We demonstrate that Zn(II) porphyrin in Zn(II)cytochrome c (Zn cyt c) is a fluorescence resonance energy transfer (FRET) donor to all Alexa660 dye acceptor. The energy transfer efficiency is dependent oil the distance between the two fluorophores as shown through( protein denaturation studies of Five Zn cyt c variants labeled with Alexa660 in different positions. The relative quantum yield, excitation and emission energies, and labeling efficiencies of this donor-acceptor pair allow for a method of analysis based on sensitized emission of the these studies show that Zn(II) porphyrin is all effective energy donor for measurement of molecular-acceptor. scale distances by FRET.