BETA-CAROTENE QUENCHES SINGLET OXYGEN FORMED BY ISOLATED PHOTOSYSTEM-II REACTION CENTERS

By measuring time-resolved luminescence emission at 1270 nm, we have d etected singlet oxygen formation by illuminated, reaction centers of p hotosystem II isolated from Pisum sativum, which is in agreement with earlier work (Macpherson, A. N., Telfer, A., Barber, J., & Truscott, T . G. (1993) Biochim. Biophys. Acta 1143, 301-309). In this paper we sh ow that the yield of singlet oxygen is significantly increased if the number of beta-carotene molecules bound per isolated complex is reduce d from two to one. We conclude, therefore, that beta-carotene can act an effective quencher of singlet oxygen in the photosystem II reaction center. This conclusion is supported by the finding that the rate of light-induced irreversible bleaching of chlorins in the reaction cente r is increased with decreasing beta-carotene levels. The results demon strate the direct intermediacy of singlet oxygen in causing photooxida tive damage within a biological environment and are discussed, specifi cally, in terms of the role of beta-carotene in protecting photosystem II against photoinhibition.