Lifetimes of the lowest excited singlet (S1) electronic states of various derivatives of the pyrimidine nucleobase cytosine (Cyt) were measured by the femtosecond transient absorption technique. The bases were excited in room-temperature aqueous solution at 265 nm using approximately 200 fs pump pulses from a titanium-sapphire laser system. The decay of excited-state absorption (ESA) at visible probe wavelengths was used to determine the S1 lifetimes of a variety of modified Cyt compounds at different pH values by global fitting. Identical lifetimes were observed for Cyt and cytidine (Cyd) within experimental uncertainty, but ESA by the ribonucleoside was considerably stronger, suggesting that the ribose group increases the oscillator strength of the S1 --> SN transition. The S1 lifetime of the important minor base 5-methylcytosine (m5Cyt) is 7.2 +/- 0.4 ps at pH 6.8. The same lifetime was measured for the ribonucleoside 5-methylcytidine, but sugar substitution again increased the strength of the ESA signal. Protonation of Cyd and m5Cyt at low pH led to a modest decrease in their S1 lifetimes. On the other hand, deprotonation of Cyt and m5Cyt significantly increased the lifetime of their respective S1 states. These trends support the intermediacy of the n,pi* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S1 lifetimes were observed for 5-fluorocytosine and N4-acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited-state dynamics in DNA and DNA components.
On- and off-road mobile sources are the dominant contributors to urban anthropogenic volatile organic compound (AVOC) emissions. Analyses of gasoline samples from California for both summer and winter indicate significant differences in liquid fuel and vapor chemical composition due to intentional seasonal adjustments. Ambient concentrations of 55 VOCs were measured via in situ gas chromatography in the 2005 Study of Organic Aerosols at Riverside (SOAR) during both summer and fall. A chemical mass balance analysis was used to differentiate vapor pressure-driven VOC emissions from other motor vehicle-related emissions such as tailpipe exhaust. Overall, fuel vapor emissions accounted for 31 +/- 2% of gasoline-related VOC in Riverside; California's emission factor model similarly estimates 31% of gasoline-related VOC emissions are fuel vapor. The diurnal pattern of vapor pressure-driven VOC source contributions is relatively stable around 10 microg/m3, while whole gasoline (i.e., tailpipe) contributions peak at approximately 60 microg/m3 during the morning commute. There is no peak in whole gasoline source contributions during the afternoon, due to rapid dilution associated with high mixing heights and wind speeds in the Riverside area. The relationship between estimated gasoline-related VOC and observed carbon monoxide concentrations in this study is similar to California's 2005 emission inventory; we calculated a VOC to CO mass ratio of 0.086 +/- 0.006 (95% CI) compared to 0.097 in the emission inventory for all gasoline-related sources.
Results of this novel analysis linking health and education data revealed that children with a history of NAS were significantly more likely to have a subsequent educational disability.
The overall rate constants of the NO reaction with hydroxyalkylperoxy radicals derived from the OH-initiated oxidation of several atmospherically abundant alkenes-ethene, propene, 1-butene, 2-butene, 2-methyl propene, 1,3-butadiene, and isoprene (2-methyl-1,3-butadiene)-were determined using the turbulent flow technique and pseudo first-order kinetic conditions with high pressure chemical ionization mass spectrometry for the direct detection of hydroxyalkylperoxy radical reactants. The individual 100 Torr, 298 K rate constants for each alkene system were found to be identical within the 95% confidence interval associated with each separate measurement. The average overall rate constant for the reaction of all alkene-derived hydroxyalkylperoxy radicals with NO was determined to be 9.1 AE 0.5 (2s) Â 10 À12 cm 3 molecule À1 s À1 . Previous studies of this reaction for the ethene, 2-methyl propene and isoprene systems reported varying rate constant values that ranged from a factor of two lower to a factor of two higher than the present result. However, the invariant nature and value of the present rate constant determination closely parallels the results obtained in the measurement of rate constants for the reactions of several alkane-derived peroxy radicals with NO.
Lifetimes of the lowest excited singlet (S1) electronic states of various derivatives of the pyrimidine nucleobase cytosine (Cyt) were measured by the femtosecond transient absorption technique. The bases were excited in room‐temperature aqueous solution at 265 nm using approximately 200 fs pump pulses from a titanium–sapphire laser system. The decay of excited‐state absorption (ESA) at visible probe wavelengths was used to determine the S1 lifetimes of a variety of modified Cyt compounds at different pH values by global fitting. Identical lifetimes were observed for Cyt and cytidine (Cyd) within experimental uncertainty, but ESA by the ribonucleoside was considerably stronger, suggesting that the ribose group increases the oscillator strength of the S1→ SN transition. The S1 lifetime of the important minor base 5‐methylcytosine (m5Cyt) is 7.2 ± 0.4 ps at pH 6.8. The same lifetime was measured for the ribonucleoside 5‐methylcytidine, but sugar substitution again increased the strength of the ESA signal. Protonation of Cyd and m5Cyt at low pH led to a modest decrease in their S1 lifetimes. On the other hand, deprotonation of Cyt and m5Cyt significantly increased the lifetime of their respective S1 states. These trends support the intermediacy of the n,π* state localized on the carbonyl oxygen in the nonradiative decay mechanism of Cyt. Longer S1 lifetimes were observed for 5‐fluorocytosine and N‐acetylcytosine. Collectively, these results illustrate the great potential of femtosecond laser spectroscopy for investigating excited‐state dynamics in DNA and DNA components.
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