Normal vibrational modes of heme have been analyzed using density functional theory, to assign a new Raman band observed at 312 cm(-1) in the CO-bound, active form of soluble guanylate cyclase. The conserved YxSxR motif is incorporated into the model to reproduce the hydrogen-bonding network around propionates. A delocalized mode that involves Arg motions as well as pyrrole tilting and propionate bending is found to be the most likely candidate accounting for the new band.