Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Biocidal activity of a light-absorbing fluorescent conjugated polyelectrolyte

Herein we describe studies that indicate a cationic conjugated polyelectrolyte shows biocidal activity against gram-negative bacteria (Escherichia coli, E. coli, BL21, with plasmids for Azurin and ampicillin resistance) and gram-positive bacterial spores (Bacillus anthracis, Sterne, B. anthracis, Sterne). These studies were carried out with aqueous suspensions of the conjugated polyelectrolyte, with the polyelectrolyte in supported formats and with samples in which the conjugated polyelectrolyte was coated on the bacteria. The results are interesting in that the biocidal activity is light-induced and appears effective due to the ability of the conjugated polyelectrolyte to form a surface coating on both types of bacteria. The effects observed here should be general and suggest that a range of conjugated polyelectrolytes in different formulations may provide a useful new class of biocides for both dark and light-activated applications.     

Computable error bounds for pointwise derivatives of a Neumann problem

In this paper we discuss the recovery of derivatives and thecomputation of rigorous and useful upper bounds for the pointwiseerror in the recovered derivatives, for finite element approximationsof the Laplace equation with Neumann boundary conditions, especiallyat points close to or on a smooth, curved boundary. We analyzethe dipole image technique for the case of curved boundaries,and show how to compute reliable recovered derivatives and errorbounds even in the limiting case of points lying on the curvedboundary. Numerical experiments show reasonably tight errorbounds for points both close to and away from a curved boundary.     

Superquenching in cyanine pendant poly(L-lysine) dyes: dependence on molecular weight, solvent, and aggregation.

A series of poly(L-lysines) ranging in number of repeat units (N(PRU)) from 6 to 900 has been synthesized and the photophysics of the series and monomer cyanine dye have been studied in solution. In water or aqueous dimethyl sulfoxide, the oligomers and polymers exhibit high sensitivity to fluorescence quenching by oppositely charged electron acceptors; in this study, 9,10-anthraquinone-2,6-disulfonate was used as a quencher for the cationic fluorescent polyelectrolytes. Quenching constants (K(SV)) measured in 50:50 (v/v) dimethyl sulfoxide-water increase monotonically with increase in N(PRU) ranging from 630 M(-1) for monomer to 1.2 x 10(9) M(-1) for dilute solutions of the polymer having N(PRU) approximately 900. The polymers having N(PRU) > 100 exhibit predominantly J-aggregate absorption and fluorescence and enhanced susceptibility to quenching. For the polymers exhibiting strong J-aggregation, the effective exciton domain quenched by a single quencher reaches approximately 100 PRU. The results of this study permit a semiquantitative analysis of superquenching of fluorescent polyelectrolytes in solution and the factors that control it.     

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers

A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of pi-conjugated oligomers and semiconductor nanocrystals.     

Femtosecond to nanosecond dynamics in fullerenes: Implications for excitedstate optical nonlinearities

We compared detailed dynamics of the excited-state absorption for C₆₀ in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C₆₀ molecules plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of our C₆₀-glass composites occur on long (ns) timescales, comparable to those in solution; thin film samples, by contrast, show rapid decay (<20 picoseconds). These results demonstrate that C₆₀-sol-gel glass composites contain C₆₀ in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time (600±100ps) and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450–950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range from 620–810 nm.