Molecular orientation and structure of the transition moments of porphyrin derivatives with various symmetries

In order to determine the anisotropy in the absorption transition moments of hemes we measured the visible and UV linear dichroism of the high-symmetry porphyrin derivatives α,β,γ,δ-tetraphenylsulfonic porphyrin (D_4h symmetry), deuterohemin III (iron deuteroporphyrin III chloride; C_2v symmetry), and protohemin III (iron protoporphyrin III chloride; C_2v symmetry) in stretched poly(vinyl alcohol) films in the 250-700-nm region. Their linear dichroism was analyzed by using either a "circular absorber" model or a linear absorption oscillator model. For α,β,γ,δ-tetraphenylsulfonic porphyrin with 4-fold symmetry the absorption anisotropy was found to be wavelength independent as for a circular absorber. Deutero- and protohemin III with C_2v symmetry had a wavelength-dependent dichroism, indicating the linear type of the allowed transition moments. As expected from theoretical calculation the transitions most affected by peripheral substituents were the relatively weak Q, L, and N bands. Linear dichroism with very similar characteristics was reported earlier by us for native protohemin IX (iron protoporphyrin IX chloride) derivatives (Gryczynski et al. Photochem. Photobiol. 1993, 58, 492). It appears that the absorption of hemes should be considered as a simple combination of linear oscillators. The linear character of heme transition moments is what regulates the radiationless tryptophanheme interactions.