Aggregation in five-coordinate high-spin natural hemins: determination of solution structure by proton NMR

Abstract

¹H NMR measurements (at 500 MHz) of nuclear spin-spin relaxation time T₂ (from NMR line width) at different temperatures are reported for aggregates of several five-coordinate high-spin iron(III) complexes of proto-, deutero-, and coproporphyrins in solution and are utilized to determine their solution structure. Extensive aggregation of these complexes in solution is observed, and the dominant form of the aggregates is shown to be dimers. The degree of aggregation for these iron(III) porphyrins follows the order proto- » deutero- > copro-. The line width of the heme methyl resonances was analyzed by using a nonlinear least-squares fit program working in finite difference algorithm. The values of T₂ were used to determine the structural details of the dimer.