Photoelectron quantum yields of hemin, hemoglobin, and apohemoglobin. Possible applications to photoelectron microscopy of heme proteins in biological membranes.

Hemoglobin is examined as a model system for intrinsic photoelectron labeling experiments. The absolute photoelectron quantum yields of hemin, hemoglobin, and apohemoglobin thin films were measured in the 180-230 nm wavelength region. Hemin exhibits a quantum yield of approximately 6 x 10(-4) electrons per incident photon at 180 nm, 9 x 10(-5) electrons per incident photon at 210 nm, and 2 x 10(-6) electrons per incident photon at 230 nm. At all wavelengths the hemin curve lies approximately a factor of 20 above that of hemoglobin and two orders of magnitude above that of apohemoglobin. High image contrast is observed between hemin and apohemoglobin in low magnification photoelectron micrographs, suggesting the feasibility of intrinsic labeling studies involving heme proteins. The quantum yield of hemoglobin is discussed in terms of linear contributions from heme groups and protein weighted by their relative surface areas. The fractional surface areas based on the known structure of hemoglobin are consistent with values derived from the quantum yields of hemin and apohemoglobin.