Variations of electrostatic interactions in myoglobin probed by UVRR spectroscopy. Effect of iron ligand and pH on tryptophan and tyrosine vibrations.

The present work investigates the variations of electrostatic interactions within the myoglobin molecule associated with azide heme binding and pH variations. Far ultraviolet (223 nm) resonance Raman spectroscopy of the tryptophan and tyrosine residues, along with acid-base titration measurements, have been used to monitor variations in the protein matrix. With previously determined mode assignments, it is shown that the Trp and Tyr residues of the globin moiety are influenced by the charge spatial distribution. Upon ligand binding or under various pH conditions, the polar interactions inside the protein appear to be modulated by the electric field generated by the charge array. It is concluded that the binding site properties of myoglobin can be modulated by the charge spatial distribution within the protein, even in the absence of measurable conformational changes of the bulk.