Nonplanar porphyrins and their biological relevance: ground and excited state dynamics

Abstract

X-ray structures solved for many tetrapyrrole-containing biomolecules have indicated that the tetrapyrrole unit adopts a nonplanar conformation and the degree of nonplanarity varies from one system to other. One of the reasons for choosing a family of tetrapyrrole macrocycles at the active sites of many biomolecules is due to the fact that they are highly flexible and can adopt a range of conformations. Recent realisation that the nonplanar conformations are essential for the biological functions has led to a surge of research activity on studies of nonplanar porphyrins. A large number of synthetic nonplanar porphyrins with varying degree of nonplanarity have been studied as models. The present article reviews the latest developments in such studies. Specifically, the structural deformation effect on various spectral properties such as optical, redox behaviour in the ground and excited singlet and triplet states are described for a series of nonplanar porphyrin systems. They include, hydroporphyrins, N-substituted porphyrins, heterosubstituted porphyrins and sterically crowded porphyrins. Furthermore, the dependence of nature of magnetic coupling on structure of porphyrin core in paramagnetic metalloporphyrin cation radicals is highlighted.