Alkaline denaturation of the light-harvesting complex II from the purple bacterium Ectothiorhodospira Sp.: Kinetic evidence of the existence of the 780 nm upper exciton component of the B850 bacteriochlorophylls

The light-harvesting complex II of the purple bacteria has two strong near- infrared electronic absorption bands around 800 (B800) and 850 (B850) nm, a rising from the Q(y) transitions of the bacteriochlorophyll a. In the prese nt work, high concentrations of NaOH were used to study the destabilization of the complex of the Ectothiorhodospira sp. The majority of the bacterioc hlorophylls were monomerized within 90 min of treatment. However, the kinet ic patterns of the two near-infrared bands were remarkably different. After an instantaneous blue shift from 853 to 828 nm, B850 showed a first-order monomerization with a rate constant of -0.016 min(-1). This instantaneous b lue shift was previously attributed to the deprotonation of a lysine and wa s independent of the monomerization process. The observed native B800 is in fact composed of two bands, one at 796 nm and the other at 780 nm. The ban d absorbing at 780 nm red shifted also instantaneously to 786-788 nm and th en disappeared in a first-order process as B850. The other band absorbing a t 796 nm has a two-step process of monomerization; after a rapid conversion a slower first-order process occurred with a rate constant of -0.025 min(- 1). The similarity between the kinetic behaviors of B850 and the 780 nm ban d indicated a strong relationship between these two bands. Our interpretati on of the results considers the 780 nm band as the upper exciton component of the B850 bacteriochlorophylls.