Various aquatic dissolved organic matter (DOM) samples produce singlet oxygen (1O2) and hydrogen peroxide (H2O2) with quantum yields of 0.59 to 4.5% (1O2 at 365 nm) and 0.017 to 0.053% (H2O2, 300-400 nm integrated). The two species' yields have opposite pH dependencies and strong, but opposite, correlations with the E2/E3 ratio (A254 divided by A365). Linear regressions allow prediction of both quantum yields from E2/E3 in natural water samples with errors ranging from -3% to 60%. Experimental evidence and kinetic calculations indicate that less than six percent of the H2O2 is produced by reaction between 1O2 and DOM. The inverse relationship between the 1O2 and H2O2 yields is thus best explained by a model in which precursors to these species are populated competitively. A model is presented, which proposes that important precursors to H2O2 may be either charge-transfer or triplet states of DOM.

Superresolution fluorescence microscopy is used to locate single copies of RNA polymerase (RNAP) in live Escherichia coli and track their diffusive motion. On a timescale of 0.1-1 s, most copies separate remarkably cleanly into two diffusive states. The "slow" RNAPs, which move indistinguishably from DNA loci, are assigned to specifically bound copies (with fractional population ftrxn) that are initiating transcription, elongating, pausing, or awaiting termination. The "mixed-state" RNAP copies, with effective diffusion constant Dmixed = 0.21 μm(2) s(-1), are assigned as a rapidly exchanging mixture of nonspecifically bound copies (fns) and copies undergoing free, three-dimensional diffusion within the nucleoids (ffree). Longer trajectories of 7-s duration reveal transitions between the slow and mixed states, corroborating the assignments. Short trajectories of 20-ms duration enable direct observation of the freely diffusing RNAP copies, yielding Dfree = 0.7 μm(2) s(-1). Analysis of single-particle trajectories provides quantitative estimates of the partitioning of RNAP into different states of activity: ftrxn = 0.54 ± 0.07, fns = 0.28 ± 0.05, ffree = 0.12 ± 0.03, and fnb = 0.06 ± 0.05 (fraction unable to bind to DNA on a 1-s timescale). These fractions disagree with earlier estimates.

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