A kinetic study of bacteriochlorophyll pheophytinization in the protein complex from a green photosynthetic bacterium

Abstract

The rate of magnesium removal from bacteriochlorophyll in a chlorophyll-protein complex containing 20 chlorophyll molecules was directly proportional to [H +]. HCl first converted the native blue-green complex to a blue intermediate (1790) in which some bacteriochlorophyll molecules were exposed to the solvent. The spectrum of the intermediate was determined from the ratio of the initial to the final absorbance values recorded in the kinetic runs. The course of the reaction was then followed in a spectrophotometer by observing either the absorbance decrease at 783 mμ or the absorbance increase at 535 mμ. The difference in the apparent rates observed at the two wavelengths suggested that a second intermediate (1770) was formed from the first intermediate. An increase in ionic strength decreased the rate of pheophytinization and accentuated the difference between 783-mμ and 535-mμ data. The kinetic data at several values of [H +], ionic strength and temperature showed that the 20 molecules of bacteriochlorophyll in the protein differ with respect to reactivity towards H +. The apparent activation energy increased with the course of the reaction and approached the value for chlorophylls in solution. The quantitative differences between the present study and previous observations of the pheophytinization of chlorophylls dissolved in polar solvents are attributed to the protein environment of bacteriochlorophyll in the complex.