Evidence for dynamic behavior of O₂ in oxyheme model compounds

We have performed Mössbauer studies on several oxy-heme model compounds, and for two of them we also have derived the threedimensional structure from X-ray studies; I: [Fe(O₂) (SC₆HF₄) TP_pivP] [Na⊂18C6] and II: [Fe(O₂) (SC₆HF₄)TP_pivP] [K⊂222]. Compound I shows a pronounced temperature-dependence of ΔE_Q, i. e. 2.17 mms^-1 at 4.2 K and 1.55 mms^-1 at 295 K, while ΔE_Q of compound II is practically temperature-independent, i.e. ca. 2.05 mms^-1. The X-ray structure analysis of these model compounds provides us with the fact that O₂ occupies three different sites in I and only two sites in II. From the experimentally known occupancies of these sites at specific temperatures we were able to apply relaxation calculations for the electric field gradient tensors in I and II, and we found for both compounds that our calculated ΔE_Q(T)-data are in agreement with our experimental results. These findings imply that the dynamic jump behavior of O₂ among the different sites in oxy-heme model compounds is responsible for the temperature-dependence (like in HbO₂) or temperature-independence of ΔE_Q (like in cytochrome P450).