CHEMILUMINESCENCE FROM THE OXIDATION OF MODEL LIPID SYSTEMS

R.N. Tilbury and H. Miller

Chemistry, School of Chemical and Physical Sciences,

Victoria University of Wellington, PO Box 600, Wellington, New Zealand

ABSTRACT

The oxidation and ultraweak chemiluminescence of linoleic acid and methyl linoleate were studied as model systems for lipids in biological organisms. Ultraweak biological chemiluminescence is extremely weak light emitted by living organisms, or their components, at photon fluxes below about 10⁴ photons cm^-2 s^-1. It is finding application as a very sensitive and non-invasive analytical tool in biochemistry, medicine and agriculture. There is growing evidence that electronically excited oxygen species, which are of interest in a variety of cellular processes in normal and pathological conditions, give rise to most of the chemiluminescence in biological systems. The process of lipid peroxidation has been particularly emphasized as playing a pivotal role.

Oxidation of the fatty acid and its methyl ester was initiated using two different methods: the first involved heating (at 60°C) over prolonged periods; while the second method utilized the addition of a catalyst, either glucose or iron (II) nitrilotriacetate. The course of oxidation was followed by measuring the absorbance of light at 234 nm as an indication of hydroperoxide concentration. Spontaneous ultraweak light emission, which was oxygen dependent, was detected from the oxidized oils with a photon counter sensitive in the wavelength range 200-630 nm. In each case, the chemiluminescence coincided with the decomposition of hydroperoxides rather than their initial increase or the later formation of secondary products, such as malondialdehyde. As expected, the linoleic acid was more readily oxidized than its methyl ester and both hydroperoxide formation and subsequent chemiluminescence occurred earlier. It was also found that glucose and Fe²⁺-NTA accelerated the oxidation of the oils, most likely through the decomposition of hydroperoxides, forming free radicals which could then initiate further peroxidative chain reactions.

It is suggested that the major species responsible for the light emission are excited triplet carbonyl and excited singlet oxygen, and that these species arise through the decomposition of hydroperoxides formed during the oxidation of lipids. Indicators of oil deterioration such as smell and colourisation were associated with the formation of hydroperoxides and not the increase in chemiluminescence, which occurred at a later stage. This indicates that the chemiluminescence, under the conditions of this study, would not be a useful measure of oil degradation in foodstuffs. However, it may still prove to be a useful tool in medical diagnostics for conditions which involve lipid peroxidation.

21 June 1999