No abstract

be delocalized in the three oxygen atoms of N03" like the resonance structure 111, which may be less electrophilic and less reactive. In the condensed phase, N03' is reported to be a Yshaped structure,36 in which an unpaired electron is localized on one oxygen atom like the canonical structures I and II. This radical center is quite reactive and electrophilic. This change of the electronic distribution may also be one of the causes for the medium effect.For the reaction of N03* with phenol in the gas phase,37 it is assumed that the addition of N03' to the benzene ring occurs first followed by the elimination of phenolic hydrogen. In the case of (36) Gundu Rao, T.

Acetaldehyde (AA), ethanol, and CO, production in red bell pepper (Capsicum annum 1.) fruit has been measured in a continu o u~ flow system as the fruit was switched between 20% O, and anaerobic conditions. Minimum gas phase concentrations of 0.5 nL L-l, 10 nL 1-', and 1 mL L-', respectively, can be detected employing a laser-based photoacoustic technique. This technique allows monitoring of low production rates and transient features in real time. At the start of anaerobic treatment respiration decreases by 60% within 0.5 h, whereas AA and ethanol production is delayed by 1 to 3 h. This suggests a direct slow-down of the tricarboxylic acid cycle and a delayed onset of alcoholic fermentation. Reexposure of the fruit to oxygen results in a 2-to 10-fold upsurge in AA production. A short anoxic period leads to a sharp transient peak lasting about 40 min, whereas after numerous and longer anoxic periods, post-anoxic AA production stays high for severa1 hours. High sensitivity of the fruit tissue to oxygen is further evidenced by a sharp decrease in post-anoxic AA production upon an early return to anaerobic conditions. Ethanol oxidatioo by the "peroxidatic" action of catalase is proposed to account for the immediate postanoxic AA upsurge.Fermentation occurs in fruits when oxygen flux to respiring cells is reduced below a critica1 value. Hypoxia occurs in bulky fruits under natural conditions during normal ripening due to impaired gas exchange with the atmosphere. The metabolic response and adaptation of plants to anaerobiosis has been extensively reviewed (Perata and Alpi, 1993; Pfister-Sieber and Braendle, 1994). Low oxygen concentrations are widely used in controlled atmosphere storage of harvested fruit, e.g. apples, with the goal of prolonging fruit shelf-life (Knee, 1991). Reduction of respiration rate and ethylene biosynthesis during controlled atmosphere storage does not, however, induce fermentation. Recently, storage in 1% O, was shown to extend the postharvest life of bell peppers (Lu0 and Mikitzel, 1996). Similarly, short-term anoxic treatments of harvested fruit improve fruit aroma and quality (Pesis, 1995). The physiological basis of the observed effects has yet to be clarified.

The consideration of the second preequilibrium is made necessary by the finding that, in the route leading to tetrathionate, the concentrations of both thiosulfate and the hydrogen ion are on the second power.The catalytic effect of thiosulfate on the oxidation of hydrogen sulfite is explained by (M8) and (M9) I03H2S203-+ HSO3--» SO42-+ S2032" + IOf + 3H+ (M8) I03H2S203 + HSO3-+ H+ ^S042" + S2032" + I02-+ 4H+ (M9)These reactions, together with the preequilibria (M4) and (M6), are responsible for the third and the fourth terms of the rate law(1"'), resp. In this set of elementary steps, (M8) is responsible for the inhibition effect of hydrogen sulfite in the oxidation of thiosulfate, that is, for the &,[ 803~] term in the denominator of rate laws (2") and (3'). In principle, a similar inhibition effect was due to (M9). According to the experiments, however, its weight is negligible. However, due to the obvious differences in the catalytic and inhibition effects, the catalytic effect due to (M9)appears to be significant and therefore even the fourth term in (V") should have been considered.The finding that with increasing hydrogen ion concentration, and in an excess of thiosulfate, the oxidation of thiosulfate to hydrogen sulfite is suppressed in comparison with the competing oxidation to tetrathionate can be explained by ( M6) and (M7). These steps can be responsible for the &/[S2032"] [H+] term in the denominator of (3')•Of course, a great number of further reactions must occur involving iodite and hypoiodite. There are good reasons to assume that these reactions are rather complex and involve the formation of adducts between the reactants. These fast reactions, however, probably are of secondary importance in the dynamics of the system.It goes without saying that reactions M1-M9 do not represent the mechanism of the reaction. However, it seems that it is the simplest and chemically plausible set of elementary and quasielementary reactions which may be responsible for the observed phenomena.