Role of the Arginine-45 Salt Bridge in Ligand Dissociation from Sperm Whale Carboxymyoglobin As Probed by Photoacoustic Calorimetry

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.