Near‐ultraviolet Modification of Escherichia Coli B Ubiquinone in Vivo and in Vitro*

Ubiquinone Q‐8 in Escherichia coli B was labeled by growing the bacteria in a synthetic growth medium containing p ‐hydroxybenzoic acid‐ 14 C(U). The cells were then irradiated with light of wavelengths 313, 334, 366, and 405 nm, and Q‐8‐ 14 C was extracted and assayed. Q‐8‐ 14 C 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.