THE EFFECT OF PERIPHERAL SUBSTITUTION ON THE BATHOCHROMIC SHIFT OF THE Q(Y) TRANSITION OF BACTERIOCHLOROPHYLL DIMERS - IN-VITRO MODELS OF THE PROTEIN EFFECT ON THE SPECTRUM OF PIGMENT CENTERS IN THE LIGHT-HARVESTING COMPLEXES

The effect of chemical modifications in the side groups of the isocycl ic ring V on the formation, optical absorption and circular dichroism of bacteriochlorophyll (Bchl) dimers was examined in a mixture of form amide and water containing Triton X-100 and variable amounts of pyridi ne. Substitution of the carbomethoxy group in the C13(2) position with a hydrogen atom, had no effect on the dimerization constant but incre ased the shift of the Q(y) transition by 1000 cm(-1) with respect to t he native Bchl. Substitution of the C13 hydrogen atom with OH decrease d the shift of the Q(y) transition by 400 cm(-1) The similarity betwee n the spectra of the modified Bchl dimers and Bchl dimers in vivo indi cates that protein binding to the side groups at Bchl dimers may profo undly affect the energy of their Q(y) transition but have minor effect s on the Q(y) transitions of the monomeric Bchl.