ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY AS A PROBE OF COORDINATION STRUCTURE IN GREEN HEME SYSTEMS - IRON CHLORINS AND IRON FORMYLPORPHYRINS RECONSTITUTED INTO MYOGLOBIN

A series of ferric low-spin derivatives of myoglobin containing its na tural prosthetic group, iron protoporphyrin IX, and reconstituted with iron heme s (a formyl-substituted porphyrin) and iron methylchlorin h ave been examined using low-temperature electron paramagnetic resonanc e (EPR) spectroscopy. Good agreement is observed between the EPR prope rties of parallel derivatives of natural myoglobin and heme s-myoglobi n. Likewise, the EPR properties of parallel adducts of three types of iron chlorins, methylchlorin-myoglobin, sulfmyoglobin (a myoglobin der ivative known to contain a chlorin macrocycle) and synthetic chlorin m odels are similar to each other. The ferric chlorin systems are shown to exhibit increased tetragonality and decreased rhombicity values rel ative to protoporphyrin/formylporphyrin systems. Thus, EPR spectroscop y is a very useful technique with which to probe the coordination stru cture of naturally occurring iron chlorin proteins and the method can be used to distinguish between proteins containing iron formylporphyri ns and iron chlorin prosthetic groups.