Time-resolved X-ray absorption determination of structural changes following photoinduced electron transfer within bis-porphyrin heme protein models

Time domain energy dispersive X-ray absorption spectroscopy provides a powe rful probe of changes in molecular structure that occur during photoinduced electron transfer reactions. In this study a diporphyrin model for electro n transfer within modified heme proteins is examined using this technique. Transient optical absorption experiments show that photoinduced electron tr ansfer from the lowest excited triplet state of a Zn, porphyrin attached at a fixed 25 Angstrom distance by means of a long spacer molecule to an Fe(I II) porphyrin occurs with a 6 ms time constant at 77 K. Time domain X-ray a bsorption measurements are consistent with transient reduction of Fe(III) t o Fe(II) that is accompanied by substantial weakening of the bond between t he Fe atom and a pyridine molecule ligated to it. Coupling of ligand loss t o reduction of Fe(III) to Fe(II) provides a means of stabilizing the reduce d intermediate.