THE CONFORMATIONAL FLEXIBILITY OF TETRAPYRROLES - CURRENT MODEL STUDIES AND PHOTOBIOLOGICAL RELEVANCE

Porphyrins and other tetrapyrroles exhibit a considerable flexibility of the macrocycle conformation. Examples of non-planar tetrapyrrole conformations have been observed in a variety of protein complexes, such as chlorophyll-reaction center and antenna systems, heme proteins and cytochromes. Different macrocycle conformations are believed to be responsible for diverse functions of the same chromophore in different protein environments. This allows an explanation of the spectral properties of antenna complexes and the unidirectionality of electron transfer in reaction centers to be given. The concept of conformational flexibility is reviewed with special attention given to the current work on model compounds for chlorophylls. Sterically strained porphyrins exhibit considerable distortions with associated changes in their physicochemical properties. The synthesis of tetrapyrroles with a varying degree of non-planarity allows detailed studies using structural and spectroscopic methods. The degree of non-planarity depends on the kind and number of substitutents, central metal, axial ligands, charge, aggregation, and packing effects in crystals. Emerging studies on chlorins and bacteriochlorins show a high degree of variability in the conformations observed in isolated pigments and indicate possibilities for fine tuning of the chromophore properties via protein interactions by axial ligation of the central metal, hydrogen bonding and electronic effects. Current methods for the preparation of model compounds are presented and a discussion of future prospects is given.