Ultrafast excited-state dynamics of adenine and monomethylated adenines in solution: implications for the nonradiative decay mechanism

The DNA base adenine and four monomethylated adenines were studied in solution at room temperature by femtosecond pump-probe spectroscopy. Transient absorption at visible probe wavelengths was used to directly observe relaxation of the lowest excited singlet state (S(1) state) populated by a UV pump pulse. In H(2)O, transient absorption signals from adenine decay biexponentially with lifetimes of 0.18 +/- 0.03 ps and 8.8 +/- 1.2 ps. In contrast, signals from monomethylated adenines decay monoexponentially. The S(1) lifetimes of 1-, 3-, and 9-methyladenine are similar to one another and are all below 300 fs, while 7-methyladenine has a significantly longer lifetime (tau = 4.23 +/- 0.13 ps). On this basis, the biexponential signal of adenine is assigned to an equilibrium mixture of the 7H- and 9H-amino tautomers. Excited-state absorption (ESA) by 9-methyladenine is 50% stronger than by 7-methyladenine. Assuming that ESA by the corresponding tautomers of adenine is unchanged, we estimate the population of 7H-adenine in H(2)O at room temperature to be 22 +/- 4% (estimated standard deviation). To understand how the environment affects nonradiative decay, we performed the first solvent-dependent study of nucleobase dynamics on the ultrafast time scale. In acetonitrile, both lowest energy tautomers of adenine are present in roughly similar proportions as in water. The lifetimes of the 9-substituted adenines depend somewhat more sensitively on the solvent than those of the 7-substituted adenines. Transient signals for adenine in H(2)O and D(2)O are identical. These solvent effects strongly suggest that excited-state tautomerization is not an important nonradiative decay pathway. Instead, the data are most consistent with electronic energy relaxation due to state crossings between the optically prepared (1)pipi* state and one or more (1)npi* states and the electronic ground state. The pattern of lifetimes measured for the monomethylated adenines suggests a special role for the (1)npi* state associated with the N7 electron lone pair.     

Order and dynamics of a liquid crystalline dendrimer by means of 2H NMR spectroscopy

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).     

Kinetically controlled phase transitions in liquid‐crystalline polymers. III. Influence of the molecular weight and annealing temperature on two‐dimensional K phase formation in a side‐chain polyacrylate

Narrow fractions of a side‐chain acrylate oligomer/polymer with phenyl benzoate side chains are separated in a broad range of the degree of polymerization (7 ≤ P_w ≤ 149). An examination of the phase behavior of the obtained fractions has shown that only the longer macromolecules can form the two‐dimensional K (TDK) mesophase, whereas oligomers of a shorter main chain form the conventional nematic phase only. A critical P_w value has been observed to be necessary for the TDK mesophase formation. The temperatures and enthalpies of liquid‐crystalline phase transitions have been studied as a function of the molar mass, and the phase‐growth kinetics for the TDK phase have been studied with an Avrami treatment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2352–2360, 2005     

Modification of some mono- and bispiperidols exhibiting growth-regulating properties

Procedures for esterification of biologically active mono-and bispiperidols by reactions with benzoyl and terephthaloyl chlorides were developed.     

Influence of thermal conductivity on the heating of a substance by a high-intensity ion beam

Relations describing the process of heat transfer from the energy-dissipation region of a monoenergetic ion beam in an obstructing target within the pulse duration are derived on the basis of an analysis of the fundamental laws governing the interaction of a high-intensity ion beam with condensed media and the solution of the heat-conduction equation. The proposed analytical expressions can be used to assess the feasibility of neglecting thermal conductivity and calculating the temperature fields in the target without solving the heat-conduction boundary-value problem, namely on the basis of the characteristics of a heat-release source approximating the real source.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 16–20, June, 1986.     

Thermodynamic properties of a liquid crystal carbosilane dendrimer

The temperature dependence of the heat capacity of a first-generation liquid crystal carbosilane dendrimer with methoxyphenyl benzoate end groups is studied for the first time in the region of 6–370 K by means of precision adiabatic vacuum calorimetry. Physical transformations are observed in this interval of temperatures, and their standard thermodynamic characteristics are determined and discussed. Standard thermodynamic functions C_p^°(T), H°(T) ? H°(0), S°(T) ? S°(0), and G°(T) ? H°(0) are calculated from the obtained experimental data for the region of Т → 0 to 370 K. The standard entropy of formation of the dendrimer in the partially crystalline state at Т = 298.15 K is calculated, and the standard entropy of the hypothetic reaction of its synthesis at this temperature is estimated. The thermodynamic properties of the studied dendrimer are compared to those of second- and fourth-generation liquid crystal carbosilane dendrimers with the same end groups studied earlier.     

DETERMINATION OF THRESHOLD N-FACTORS OF THE LAMINAR–TURBULENT TRANSITION IN A SUBSONIC BOUNDARY LAYER ON A PROLATE SPHEROID

Journal of Applied Mechanics and Technical Physics - Results of a numerical study of the laminar-turbulent transition in the boundary layer of a prolate spheroid obtained with the use of the ANSYS...