Absorption and magnetic circular dichroism of chlorophyll a and b dimers

The dimerization of chlorophyll a to the so-called special pair, in which the two monomers are linked together by two nucleophilic molecules (alcohol or water), leads to shifts and splittings of the absorption and magnetic circular dichroism (MCD) spectral bands. The changes in the Q-band region are described starting from a model proposed previously (L.L. Shipman. J.R. Morris and J.J. Katz. J. Phys. Chem. 80 (1976) 877). and which we extended to include the MCD. The parameters alpha(x) and alpha(y), containing the exciton and environmental parameters (L.L. Shipman. J.R. Norris and J.J. Katz, J. Phys. Chem. 80 (1976) 877) and the relative orientation of the monomers in the dimer, determine the spectral features. Spectral simulation leads to the conclusion that in the special pair alpha(x) and alpha(y), are in the region of 0.6-0.8 and that the dimer has C2 symmetry. The model was also applied to the case of the pure dimer of chlorophyll b where the monomers are bound together directly. With similar values for alpha(x) and alpha(y) the spectra could be reconstructed assuming almost parallel monomers in the dimer, the equilibrium constant for the association 2M <==> M2 was determined as 0.8(+/-0.2) x 10(6) mol(-1)/I. The present choice of compounds was based merely on practical reasons. The model may be applied equally well to other similar cases.