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Tetracycline molecules offer several sites for peroxidative metabolism of the type known to lead to oxygen consumption and electronic excitation. Accordingly, when tetracycline and chlortetracycline were exposed to horseradish peroxidase in the presence of hydrogen peroxide, oxygen was taken up and light emission was observed. The overall quantum yield of chemiluminescence is on the order of 10(-6), but that of chemiexcitation may be orders of magnitude higher as suggested by studies of sensitized emission. Given the widespread distribution of peroxidases, the formation of highly reactive metabolites of tetracycline may have biological importance.

Efficient excited state formation - much higher than that hitherto expected - may occur in organelles and in intact cells. Excited triplet states can be enzymatically generated in high yields by different routes. An example is the oxidation of isobutanal to acetone and formic acid, catalyzed by horseradish peroxidase. Other enzymatic systems that generate triplet carbonyls are linear aliphatic aldehydes when oxidized by peroxidase/O2, or the indole-3-acetic acid/peroxidase/O2-reaction. The latter is widespread in plants. This new field - photobiochemistry without light - has led to a growing awareness of the idea that cells may utilize excited states to trigger photochemical processes even in the dark. Such phenomena are of considerable importance, also for the understanding of weak photon emission from biological systems.

Lo spettro di assorbimento U.V. di esteri e tioesteri aromatici indica che nello stato eccitato di certi tioesteri l'atomo di zolfo fa uso d'una orbitale 3d.