Alan D. Adler

ELECTRON PARAMAGNETIC RESONANCE STUDIES OF IRON PORPHIN AND CHLORIN SYSTEMS

In any discussion of porphins and chlorins, one would like to limit oneself to the discussion of nonprotein problems. However, other participants have cited work on metal porphins as models for the understanding of protein-catalyzed reactions.’ We should like to present the results of experiments on metal porphins and chlorins that were undertaken in order to elucidate structural aspects of heme proteins. As a means of orienting oneself, the first Figure (FIGURE 1) is a representation, after Dickersoq2 of the heme protein molecule, myoglobin, showing the helical content and the region of heme binding to the protein moiety. This Figure can also serve as an approximate representation of the tertiary structure of a chain of the tetrameric molecule, hemoglobin. In the upper right portion, note the prosthetic group, iron protoporphyrin IX, bound to the protein via covalent linkage to a nitrogen atom of histidine imidazole F8, often called the proximal histidine. Here, the imidazole is a ligand for the iron not only when the molecule is in its functional high-spin ferrous form,’ but also when the molecule is in the reversibly and irreversibly denatured ferric form^.^.^ On the distal side of the iron is the position where oxygen and other endogenous ligands such as CN-, F-, etc., bind. Pointed at this spot, but too far away to bind to it, is the distal histidine imidazole E7. During the last four years we have been studying the EPR of various high-6 and low-spin’ forms of heme proteins. Of special interest are those compounds which can be formed from specific denaturation reactions of hemoglobins and myoglobins in which the ligands of the heme are endogenous to the protein. One can consider a protein as a medium for maintaining iron protoporphyrin IX as a mononuclear species. The mononuclear iron protoporphyrin, at least in the ferric state, has properties which lend themselves to study by electron paramagnetic resonance (EPR) spectroscopy, and this will be the basis for this paper.

PORPHYRINS, POWER, AND POLLUTION

Several avenues of research in the areas of pollution, ecology and energy that are open to investigators of porphyrins are discussed. The versatility of these structures for such structure-function studies equals that of their usefulness in physical investigations of structure-reactivity and structure-property studies.

PORPHYRINS AS MODEL SYSTEMS FOR STUDYING STRUCTURAL RELATIONSHIPS

Porphyrins, porphyrin derivatives, and porphyrin-like materials play a large number of well-diversified and well-established biological roles. 1-4 Some of these are shown in TABLE 1. The significance and importance of these biological functions of porphyrinic materials has always been well emphasized. They have been most strongly appreciated in those physiologic processes associated with photosynthesis and hemeprotein phenomenology, 4-110ften illustrated as in FIGURE 1.