The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of approximately (87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm2 molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of approximately 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.

In this communication we report the first use of the Raman laser tweezers technique to trap and hold a mixed droplet of oleic acid and water at atmospheric pressure for 30 min, oxidize the oleic acid on the droplet, follow the decay of reactants and the growth of chemical products using Raman spectroscopy, and monitor the growth in size of the droplet as it becomes more hydrophilic. We demonstrate that the oxidation of organic films on water droplets could have large climatic effects in the atmosphere. We show that cloud-droplet growth and activation of cloud condensation nuclei (to become cloud droplets) is retarded by the presence of an organic film and that chemical oxidation of this film would allow a cloud droplet to grow, reducing cloud albedo by inducing precipitation, and would allow a cloud condensation nucleus to grow to a cloud droplet, thus forming a cloud and increasing the albedo of the Earth.

Fluorescence resonance energy transfer, FRET, can be used to obtain long-range distance information for macromolecules and is particularly powerful when used in single-molecule studies. The determination of accurate distances requires knowledge of the fluorophore position with respect to the macromolecule. In this study we have used NMR to determine the structure of the commonly used fluorophore indocarbocyanine-5 (Cy5) covalently attached to the 5'-terminus of double-helical DNA. We find that Cy5 is predominantly stacked onto the end of the duplex, in a manner similar to an additional base pair. This is very similar to the behavior of Cy3 terminally attached to DNA and suggests that the efficiency of energy transfer between Cy3 and Cy5, that are attached to nucleic acids in this way, will exhibit significant dependence on fluorophore orientation.

Fluorescent nucleobase analogues are used extensively to probe the structure and dynamics of nucleic acids. The fluorescence of the adenine analogue 2-aminopurine and the cytosine analogue pyrrolocytosine is significantly quenched when the bases are located in regions of double-stranded nucleic acids. To allow more detailed structural information to be obtained from fluorescence studies using these bases, we have studied the excited-state properties of the bases at the CIS and TDB3LYP level in hydrogen-bonded and base-stacked complexes. The results reveal that the first excited state (the fluorescent state) of a hydrogen-bonded complex containing 2-aminopurine and thymine is just the first excited state of 2-aminopurine alone. However, the same cannot be said for structures in which 2-aminopurine is base stacked with other nucleobases. Stacking causes the molecular orbitals involved in the fluorescence transition to spread over more than one base. The predicted rate for the fluorescence transition is reduced, thus reducing the fluorescence quantum yield. The decrease in radiative rate varies with the stacking arrangement (e.g., A- or B-form DNA) and with the identity of the nucleobase with which 2-aminopurine is stacked. Stacking 2-aminopurine between two guanine moieties is shown to significantly decrease the energy gap between the first and second excited states. We do not find reliable evidence for a low-energy charge-transfer state in any of the systems that were studied. In the case of pyrrolocytosine, base stacking was found to reduce the oscillator strength for the fluorescence transition, but very little spreading of molecular orbitals across more than one base was observed.

