A series of 8 pyrazines (pyrazine, 3-ethyl-2-methoxy-, 2-ethoxy-3-ethyl-, 3,6-dimethyl-2-propyl-, 2-butyl-3,5-dimethyl-, 2-butyl-3,6-dimethyl-, 2-acetyl-3-methyl-, 2-acetyI-3,S-dimethylpyrazine) were identified in the smoke generated from 17 different wood sources (white and red oak, walnut, chestnut, apple, redwood, red alder, cherry, aspen, birch, hard maple, eastern cedar, hickory, Lodgepole pine wood and bark, Douglas-Fir heartwood and sapwood).Hickory produced the largest amount (41.03 mg 100 g wood) of total pyrazines while redwood had the least (10.33 mg). Total and individual pyrazine levels did not appear to correlate with wood nitrogen content nor with the levels of cellulose, hemicellulose or lignin.Smoke generation variables relative to pyrazine formation were evaluated with hickory smoke. Increasing the moisture content from 4 per cent to 20 or 30 per cent resulted in decreased total pyrazine levels. The absence of air decreased total pyrazine concentration while increasing smoke generation temperature from 290 to 450°C increased the total amount of pyrazines formed. Collection of hickory smoke volatiles in an alkaline solution resulted in lower total pyrazine levels than condensate trapped in water.

Nucleosomes represent mechanical and energetic barriers that RNA Polymerase II (Pol II) must overcome during transcription. A high-resolution description of the barrier topography, its modulation by epigenetic modifications, and their effects on Pol II nucleosome crossing dynamics, is still missing. Here, we obtain topographic and transcriptional (Pol II residence time) maps of canonical, H2A.Z, and monoubiquitinated H2B (uH2B) nucleosomes at near base-pair resolution and accuracy. Pol II crossing dynamics are complex, displaying pauses at specific loci, backtracking, and nucleosome hopping between wrapped states. While H2A.Z widens the barrier, uH2B heightens it, and both modifications greatly lengthen Pol II crossing time. Using the dwell times of Pol II at each nucleosomal position we extract the energetics of the barrier. The orthogonal barrier modifications of H2A.Z and uH2B, and their effects on Pol II dynamics rationalize their observed enrichment in +1 nucleosomes and suggest a mechanism for selective control of gene expression. Highlights A single-molecule unzipping assay mimics DNA unwinding by Pol II and maps the topography of human canonical, H2A.Z and uH2B nucleosome barriers at high resolution Real-time dynamics and full molecular trajectories of Pol II crossing the nucleosomal barrier reveal the transcriptional landscape of the barrier at high accuracy H2A.Z enhances the width and uH2B the height of the barrier A unified mechanical model links position-dependent dwell times of Pol II on the nucleosome with energetics of the barrier

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