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Molecular dynamics simulations and time-resolved linear dichroism measurements have been employed to investigate rotational diffusion of perylene in two organic solvents, cyclohexane, a nonpolar solvent, and 2-propanol, a polar solvent. Both experiments and simulations yield a biexponential rotational anisotropy decay for the long in-plane axis. The calculated time constants were 9 and 44 ps in cyclohexane at 300 K, 13 and 75 ps in 2-propanol at 300 K, and 25 and 126 ps in 2-propanol at 263 K, in excellent agreement with corresponding time-resolved linear dichroism measurements of 14 and 52 ps, 10 and 51 ps, and 22 and 240 ps respectively. Although the viscosity of 2-propanol is more than two times that of cyclohexane at room temperature, the measured rotational reorientation times and the calculated average rotational diffusion coefficients of perylene are similar in the two solvents, demonstrating a breakdown of simple hydrodynamic theory. Analysis of the calculated rotational diffusion coefficients for the individual molecular axes showed that diffusion was highly anisotropic, with the fastest rotation around the out-of-plane axis z. This dominant motion occurred at comparable rates for perylene in cyclohexane and 2-propanol, leading to similar values of average rotational diffusion coefficients in the two solvents. The hindered spinning of perylene in cyclohexane relative to 2-propanol could be rationalized in terms of tighter packing of the former solvent around the solute in the molecular plane.

One-and two-photon excited ultrafast transient absorption measurements are reported for all-trans retinal in polar, nonpolar, and hydrogen-bonding solvents. A comparison of the response following one-and twophoton excitation allows assignment of the observed decays to the 1 B u + -like and 1 A g --like excited states. In the polar, hydrogen-bonding solvent ethanol, the transient absorption changes following one-photon excitation are dominated by a 2-ps response observed both in the decay of transient absorption and in the decay of stimulated emission observed in the region from 600 to 750 nm. Because of the strong stimulated emission gain, this response can be assigned to decay of the strongly optically allowed " 1 B u + " state. With two-photon excitation of all-trans retinal in ethanol, a 200-300 fs decay component appears as well and is assigned to decay of the " 1 A g -" state, which has a large two-photon cross section. In contrast, for all-trans retinal in hexane, both 200-300 fs and 2-ps decay components are observed following one-photon excitation, and no stimulated emission is detected. A comparison of one-and two-photon induced transient absorption leads to an assignment of the decay events. The ultrafast responses of all-trans retinal in the polar, aprotic solvents acetonitrile and propionitrile are found to be similar to the response in ethanol, indicating that protic solvents act through solvent polarity rather than a specific effect of hydrogen bonding. The effect of hydrogen bonding was studied further for all-trans retinal in mixed solvents with varying concentrations of trifluoroacetic acid, a strong hydrogen-bond donor, in hexane. As the ratio of trifluoroacetic acid to retinal increased from 1:5 to 10:1, the ultrafast response changed smoothly from "hexane-like" to "ethanol-like", with an increasingly strong stimulated emission component having a 2-ps decay, in agreement with the increased fluorescence quantum yield observed in polar and hydrogen-bonding solvents. A model is proposed incorporating two populations of retinal having different electronic relaxation pathways to explain the solvent-dependent photophysics of all-trans retinal.

Background A new method for rapid discrimination among bacterial strains based on DNA fragment sizing by flow cytometry is presented. This revolutionary approach combines the reproducibility and reliability of restriction fragment length polymorphism (RFLP) analysis with the speed and sensitivity of flow cytometry. Methods Bacterial genomic DNA was isolated and digested with a rare‐cutting restriction endonuclease. The resulting fragments were stained stoichiometrically with PicoGreen dye and introduced into an ultrasensitive flow cytometer. A histogram of burst sizes from the restriction fragments (linearly related to fragment length in base pairs) resulted in a DNA fingerprint that was used to distinguish among different bacterial strains. Results Five different strains of gram‐negative Escherichia coli and six different strains of gram‐positive Staphylococcus aureus were distinguished by analyzing their restriction fragments with DNA fragment sizing by flow cytometry. Fragment distribution analyses of extracted DNA were ∼100 times faster and ∼200,000 times more sensitive than pulsed‐field gel electrophoresis (PFGE). When sample preparation time is included, the total DNA fragment analysis time was approximately 8 h by flow cytometry and approximately 24 h by PFGE. Conclusions DNA fragment sizing by flow cytometry is a fast and reliable technique that can be applied to the discrimination among species and strains of human pathogens. Unlike some polymerase chain reaction (PCR)‐based methods, sequence information about the bacterial strains is not required, allowing the detection of unknown, newly emerged, or unanticipated strains. Cytometry 41:203–208, 2000. Published 2000 Wiley‐Liss, Inc.