Variations of the Differential Extinction Coefficient of P-700 and Re-Estimation of Stoichiometry of Constituents in Photosystem I Reaction Center Complexes from Synechococcus elongatus

Photosystem I (PS I) reaction center complex consists of two large subunits which carry antenna chlorophyll a, P-700 and early electron acceptors and several small subunits, one of which contains the iron-sulfur centers, FA and FB. Because the PS I complexes are usually isolated with detergents, careful evaluation of detergent-effects, such as solubilization of chlorophyll a, or inactivation of P-700, is essential for determination of stoichiometry of subunit polypeptides and functional constituents in the complex. Recently, we have shown that the differential extinction coefficient of P-700 is markedly affected by sodium dodecyl sulfate (SDS) which induces a band-shift of chlorophyll a molecules at 690 nm [1]. Herein, we report effects of other detergents on the extinction coefficient of P-700 and provide a simple method to estimate the extinction coefficient in PS I preparations treated with various detergents. Using the extinction values thus determined, stoichiometry of chlorophyll a and the two large subunits in PS I reaction center complexes were estimated.