Singlet → triplet intersystem crossing quantum yields of photosynthetic and related polyenes

Abstract

The extinction coefficients for the triplet–triplet maxima of all-trans-β-carotene, 15,15′-cis-β-carotene, all-trans-lycopene, 7,7′-dihydro-β-carotene, all-trans-retinol, all-trans-retinal, 13-cis-retinal and all-trans-retinylidene-N-butylamine have been determined via pulse-radiolysis using a refinement of the energy transfer technique introduced previously. Together with Wolff and Witt's data, the coefficient found for β-carotene suggests that ∼10 % of the total carotene present in photosynthesising spinach chloroplasts is available as a protective valve against overillumination.

These extinction coefficients were employed to estimate, via laser flash photolysis, the corresponding singlet → triplet intersystem crossing quantum yields using 265 and 353 nm excitation. The very low yields ( < 0.001) found for β-carotene and derivatives mean that carotenoid triplets formed in chloroplasts during the course of the valve function could not have been formed directly, but only via triplet–triplet energy transfer sensitation via another triplet (chlorophyll^T?), or via¹O₂ quenching. The low cross-over efficiencies also mean that in their accessory pigment role no carotenoid excited singlets are wasted in crossing over to the triplet state.