PHOTOCHEMISTRY OF URACIL. INTERSYSTEM CROSSING AND DIMERIZATION IN AQUEOUS SOLUTION

Abstract— Ultraviolet irradiation of ¹⁴C-uracil in aqueous solution results in the formation of hydrate and dimer photoproducts. The rate of dimerization increases with increasing uracil concentration, and decreases with increasing concentration of oxygen in solution. The kinetics are in agreement with a model previously proposed to account for the reactions, in which dimerization occurs by a reaction involving the triplet state of uracil, but hydration occurs from an excited singlet state. Oxygen reduces dimer formation by quenching the triplet. The quantum yield for intersystem crossing (ISC) to the triplet depends on the irradiation wavelength, increasing from 0.0014 at 280 nm to 0.016 at 230 nm. The ratio of rate constants for reaction of the triplet with oxygen and for dimerization is 1.1; the ratio of rate constants for triplet decay and for dimerization is 5.9 × 10^-5 M. The increase in ISC with photon energy suggests that ISC is favoured from excited vibrational levels. The quantum yield for hydration is about 0.002 at pH 4.5 for all wavelengths, but increases as the pH is decreased.     

PROPERTIES OF THYMINE DIMERS

Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross-sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross-sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.     

The amide rotational barriers in picolinamide and nicotinamide: NMR and ab initio studies

Pyridine carboxamides are a class of medicinal agents with activity that includes the reduction of iron-induced renal damage, the regulation of nicotinamidase activity, and radio- and chemosensitization. Such pharmacological activities, and the prevalence of the carboxamide moiety and the importance of amide rotations in biology, motivate detailed investigation of energetics in these systems. In this study, we report the use of dynamic nuclear magnetic resonance to measure the amide rotational barriers in the pyridine carboxamides picolinamide and nicotinamide. The activation enthalpies and entropies of DeltaH++ = 12.9 +/- 0.3 kcal/mol and DeltaS++ = -7.7 +/- 0.9 cal/mol K for nicotinamide and DeltaH++ = 18.3 +/- 0.4 kcal/mol and DeltaS++ = +1.3 +/- 1.0 cal/mol K for picolinamide report a substantial energetic difference for these regioisomers. Ab initio calculations of the rotational barriers are in good agreement with the experimentally determined values and help partition the 5.4 kcal/mol enthalpy difference into its major contributions. Of principal importance are the variations in steric interactions in the ground states of picolinamide and nicotinamide, superior pi electron donation from the pyridine ring in the transition state of nicotinamide, and an intramolecular hydrogen bond in the ground state of picolinamide.     

Imperfection sensitivity of coincident buckling systems

A two-degree of freedom structural system with both buckling loads equal or nearly so is examined, allowing for the presence of small imperfections specified by the values of two general imperfection parameters. A discrete generalized coordinates energy method is used, and the system considered is non-symmetric in the two modes of deflection. Conclusions regarding the types of behaviour possible are obtained by examination of all possible equilibrium path shapes. Comparison is made to behaviour of idealized systems. Change in buckling load value with imperfection size, or imperfection sensitivity, is established and imperfection sensitivity surface forms are sketched.     

Directed distance in digraphs: Centers and medians

The directed distance d_D(u, v) from a vertex u to a vertex v in a strong digraph D is the length of a shortest (directed) u - v path in D. The eccentricity of a vertex v in D is the directed distance from v to a vertex furthest from v. The distance of a vertex v in D is the sum of the directed distances from v to the vertices of D. The center C(D) of D is the subdigraph induced by those vertices of minimum eccentricity, while the median M(D) of D is the subdigraph induced by those vertices of minimum distance. It is shown that for every two asymmetric digraphs D₁ and D₂, there exists a strong asymmetric digraph H such that C(H) ? D₁ and M(H) ? D₂, and where the directed distance from C(H) to M(H) and from M(H) to C(H) can be arbitrarily prescribed. Furthermore, if K is a nonempty asymmetric digraph isomorphic to an induced subdigraph of both D₁ and D₂, then there exists a strong asymmetric digraph F such that C(F) ? D₁, M(F) ? D₂ and C(F) ∩ M(F) ? K. © 1993 John Wiley & Sons, Inc.     

A novel approach to phosphopeptide synthesis-preparation of glu-pser-leu

A synthesis of the phosphotripeptide Glu-PSer-Leu is reported via the stepwise incorporation of the protected phosphoamino acid N^α-tert-butoxycarbonyl-o-dibenzylphosphono-L-serine.     

Correlation of the Voice Handicap Index (VHI) and the Voice-Related Quality of Life Measure (V-RQOL)

The purpose of this study was to investigate the correlation between the Voice Handicap Index (VHI) and the Voice-Related Quality of Life Measure (V-RQOL), and to test conversion of scores between the two instruments. Understanding the relationship between instruments will facilitate comparison of voice outcome studies using different measures. A retrospective medical chart review of 140 consecutive patients with a chief complaint related to their voice presenting for speech pathology voice evaluation following laryngology evaluation and diagnosis was adopted. Each patient who filled out the VHI and V-RQOL within a 2-week period with no intervening treatment was included in the study. Correlation analysis for total scores was performed for the patients meeting inclusion criteria (n=132). Correlations were also performed as a function of diagnosis. Calculated VHI score based on measured V-RQOL score was compared to measured VHI score. Pearson correlation between scores on the VHI and V-RQOL was -0.82. There was no significant difference between the mean measured and mean calculated VHI scores. For individual scores, however, regression analysis did reveal a significant difference between calculated and measured VHI. The VHI and V-RQOL are highly correlated; however, this study suggests that the two instruments are not interchangeable for individuals.