Reassociation of All-trans-3,4-Dihydroanhydrorhodovibrin with LH1 Subunits Isolated from Rhodospirillum rubrum: Selective Binding of All-trans Isomer from Mixture of cis- and trans-Isomers

Abstract

Abstract The reassociation of carotenoids with the photosynthetic light-harvesting 1 (LH1) pigment–protein complex has proven to be a powerful technique to investigate the function of carotenoids. We have recently investigated the reassociation of the carotenoids, spirilloxanthin (number of conjugated C=C bonds (n) = 13), anhydrorhodovibrin (n = 12), rhodopin (n = 11), and spheroidene (n = 10)) with carotenoid-depleted LH1 subunits isolated from Rhodospirillum rubrum. In the case of the rhodopin complex, small anomalies were observed in the absorption maximum of the Qy band of bacteriochlorophyll a (BChl a Qy) and the singlet–singlet energy-transter efficiency of 1carotenoid* → 1BChl a* within the LH1–carotenoid complex. The reason for these anomalies could probably be ascribed to the rhodopin hydroxy group. To investigate this further, reassociation experiments of the LH1 complex with hydroxy-protected rhodopin, all-trans-3,4-dihydroanhydrorhodovibrin (1), were conducted. We observed a normal absorption maximum (883 nm) of the BChl a Qy band in the LH1–1 complex. The energy-transter efficiency of 1carotenoid* (11*) → 1BChl a* was found to be 53%, giving a good correlation with the other carotenoids. Furthermore, we found a preferential complexation of all-trans-1 to carotenoid-depleted LH1 from a 1:1 mixture of all-trans-1 and cis-1 (cis-1 consists of a 1:1 mixture of 5-cis-5′-trans-1 and 5-trans-5′-cis-1).