The rates of duplex formation for two octamers of DNA (5' d-CACGGCTC/5' d-GAGCCGTG and 5' d-CACAGCAC/5' d-GTGCTGTG), the homologous RNA, and both sets of hybrids in 1 M NaCl buffer have been measured using stopped-flow spectroscopy. In addition, the thermodynamic parameters, ΔH° and ΔS°, have been determined for the same sequences under the same buffer conditions using optical melting techniques. These data reveal a linear free energy relationship between the free energy of activation for denaturation and the change in free energy for formation of the duplexes. This relationship indicates that these duplex formation reactions occur through a common unstructured transition state that is more similar to the single strands in solution than to the ensuing duplex. In addition, these data confirm that the greater stability of RNA duplexes relative to that of homologous DNA and hybrid duplexes is controlled by the denaturation rate and not the duplex formation rate.

The surface diffusion of n-alkanes on Ru(OOI) was measured using laser-induced thermal desorption (LITD) techniques. The surface diffusion coefficients for propane, n-butane, npentane, and n-hexane all displayed Arrhenius behavior. The surface diffusion activation energies increased linearly with carbon chain length from Edif = 3.0 ± 0.1 kcallmol for propane to Edif = 4.8 ± 0.2 kcallmol for n-hexane. In contrast, the surface diffusion preexponentials remained nearly constant at Do =0.15 cm 2 Is. Measurements performed at different coverages also revealed that the surface diffusion coefficients were coverageindependent for all the n-alkanes on Ru(OOI). The surface corrugation ratio 0 was defined as the ratio of the diffusion activation energy to the desorption activation energy, 0 = Edif IE des ' The surface corrugation ratio was observed to be remarkably constant at 0=0.3 for all the nalkanes. This constant corrugation ratio indicated a linear scaling between the diffusion activation energy and the desorption activation energy. This behavior also suggested that the nalkanes move with a rigid configuration parallel to the Ru(OOI) surface.5136

The first quantitative model for the partially reversible thermochromism in Langmuir-Blodgett films of the polymerized Cd2+ salts of 10,12-tricosadiynoic acid (TCDA) and 10,12-pentacosadiynoic acid (PCD A) is presented. The visible spectrum as a function of temperature provides evidence for two parallel processes, one of which is reversible. The following kinetic model is proposed which qualitatively and quantitatively accounts for the observed reversible thermochromism: B R (kf, fer); P -* R (fe2). B and P stand for two distinct forms of the blue polymer, and R stands for the red form of the polymer. Activation barriers of E{ = 22.5 kcal/mol, Et = 21.4 kcal/mol, and E2 -23.0 kcal/mol are obtained from the TCDA spectra as a function of temperature using a "normal" preexponential factor of 1012 s-1 and Et and Et as adjustable parameters. The same model can be fit to films of PCDA and gives activation barriers of Et = 21.5 kcal/mol, Et = 21.0 kcal/mol, and E2 = 22.5 kcal/mol.

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The surface diffusion coefficient for hydrogen on Ru(001) at low coverage was measured using laser-induced thermal desorption techniques. In the temperature range between 260 and 330 K, the diffusion coefficients displayed Arrhenius behavior with an activation barrier Ediff=4.0±0.5 kcal and a preexponential factor D0=6.3×10−4 cm2/s. Agreement between the experimental and theoretical parameters suggests that hydrogen diffuses on the surface by moving from a threefold site to a neighboring threefold site via a twofold site. Surface contaminants such as carbon and oxygen were observed to produce dramatic effects on the hydrogen surface diffusion rate.