Evidence for a light-induced blue band shift of part of the p515 pigment pool in intact chloroplasts

Abstract

The difference spectrum of light-induced absorbance changes in the 460-540 nm wavelength region, measured in algae and chloroplasts, is characterized by a maximum around 518 nm, a minimum at about 480 nm and an inflection in the 490-500 nm wavelength region [l-4]. Evidence has been given that this spectrum is the result of an electrochromic response of chlorophyll b and carotenoid pigments to the electric field generated by the primary light-induced charge separation in the membrane [5,6]. It has been suggested that, at least in bacterial chromatopho:ts, the pool of carotenoids that shows electrochromism may not be completely homogeneous and consists of molecules with slightly different peak wavelengths and bandshifts depending on the strength and orientation of local electric fields [7]. In this paper we discuss an effect of cations on the light-induced difference spectrum of P5 15 in chloroplasts. It is shown that in intact chloroplasts as well as in broken chloroplasts in the presence of cations part of the pigments in the pool respond in a way that upon flash illumination their absorption band is shifted towards a shorter wavelength. This blue wavelength shift, which gives rise to the inflection around 500 nm in the overall difference spectrum, is interpreted in terms of a cation-dependent change in the orientation of the pigment complex with respect to an induced photoelectric field, or of a cation-induced migration of the primary acceptor of PS II into the hydrophobic region of the membrane. This would suggest that a small part of the pigments might become exposed in the light to an oppositely oriented