Individual DNA molecules undergoing agarose gel electrophoresis were viewed with the aid of a fluorescence microscope. Molecular shape and orientation were studied in both steady and pulsed electric fields. It was observed that (i) DNA macromolecules advanced lengthwise through the gel in an extended configuration, (ii) the molecules alternately contracted and lengthened as they moved, (iii) the molecules often became hooked around obstacles in a U-shape for extended periods, and (iv) the molecules displayed elasticity as they extended from both ends at once. A computer model has been developed that simulates the migration of the molecules in a rotating-field gel electrophoresis experiment.

Europium beta diketonates are easily synthesized highly luminescent complexes with high temperature sensitivity. We report on the temperature dependence of the luminescence of recently synthesized europium complexes originally prepared for use as light emitting diodes. It has been discovered that when incorporated in a polymer matrix, their decay lifetime can provide accurate measurement of temperature. Their lifetime as a function of temperature depends on three factors: (i) the type and number of ligands in the complex, (ii) the particular polymer used for the matrix, and (iii) the europium chelate to polymer matrix concentration ratio. Various tris and tetrakis europium chelates are used to study ligand effects, while the polymers FIB, polycarbonate, and Teflon© are used to analyze matrix effects. In all cases studied, higher concentrations give rise to shorter lifetimes and higher temperature sensitivities, with sensitivity defined as ΔI/(IrefΔT). We propose to explain this phenomenon by using the following equation: 1/τobs≡Ktotal=kr+knr(T)+kc([Eu]). Here Ktotal is the observed decay rate, which is the inverse of the observed lifetime, while kr and knr(T) are the radiative and nonradiative decay rates, respectively. As well as being dependent on temperature, knr(T) for these complexes is very dependent on the environment, i.e., solvent or polymer, and can be considered as ken(T). The rate kc([Eu]) is the quenching term dependent on the concentration of the europium complex.

No abstract

A series of dimethyl-n -alkyl bonded phase sorbents ranging in length from C, to C,, have been prepared and studied by use of Fourier transform infrared spectrometry (FT-IR). Spectra have been produced for each of the bonded phases, and comparisons are made to the Corresponding silane reagents. By use of assignments reported for nonplanar conformations of normal alkanes, the supramolecular structure of these bonded phase systems is examined. The presence of absorption bands at 1341, 1354, and 1367 cm-' In the bonded phase spectra confirms that bonded alkyl chains are bent, i.e., a significant fraction of the carbon-carbon bonds are in the "gauche" configuration. The degree of conformational disorder in chemically bonded statlonary phases Is shown to be comparable to that observed in the corresponding n -alkane liquids at room temperature and above. Phase transitions for the bonded phases are not observed at temperatures near or below the melting points of alkane and silane reagents of comparable length. Instead, the bonded phase chains are seen to retain a notable degree of disorder at low temperature. Finally, the effect of mobile phase environment on chain conformation Is examined for a range of deuterated water/methanoi mixtures varying from 70 % to 100% methanol. Evidence is presented that in this range the mobile phase effects a signlflcant increase in the order of the stationary phase.'The popularity of reversed-phase separations, particularly with CI8 bonded packing materials, can be attributed to the success of this technique in solving a diverse m a y of problems. Surprisingly though, a detailed understanding of the mechanisms of solute retention is lacking. The difficulty in describing the retention process originates in the nature of the bonded phase itself. Almost all current types of bonded phases for use in liquid chromatography are produced from the reaction of chloro-or alkoxyalkylsilanes with silanol groups at the silica surface. In most instances, reaction conditions are such that a bonded monomeric layer results, with an overall density of chains of about half that of the corresponding liquids (1-3). As noted by Guiochon (4), the properties of bonded phases are thus expected to differ from bulk liquids, not only because the distance between chains is greater than in liquids but also because translational and rotational motions are absent.The question of whether a bonded phase should be considered a surface or a thin liquid film has given rise to two theories of solute retention: adsorption and partitioning. Considerable effort has been expended by workers trying to 'Current address: National Bureau of Standards, Bldg. 222, Rm Current address: Department of Chemistry, Southern Methodist A113, Washington, DC 20234. University, Dallas, TX 75275.classify retention as one of the two categories (5-7), but evidence for such a classification remains inconclusive. Clearly, before an accurate picture of solute retention can be built up, the microscopic properties of the anchored hydrocarbons must be eluc...

Abstract— The fluorescence yields and lifetimes of fluorescein and nine brominated derivatives in basic ethanol are reported. Calorimetric. photometric, and lifetime methods are used independently to measure the yields. A new and simple calorimetric method is presented for this purpose, and the accuracy of the techniques is assessed. There is good agreement between the calorimetric and photometric results. The importance of parameters such as purity, pH, and fluoresence reabsorption is illustrated. The theoretical determination of the natural radiative lifetime from the absorption spectrum is shown to have inherent ambiguities, so that only calorimetric methods provide a reliable, independent check for photometric yield measurements.