FTIR and optical spectroscopic studies of the reactions of heme models with nitric oxide and other NOx in porous layered solids

Porous layered solids can be prepared by the vacuum sublimation of the ferrous porphyrin complexes Fe(Por) onto a CaF2 or KBr substrate in a cryostat. The facile reactions of these heme models with volatile reactants can be studied in detail using FTIR and optical spectroscopy. The power of this technique draws from the ability to investigate such processes under carefully controlled and tunable conditions, especially temperature. Furthermore, the solvent-free medium gives relatively sharp bands in the FTIR spectra that, combined with isotopic labeling experiments, provide structural information regarding otherwise elusive intermediates and insight into key mechanistic steps. Reviewed here are investigations of the reactions between various Fe(Por) and the nitrogen oxides NO and NO2 to give (initially) such species as Fe(Por)(NO), Fe(Por)(η1-ONO) and Fe(Por)(η2-O2NO). Also described are subsequent transformations of these upon exposure to NO, NO2 and various Lewis bases. These simple models provide fundamental information relevant to the reactions of heme proteins with the biologically important nitrogen oxides.