Lisa A. Kelly

Mechanisms of Photoinitiated Cleavage of DNA by 1,8-Naphthalimide Derivatives¶

Abstract Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were <105 M−1 s−1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 × 109 and 8.8 × 109 M−1 s−1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.

Molecularly imprinted ionically permeable membrane for uranyl ion

A polymeric membrane for the separation of specific metal ions can be designed by the application of the molecular imprinting technique.

Standoff detection using coherent backscattered spectroscopy

Intense laser pulses may be used for standoff detection of energetic materials. Coherent backscattered spectroscopy offers a tremendous advantage over other spectroscopic detection techniques in that it uses stimulated or amplified spontaneous emission from the sample to produce a minimally divergent, directional beam back to the detection platform. The characteristics of the backscattered beam depend largely on the intensity and pulse width of the laser source as well as the concentration and photo-physical characteristics of the target molecule. Different target molecules will exhibit different backscattered emission signals, allowing differential detection of energetic materials in the vapor phase. Because of the highly directional nature of the coherent backscattered beam, detection limits in the vapor of less than 1 ppm at ranges up to 100 meters can be anticipated.

Synthesis and characterization of molecularly imprinted uranyl ion exchange resins

Abstract Ion exchange resins with enhanced capacity and selectivity for UO2 2+ have been prepared using the molecular imprinting method. The imprinted structure was introduced by copolymerization of styrene with uranyl vinylbenzoate in the presence of divinylbenzene as the cross‐linking agent. The polymerization was carried out in bulk with free‐radical initiation using 2‐methoxyethanol as a solvent and porogen. Upon removal of the template, specific sites for UO2 2+ rebinding remained. In equilibrium binding experiments with a variety of metal ions, the polymers preferentially bound UO2 2+. Control experiments were performed with reference polymers prepared without the template (unimprinted) and Ni2+ imprinted polymers. Selectivity data show that the imprinted polymers have a higher affinity for UO2 2+ than any of the competing metal ions tested. Several variables, that are crucial in improving the capacity and selectivity of imprinted polymers, were investigated in detail. The variables included (1) the effects of template monomer content, (2) the degree of covalent cross‐linking, and (3) the particle size. The effects of the variables on capacity and selectivity were examined and are discussed.

Sequence-dependent interactions of cationic naphthalimides and polynucleotides.

The binding interactions of three naphthalimide derivatives with heteropoly nucleic acids have been evaluated using fluorescence, absorption and circular dichroism spectroscopies. Mono‐ and bifunctionalized naphthalimides exhibit sequence‐dependent variations in their affinity toward DNA. The heteropoly nucleic acids, [Poly(dA‐dT)]2 and [Poly(dG‐dC)]2, as well as calf thymus (CT) DNA, were used to understand the factors that govern binding strength and selectivity. Sequence selectivity was addressed by determining the binding constants as a function of polynucleotide composition according to the noncooperative McGhee–von Hippel binding model. Binding affinities toward [poly(dA‐dT)]2 were the largest for spermine‐substituted naphthalimides (Kb = 2−6 × 106 m−1). The association constants for complex formation between the cationic naphthalmides and [poly(dG‐dC)]2 or CT DNA (58% A‐T content) were 2–500 times smaller, depending on the naphthalmide–polynucleotide pair. The binding modes were also assessed using a combination of induced circular dichroism and salt effects to determine whether the naphthalimides associate with DNA through intercalative, electrostatic or groove‐binding. The results show that the monofunctionalized spermine and pyridinium‐substituted naphthalimides associate with DNA through electrostatic interactions. In contrast, intercalative interactions are predominant in the complex formed between the bifunctionalized spermine compound and all of the polynucleotides.

