LINEAR DICHROISM STUDY OF METALLOPORPHYRIN TRANSITION MOMENTS IN VIEWOF RADIATIONLESS INTERACTIONS WITH TRYPTOPHAN IN HEMOPROTEINS

We measured the linear dichroism of several metalloporphyrins embedded in stretched polyvinyl alcohol (PVA) films to estimate the orientatio n of the absorption transition moments, which in hemoproteins are rele vant to the radiationless energy transfer between tryptophan and heme. The metalloporphyrins were derivatives of protoporphyrin IX (PPIX), n amely Fe3+-PPIX (ferric-heme) and Fe2+CO-PPIX (CO-heme), Mg-PPIX (Mg-h eme) and Zn-PPIX (Zn-heme). Measurements were conducted between 300 an d 700 nm. In all cases the linear dichroism was wavelength dependent, indicating the presence of several transition moments with different o rientations. We focused our attention on the near-UV (300-380 nm) and Soret (380-450 nm) absorption bands. Deconvolution in terms of Gaussia n components gave three components between 380 and 450 nm and only one in the 300-380 nm region. Deconvolution of the near-UV and Soret spec tra of oxy-, deoxy- and carbonmonoxyhemoglobin gave very similar resul ts, suggesting a very similar orientation of the various transition mo ments in the free and protein-embedded hemes. It should be stressed th at the single 300-380 nm band is the only one responsible for the over lap integral that regulates the energy transfer from tryptophan to hem e in hemoproteins (Gryczynski et al., Biophys. J. 63, 648-653, 1992). The dichroism of this single band indicated that its transition moment is oriented at about 60-degrees from the alpha-gamma meso-axis of the heme moiety. We conclude that the heme should be considered a linear oscillator when it acts as acceptor of energy transfer from tryptophan s.