Abstract: Conformationally designed biomimetic systems and the investigation of the conformation control of biological reactions is becoming a topic of active research at the interface of chemistry and biology.[1] In this context the interrelationship between macrocycle conformation and physicochemical properties in porphyrins is emerging as a topic of special biological relevance. Various nonplanar conformations have been observed for the porphyrinoid cofactors involved in photosynthesis, electron transfer and respiration and it is now believed that the physicochemical properties of the natural tetrapyrroles are fine-tuned via steric interactions with the protein skeleton in a conformational control process.[1b,c,2] For example, there is considerable structural heterogeneity in the accessory bacteriochlorophylls of photosynthesis[3] and a conformational asymmetry is one possible explanation for the unidirectonality of the light-induced electron transfer in the photosynthetic reaction center.[4] Similarly different coactor conformations have been observed for and implied in the action of various heme proteins such as respiratory proteins,[5a] cytochromes,[5b] cytochrome P450[5c] and catalases.[5d] Indeed, dynamic conformation changes and redox dependent changes of the porphyrin conformation in proteins have now been established.[6] Different macrocycle conformations have also been described for diverse tetrapyrrole classes like corrins in B12 and F430[7a] and hydroporphyrins[7b] in sulfite and nitrite reductase[7c].[8