The results of an experimental study into the oxidative degradation of proxies for atmospheric aerosol are presented. We demonstrate that the laser Raman tweezers method can be used successfully to obtain uptake coefficients for gaseous oxidants on individual aqueous and organic droplets, whilst the size and composition of the droplets is simultaneously followed. A laser tweezers system was used to trap individual droplets containing an unsaturated organic compound in either an aqueous or organic (alkane) solvent. The droplet was exposed to gas-phase ozone and the reaction kinetics and products followed using Raman spectroscopy. The reactions of three different organic compounds with ozone were studied: fumarate anions, benzoate anions and alpha-pinene. The fumarate and benzoate anions in aqueous solution were used to represent components of humic-like substances, HULIS; alpha-pinene in an alkane solvent was studied as a proxy for biogenic aerosol. The kinetic analysis shows that for these systems the diffusive transport and mass accommodation of ozone is relatively fast, and that liquid-phase diffusion and reaction are the rate determining steps. Uptake coefficients, gamma, were found to be (1.1 +/- 0.7) x 10(-5), (1.5 +/- 0.7) x 10(-5) and (3.0-7.5) x 10(-3) for the reactions of ozone with the fumarate, benzoate and alpha-pinene containing droplets, respectively. Liquid-phase bimolecular rate coefficients for reactions of dissolved ozone molecules with fumarate, benzoate and alpha-pinene were also obtained: kfumarate = (2.7 +/- 2) x 10(5), kbenzoate = (3.5 +/- 3) x 10(5) and kalpha-pinene = (1-3) x 10(7) dm3 mol(-1) s(-1). The droplet size was found to remain stable over the course of the oxidation process for the HULIS-proxies and for the oxidation of alpha-pinene in pentadecane. The study of the alpha-pinene/ozone system is the first using organic seed particles to show that the hygroscopicity of the particle does not increase dramatically over the course of the oxidation. No products were detected by Raman spectroscopy for the reaction of benzoate ions with ozone. One product peak, consistent with aqueous carbonate anions, was observed when following the oxidation of fumarate ions by ozone. Product peaks observed in the reaction of ozone with alpha-pinene suggest the formation of new species containing carbonyl groups.

The reaction between gas-phase ozone and monolayers of the unsaturated lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC, on aqueous solutions has been studied in real time using neutron reflection and surface pressure measurements. The reaction between ozone and lung surfactant, which contains POPC, leads to decreased pulmonary function, but little is known about the changes that occur to the interfacial material as a result of oxidation. The results reveal that the initial reaction of ozone with POPC leads to a rapid increase in surface pressure followed by a slow decrease to very low values. The neutron reflection measurements, performed on an isotopologue of POPC with a selectively deuterated palmitoyl strand, reveal that the reaction leads to loss of this strand from the air-water interface, suggesting either solubilization of the product lipid or degradation of the palmitoyl strand by a reactive species. Reactions of (1)H-POPC on D(2)O reveal that the headgroup region of the lipids in aqueous solution is not dramatically perturbed by the reaction of POPC monolayers with ozone supporting degradation of the palmitoyl strand rather than solubilization. The results are consistent with the reaction of ozone with the oleoyl strand of POPC at the air-water interface leading to the formation of OH radicals. The highly reactive OH radicals produced can then go on to react with the saturated palmitoyl strands leading to the formation of oxidized lipids with shorter alkyl tails.

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An investigation into the reactions between Cl atoms and acrolein (1), methacrolein (2) and methyl vinyl ketone (3)is presented. Values of the rate constants for the reactions have been determined by an absolute technique for the Ðrst time. At a pressure of 1.6 Torr, the rate constants obtained were :6 in units of 10~10 cm3 molecule~1 s~1. was also determined at atmospheric pressure using k 3 \ 0.99 ^0.20 k 1 a relative-rate technique. The rate constant obtained was (2.2 ^0.3) ] 10~10 cm3 molecule~1 s~1 ; the larger value compared with that for 1.6 Torr is thought to reÑect a true dependence of the reaction rate on pressure. The Ðnal products of the reactions performed under an atmosphere of synthetic air were investigated using FTIR spectroscopy. The only chlorinated organic species identiÐed as products of the reactions were chloroacetaldehyde in the case of reaction of Cl atoms with acrolein ; chloroacetone with methacrolein ; and chloroacetaldehyde with methyl vinyl ketone. Branching ratios for abstraction (the fraction of reactions occurring by abstraction relative to the sum of addition and abstraction processes) were found to be 0.22 ^0.13 for acrolein, 0.18 ^0.02 for methacrolein and \0.03 for methyl vinyl ketone. The reaction of Cl atoms with methacrolein proceeds via a mechanism that involves the decomposition of the methyl vinyl radical. The decomposition of this radical in synthetic air, and in the absence of NO, appears to lead to the formation of signiÐcantly more CO than previously thought. This observation is in agreement with the work of