Emission lifetimes of human hemoglobin computed from the atomic coordinates

Using the Forster equations we have estimated the rate of energy transfer from tryptophans to hemes in hemoglobin. We computed the orientation factors and the consequent transfer rates from the crystallographic coordinates of human oxy- and deoxyhemoglobin. The intrasubunit distances between hemes and tryptophans allow lifetimes between 5 and 15 ps per each ns of tryptophan lifetime. Lifetimes of several hundred ps would be allowed by the intersubunit distances. However, these distances become operative only when one heme per tetramer does not accept transfer. If more than one heme per tetramer does not function as an acceptor, lifetimes of several ns would be allowed.