conformational heterogeneity of the 10S (+) adduct may play a role in its enhanced susceptibility to nucleotide excision repair.     

Field splitting of a nonabsorbing state

This paper analyzes a four-level system interacting with four strong fields with frequencies combining into a cycle. The conditions for coherent population trapping, when a nonabsorbing superposition of states forms, are calculated. It is found that, in contrast to a three-level system, the nonabsorbing state splits, i.e., is realized at two values of the detuning. Such splitting manifests itself as narrow dips in the frequency dependence of the upper-level population. Similar dips are found in the spectrum of the nonlinear susceptibility, which is responsible for the conversion efficiency in the process of four-wave mixing. Zh. éksp. Teor. Fiz. 115, 1961–1972 (June 1999)     

What Do Composition Operators Know About Inner Functions?

 This paper gives several different ways in which operator norms characterize those composition operators that arise from holomorphic self-maps ϕ of the unit disc that are inner functions. The setting is the Hardy space H ² of the disc, and the key result is a characterization of inner functions in terms of the asymptotic behavior of the Nevanlinna counting function. The case offers an interesting surprise. (Received 25 June 1999; in revised form 29 September 1999)     

Triplet cooper pairing in nuclear matter and rotational states of superdeformed nuclei

One hundred and sixty-one rotational bands of superdeformed states in nuclei are considered on the basis of a model that admits triplet Cooper pairing in superfluid nuclear matter. The behavior of the dynamical moment of inertia for such states is investigated within this model, which is shown to comply well with available experimental data and to describe successfully the rotational spectra of superdeformed states.     

Second harmonic generation in polymers containing dithiosquarylium dye

Polymer layers containing the dye 2,4-bis(1,3,3-trimethyl-2-indolinylidenemethyl)-1,4-dithiosquaraine display second-order nonlinear optical properties. Depending on conditions of preparation of the layers, the monomer form of the dye or the intermolecular charge transfer complex between two dye molecules are responsible for the second harmonic generation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 86–91, January, 1999.     

A minimal area problem in conformal mapping II

LetS denote the usual class of functionsf holomorphic and univalent in the unit diskU. For 0<r<1 andr(1+r)⁻²<b<r(1−r)⁻², letS(r, b) be the subclass of functionsf∈S such that |f(r)|=b. In Theorem 1, we solve the problem of minimizing the Dirichlet integral inS(r, b). The first main ingredient of the solution is the establishment of sufficient regularity of the domains onto whichU is mapped by extremal functions, and here techniques of symmetrization and polarization play an essential role. The second main ingredient is the identification of all Jordan domains satisfying a certain kind of functional equation (called “quadrature identities”) which are encountered by applying variational techniques. These turn out to be conformal images ofU by mappings of a special form involving a logarithmic function. In Theorem 2, this aspect of our work is generalized to encompass analogous minimal area problem when a larger number of initial data are prescribed. The third author thanks for its hospitality the Mittag-Leffler Institute of Royal Swedish Academy of Sciences where this work was finalized. This author was supported in part by the Swedish Institute and by the Russian Fund for Fundamental Research, grant no. 97-01-00259.     

The superconductive states in the dislocation structures

On the basis of an analysis of the Ginzburg-Landau equations it is shown that in the dislocation structures above the critical temperature and the critical magnetic field of homogeneous superconductor there is an unhomogeneous superconductive order parameter. Such states may be either localized for the single dislocation and the dislocations pile-up or delocalized for periodical systems. The temperature shift, the critical field and the current are investigated.