Spin state transitions upon visible and infrared excitation of ferric MbN3

►Visible heme and infrared ligand excitation change the spin state equilibrium of MbN3. ► The difference in ligand binding angle between high and low spin protein was measured. ► Spin state changes take place on the time scale of vibrational relaxation of heme. ► Heme vibrations may modulate the electronic and functional properties of the protein.