The absorption spectrum of chlorophyll a and the chlorophyll/dioxane and chlorophyll/benzoquinone interactions

A molecular orbital calculation, using the "free-electron-network" method, is reported for the absorption spectrum of chlorophyll a and the complexes chlorophyll/p-dioxane and chlorophyll/p-benzoquinone. For chlorophyll a, our results are in good agreement with experiment and are substantially the same as those obtained by Weiss using the self-consistent molecular-orbital method. Our results seem to indicate: (i) that chlorophyll a has 26 rather than 28 π electrons, (ii) that, in a chlorophyll complex, the central magnesium atom has a coordination number of 6, and (iii) that the chlorophyll/benzoquinone complex has an intense absorption band centered at 713 nm. If present in vivo this complex can provide a model for the P-700 energy trap.