Cryo IR Spectroscopy of [Hemin]+ Complexes in Isolation

We present cryo IR spectra of isolated [Hemin] + adducts with CO, N 2, and O 2 ([Hemin(CO) 1 ] +, [Hemin(CO) 2 ] +, [Hemin( 14 N 2 )] +, [Hemin( 15 N 2 )] +, and [Hemin(O 2 )] + ). Well resolved bands allow for the elucidation of structure and spin multiplicity in conjunction with density functional (DFT) calculations. There is a quartet spin state for the N 2 and CO adducts and a sextet spin state for the O 2 adduct, where the O 2 retains its triplet state. The double CO adsorption induces significant changes in the vibrational patterns of the IR spectra, which we take as strong evidence for a spin quenching into a doublet. Our study characterizes [Hemin] +, which is the Fe 3+ oxidation product of heme that is of ubiquitous presence in hemeproteins.