MOLECULAR-ORIENTATION AND STRUCTURE OF THE TRANSITION MOMENTS OF PORPHYRIN DERIVATIVES WITH VARIOUS SYMMETRIES

In order to determine the anisotropy in the absorption transition mome nts of hemes we measured the visible and UV linear dichroism of the hi gh-symmetry porphyrin derivatives alpha,beta,gamma,delta-tetraphenylsu lfonic porphyrin (D-4h symmetry), deuterohemin III (iron deuteroporphy rin III chloride; C-2v symmetry), and protohemin III (iron protoporphy rin III chloride; C-2v symmetry) in stretched poly(vinyl alcohol) film s in the 250-700-nm region. Their linear dichroism was analyzed by usi ng either a ''circular absorber'' model or a linear absorption oscilla tor model. For alpha,beta,gamma,delta-tetraphenylsulfonic porphyrin wi th 4-fold symmetry the absorption anisotropy was found to be wavelengt h independent as for a circular absorber. Deutero- and protohemin III with C-2v symmetry had a wavelength-dependent dichroism, indicating th e linear type of the allowed transition moments. As expected from theo retical calculation the transitions most affected by peripheral substi tuents were the relatively weak Q, L, and N bands. Linear dichroism wi th very similar characteristics was reported earlier by us for native protohemin IX (iron protoporphyrin IX chloride) derivatives (Gryczynsk i et al. Photochem. Photobiol. 1993, 58, 492). It appears that the abs orption of hemes should be considered as a simple combination of linea r oscillators. The linear character of heme transition moments is what regulates the radiationless tryptophanheme interactions.