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Abstract— Ubiquinone Q‐8 in Escherichia coli B was labeled by growing the bacteria in a synthetic growth medium containing p‐hydroxybenzoic acid‐ 14C(U). The cells were then irradiated with light of wavelengths 313, 334, 366, and 405 nm, and Q‐8‐ 14C was extracted and assayed. Q‐8‐ 14C was also isolated from unirradiated bacteria, partially purified, and then irradiated similarly in hexane and assayed. Ubiquinone in the cells was four to eight times as resistant to radiation as ubiquinone in hexane. From 313 to 366 nm, the in vivo and in vitro action spectra were similar to each other and to the absorption spectra of synthetic Q‐8 dissolved in hexane or ethanol, but at 405 nm the action spectra deviated from the absorption spectra. Quantum yields for the modification (loss) of ubiquinone in hexane decreased from 0·021. at 313 nm to 0·005 at 405 nm. The chromenol of Q‐8 was not detected in labeled cells irradiated at 334 nm, but several more‐polar photoproducts were separated by thin‐layer chromatography.

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Near-ultraviolet (near-UV) light (300 to 380 nm) is a significant component of sunlight and has a variety of effects on biological systems. The present work is an attempt to identify chromophores (molecular absorbers of light) and targets (critical damaged molecules) for inhibition of adenosine triphosphate (ATP) synthesis in Escherichia coli by near UV. The fluence of 334 nm required for 37% survival of net ATP synthesis (F37) in E. coli AB2463 in succinate medium is 140 kJ/m2. The action spectrum for this inactivation is almost structureless, exhibiting a smooth transition from high efficiency at 313 nm to low efficiency at 405 nm. The action spectrum for inhibition of net ATP synthesis is consistent with the chromophore being either ubiquinone Q-8 or vitamin K2. The fluence required is consistent with ubiquinone Q-8 also being a target molecule. The activity of reduced nicotinamide adenine dinucleotide dehydrogenase in extracts of E. coli B is also inactivated by near UV and shows an F37 of about 40 kJ/m2. The action spectrum for this effect is quite structureless; it shows high efficiency at 313 nm and low efficiency at 435 nm. The data do not suggest a target molecule for this action, although it is possible that ubiquinone Q-8 absorbs the near-UV energy and then passes it on to some other target molecule. The data further indicate that inactivation of the oxidative phosphorylation system is not a primary factor in near-UV-induced growth delay in E. coli.