The dynamics of the enthalpy and volume changes found in the photodissociation of CO from sperm whale carboxymyoglobin and two site-directed mutants in which arginine-45 is replaced by glycine and asparagine are examined by photoacoustic calorimetry. An intermediate is observed whose lifetime at 20°C is 700 ns. The enthalpy of the intermediate increases by ~7 kcal/mol upon replacing arginine-45 with either asparagine or glycine. These observations support recent proposals that an arginine-45 salt bridge is broken upon ligand dissociation.
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