A de novo synthesized heme protein is characterized with EPR (electron paramagnetic resonance) and ENDOR (electron nuclear double resonance) spectroscopy. It consists of a four-helix bundle providing bis-histidine binding pockets for the paramagnetic cofactor heme. The EPR spectrum shows Fe3+ low-spin signals that were simulated on the basis of a g-strain line broadening mechanism. Pulsed-ENDOR spectra revealed proton and nitrogen resonances of axial histidine ligands. Simulation of histidine proton ENDOR signals yielded detailed structural information about the heme binding situation complementing the EPR spectroscopic observations.