Binding of alkylisocyanides of different bulkiness to the two major components of trout hemolysate is presented. In the case of trout hemoglobin I isocyanide binding is pH-independent, similar to O2 and CO, and the bulkiness of the ligand is related to the endothermicity of ligand binding to the T quaternary state. On the other hand, in trout hemoglobin IV the size of the ligand seems to affect the pH dependence of affinity and cooperativity. A comparison with other ligands, like O2, allows us to hint at possible stereochemical determinants of ligand binding in these two hemoglobins.
