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Food Lipids: Chemistry, Nutrition, and Biotechnology

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tween substrate <strong>and</strong> triplet oxygen for the excited triplet sensitizer largely determines<br />

whether the reaction pathway is type 1 or type 2. Photosensitized oxidation may<br />

change the types of pathway during the course reaction as the concentration of<br />

substrate <strong>and</strong> oxygen changes. In aqueous–lipid biphasic systems, the longer halflife<br />

of singlet oxygen in the lipid phase favors the oxidation of compounds that<br />

partition into the lipids.<br />

V. ELECTRON SPIN RESONANCE SPECTROSCOPY OF<br />

SINGLET OXYGEN<br />

The detection of singlet oxygen during photosensitized oxidation is very difficult in<br />

that its lifetime is only microseconds. Electron spin resonance (ESR) spectroscopy<br />

is a highly sensitive analytical method for the detection of free radicals. ESR detected<br />

the formation of singlet oxygen in meat <strong>and</strong> milk using a spin trapping technique<br />

[29,30]. A spin trapping agent, such as 2,2,6,6-tetramethyl-4-piperidone (TMPD), can<br />

react with singlet oxygen to form a stable nitroxide radical adduct, 2,2,6,6-tetramethyl-4-piperidone-N-oxyl<br />

(TAN), which is easily measured by ESR. The reaction<br />

of TMPD with singlet oxygen for 2,2,6,6-4-piperidone-N-oxyl formation is shown<br />

in Figure 6.<br />

Addition of TMPD to both skim <strong>and</strong> whole milk during illumination produced<br />

TAN after only 5 minutes, confirming the formation of singlet oxygen in milk under<br />

light [13]. The limitation of this method is that concentrations of TAN must remain<br />

over 10 �8 M for detection <strong>and</strong> over 10 �6 M for good spectral resolution. Since the<br />

lifetime of singlet oxygen is less than 1 ms, steady-state concentrations greater than<br />

10 �7 M are rarely maintained. Coupling spin trapping with ESR spectroscopy could<br />

improve this technique of measuring singlet oxygen concentration. Effects of 0, 5<br />

<strong>and</strong> 15 minutes illumination on ESR spectrum of 2,2,6,6-4-piperidone-N-oxyl in<br />

water solution of riboflavin <strong>and</strong> 2,2,6,6-tetramethyl-4-piperidone is shown in Figure<br />

7 [5].<br />

VI. REACTIONS OF SINGLET OXYGEN<br />

Singlet oxygen, an electrophilic molecule, is seeking electrons to fill its vacant molecular<br />

orbital. One of the most important reaction characteristics of singlet oxygen<br />

is that it can directly react with the electron-rich double bonds of unsaturated molecules<br />

[31]. The reaction rates of singlet oxygen with oleic, linoleic, linolenic, <strong>and</strong><br />

arachidonic acids are 0.74, 1.3, 1.9, <strong>and</strong> 2.4 � 10 5 M �1 s �1 , respectively, which is<br />

Figure 6 Reaction of 2,2,6,6-tetramethyl-4-piperidone with singlet oxygen for 2,2,6,6-4piperidone-N-oxyl<br />

formation [30].<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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