ELECTROCHROMIC RESPONSES OF BACTERIOCHLOROPHYLL ABSORBENCY BANDS IN PURIFIED LIGHT-HARVESTING COMPLEXES OF RHODOBACTER-CAPSULATUS RECONSTITUTED INTO LIPOSOMES

Purified light harvesting complexes I and II (LHI and LHII) from Rhodo bacter capsulatus were purified and separately incorporated into lipos omes. Electrochromic absorbance changes of bacteriochlorophyll bands i n the proteoliposomes in response to K+-diffusion potentials were reco rded. In LHII proteoliposomes the application of positive-inside poten tials led to red shifts in the bacteriochlorophyll absorbance bands ce ntred at 377, 801 and 858 nm. Negative-inside potentials caused blue s hifts of these bands. Electrochromism of the 590 nm band was too small to detect. The band at 858 nm was considerably more electrochromic th an that at 801 nm. Electrochromic absorbance changes measured at 865-8 50 nm were linear with the applied diffusion potential. In LHI proteol iposomes positive-inside diffusion potentials caused red shifts of the bands centred at 374 nm and 880 nm. At 880 nm the response was linear with the applied diffusion potential and was equivalent in amplitude to that of the 858 nm band in LHII proteoliposomes. If it is assumed t hat the permanent dipole moment differences between the ground state a nd excited state of B800, B850 and B870 are similar and that polarisab ility effects are negligible, it follows that (a) the plane of the bac teriochlorin ring of B850 is more perpendicular to the membrane plane than that of B800 and (b) the orientations of B850 and B870 relative t o the membrane plane are similar.