Nucleotide Oxidation Mediated by Naphthalimide Excited States with Covalently Attached Viologen Cosensitizers

The ground‐ and excited‐state interactions of polymethylene‐linked 1,8‐naphthalimide–viologen dyads with calf‐thymus DNA have been investigated. By virtue of the covalently attached viologen, the compounds represent the first example of linked chromophore/cosensitizer systems in the photooxidation of duplex DNA. The compounds associate strongly with DNA. Analysis of ground‐state spectral changes yield binding constants of 0.7–2.5 × 106M−1. Upon 355 nm pulsed irradiation of the compounds in the presence of calf‐thymus DNA, reduced viologen is observed within the laser pulse. Photoproducts are not observed on this time scale in the absence of DNA. Since ground‐state bleaching of the naphthalimide was not observed, the results suggest that DNA nucleobases are the species being oxidized. The quantum efficiency of radical production increases with the extent of binding to DNA. Under conditions where the compounds are bound predominantly to DNA, the quantum efficiencies were found to range from 0.02 to 0.03. Although small, the values represent a substantial increase in charge‐separation yield compared to 1,8‐naphthalimide compounds that lack the covalently attached viologen. The mechanism of radical production and effect of number of intervening methylenes are discussed.

Standoff detection of nitrotoluenes using 213-nm amplified spontaneous emission from nitric oxide

A method of standoff detection based on the observation of laser-induced fluorescence–amplified spontaneous emission (LIF-ASE) is described. LIF-ASE generates uniaxial intensity distributions of the observed fluorescence with the majority of intensity propagating along the excitation axis in both the forward and backward directions. The detection of bulk vapor at significant standoff distances is readily achieved. This method was used to detect NO directly and as a photoproduct after 213-nm excitation of 2-, 3-, and 4-nitrotoluene. The NO LIF-ASE spectra were studied as a function of buffer gas. These studies showed that the emission from different vibrational states was dependent upon the buffer gas used, suggesting that the populations of vibrational states were influenced by the environment. A similar sensitivity of the vibrational populations was observed when the different nitroaromatic precursors were used in nitrogen buffer gas. Such sensitivity to environmental influences can be used to distinguish among the different nitroaromatic precursors and facilitate the identification of the bulk vapor of these analytes.

Crystal structure and optical properties of erbium- and neodymium-doped zirconia nanoparticles

We report the synthesis, characterization, and optical properties of high-temperature stable lanthanide-doped luminescent zirconia nanoparticles via a novel method using carbon black as template. Dopant concentrations were varied from 1 to 5% of Er 3+ or Nd 3+ and annealing temperatures were varied from 650 to 1100 °C. The effects of the dopant concentration on crystal structure and emission properties were evaluated using x-ray powder diffraction and fluorescence spectroscopy, respectively. The lanthanide cations were found to stabilize the tetragonal phase of zirconia over the monoclinic phase as dopant concentration was increased to 5%. Increasing the annealing temperature to 1100 °C had the opposite effect and was found to stabilize the monoclinic phase of zirconia. The luminescence intensity of the Nd-doped zirconia was enhanced by two orders of magnitude over the undoped or Er-doped zirconia. In all cases, the luminescence spectra revealed increasing intensity with increasing annealing temperature. Zirconia luminescence at near-infrared wavelengths is likely caused by oxygen vacancies. This work demonstrates that the spectral signatures of fluorescent zirconia nanoparticles can be modified with small lanthanide dopant concentration. These particles will have utility in fluorescent sensors and tags, as well as new in refractory materials.

Progress Towards Fluorescent Molecular Thermometers

Material-based sensors have attracted an enormous amount of recent interest. Biological1 and chemical sensors2 are being rapidly developed to offer sensitivity and specificity for the analyte of choice. In addition, there has been growing material science interest in the creation of “smart materials,”3,4 capable of fast, reversible responses to environmental stimuli.

Optical characteristics of novel bulk and nanoengineered laser host materials

The hexagonal apatite single crystals have been investigated for their applications as laser host materials. Czochralksi and flux growth methods have been utilized to obtain single crystals. For low temperature processing (<100 0C), several techniques for crystal growth have been developed. The hexagonal apatite structure (space group P63/m) is characteristic of several compounds, some of which have extremely interesting and useful properties as laser hosts and bone materials. Calcium lanthanum silicate (Nd-doped) and lanthanum aluminate material systems were studied in detail. Nanoengineered calcium and lanthanum based silicates were synthesized by a solution method and their optical and morphological characteristics were compared with Czochralski grown bulk hydroxyapatite single crystals. Materials were evaluated by absorbance, fluorescence and Raman characteristics. Neodymium, iron and chromium doped crystals grown by a solution method showed weak but similar optical properties to that of Czochralski grown single crystals.