Antenna chlorophyll a and P700. Exciton transitions in chlorophyll a arrays

Abstract

Exciton calculations have been carried out for 16 distinct cases of randomly generated chlorophyll a arrays to assist in the assessment of various possibilities for the chlorophyll distribution in the thylakoid membrane of green plants. Each of the cases is distinguished by its packing density and by the constraints applied to the orientation of transition dipoles. Each array consists of 40 antenna chlorophyll a molecules with an absorption maximum at 670 nm before transition density coupling and one P700 chromophore with an absorption maximum at 700 nm. Transition density coupling always broadens the absorption bands and sometimes shifts the position of the absorption maxima. Transition density coupling between the P700 chromophore and the antenna chlorophyll molecules shifts the P700 absorption band to lower energies and, for some cases, increases or decreases the dipole strength of the P700 absorption band by as much as 50 percent. In general, for comparable packing densities, the effects of transition density coupling are more pronounced for two-dimensional arrays than for three-dimensional arrays. The present exciton calculations show that if the various in vivo ''forms'' of chlorphyll a correspond to exciton transitions in chlorophyll a arrays, then the local chlorophyll concentration in the arrays must bemore » much greater than the average chlorophyll concentration of approximately 0.1 M in the thylakoid membrane; perhaps the local chlorophyll concentration could be as high as 1 M.« less