The hemoglobin of the marine annelid Glycera dibranchiata possesses several unique features: the hemoglobin consists of multiple monomeric and polymeric components, quaternary structure is lacking, the distal histidine is replaced by leucine in at least one monomeric constituent, and 4) the protein exhibits extremely rapid ligand binding kinetics. The effect of these structural modifications on the ligand binding process has been evaluated using resonance Raman spectroscopy to examine the vibrational modes of the porphyrin macrocycle in deoxy and carbonmonoxy equilibrium species of hemoglobin G. dibranchiata in both the unseparated monomeric and polymeric forms and in a single monomeric component designated Fraction II. Significant differences relative to hemoglobin were found in porphyrin pi electron density, vinyl environment, low frequency vibrational modes, and, in particular, the Fe-proximal histidine stretching mode. Spectra of the deoxy heme transients generated within 10 ns of ligand photolysis have also been examined. These clearly indicate large differences in the heme pocket dynamics subsequent to CO photolysis in G. dibranchiata hemoglobins relative to other hemoglobins. The significance of these results in terms of the kinetics and thermodynamics of ligand binding is discussed.