Kinetics and Product Yields of the OH Initiated Oxidation of Hydroxymethyl Hydroperoxide.

Hydroxymethyl hydroperoxide (HMHP), formed in the reaction of the C1 Criegee intermediate with water, is among the most abundant organic peroxides in the atmosphere. Although reaction with OH is thought to represent one of the most important atmospheric removal processes for HMHP, this reaction has been largely unstudied in the laboratory. Here, we present measurements of the kinetics and products formed in the reaction of HMHP with OH. HMHP was oxidized by OH in an environmental chamber; the decay of the hydroperoxide and the formation of formic acid and formaldehyde were monitored over time using CF3O- chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF). The loss of HMHP by reaction with OH is measured relative to the loss of 1,2-butanediol [ k1,2-butanediol+OH = (27.0 ± 5.6) × 10-12 cm3 molecule-1s-1]. We find that HMHP reacts with OH at 295 K with a rate coefficient of (7.1 ± 1.5) × 10-12 cm3 molecule-1s-1, with the formic acid to formaldehyde yield in a ratio of 0.88 ± 0.21 and independent of NO concentration (3 × 1010 - 1.5 × 1013 molecules cm-3). We suggest that, exclusively, abstraction of the methyl hydrogen of HMHP results in formic acid, while abstraction of the hydroperoxy hydrogen results in formaldehyde. We further evaluate the relative importance of HMHP sinks and use global simulations from GEOS-Chem to estimate that HMHP oxidation by OH contributes 1.7 Tg yr-1 (1-3%) of global annual formic acid production.

Browning Index of Anthocyanin-Rich Fruit Juice Depends on pH and Anthocyanin Loss More Than the Gain of Soluble Polymeric Pigments.

Browning index (BI, ABS520 nm /ABS420 nm ) is a measure of anthocyanin-rich fruit juice pigmentation quality. This study sought to determine the extent to which BI describes anthocyanin quality and degradation in fruit juices. Commercial fruit juices were assayed for monomeric anthocyanin (MA) content, percent polymeric color (%PC), pH, and BI. BI varied, 0.29 to 1.72, among cranberry, cherry, grape, aronia, and pomegranate juices. Principal component analysis (PCA) revealed that BI was strongly inversely associated with %PC, and positively correlated with MAs to a lesser extent. The BI of grape and cherry juices varied linearly with pH from 2.0 to 4.0 in pH-adjusted juices. Cherry and grape juices at pH approximately 2.0 to 4.0 were incubated at 50 °C to induce juice browning. BI and MA decreased, and %PC increased, but the amount of MA degradation was not explained by %PC. In the aged juices, BI and MA were strongly correlated using PCA. In aged grape juice, chromatographic analysis was used characterize anthocyanins, proanthocyanidins, and anthocyanin scission products. Anthocyanin loss and a gain of unresolved components absorbing at 420 nm decreased BI. Proanthocyanidins and co-eluting pigments with varying BI decreased during aging. Scission products did not account for anthocyanin loss. Thus, MA loss more so than the gain in pigments associated with juice proanthocyanidins contribute to the increase in %PC and decline of the BI during accelerated aging of grape juice. Thus, BI is a useful marker of fruit juice quality within juices of the same pH and anthocyanin composition. PRACTICAL APPLICATION: Fruit juice pigmentation depends on anthocyanins, pH, and other matrix components. Spectrophotometric methods to determine pigmentation include the browning index (ABS520 nm /ABS420 nm ), pH differential method for monomeric anthocyanin (MA) content, and bisulfite bleaching to determine percent polymeric color (%PC). In this study, anthocyanin-rich fruit juice browning index was strongly dependent on pH and MA content. MA loss, and to a lesser extent, a gain in newly-formed pigments at 420 nm contributed to the browning index change during aging. Therefore, browning index is strongly associated with MA content and is useful for assessing fruit juice quality.

pH-dependent spectral properties of para-aminobenzoic acid and its derivatives.

The local environment dictates the structural and functional properties of many important chemical and biological systems. The impact of pH on the photophysical properties of a series of para-aminobenzoic acids is examined using a combination of experimental spectroscopy and quantum chemical calculations. Following photoexcitation, PABA derivatives may undergo an intramolecular charge transfer (ICT) resulting in the formation of a zwitterionic species. The thermodynamics of the excited state reaction and temperature-dependence of the radiative emission processes are evaluated through variable temperature fluorescence spectroscopy carried out in a range of aqueous buffers. Quantum chemical calculations are used to analyze structural changes with modifications at the amine position and different protonation states. The ICT is only observed in the tertiary amine, which calculations show has more sp(2) character than the primary or secondary amines. Thermodynamic analysis indicates the ICT reaction is driven by entropy.