A dramatic change occurs in the vibrational properties of the iron-histidine bond, trans to the oxygen binding site, on freezing deoxyhemoglobin. The large, quaternary structure-dependent differences in the shape and frequency of the iron-histidine mode observed in resonance Raman scattering measurements above freezing are significantly diminished by the freezing event and the scattering intensity increases substantially. On further reduction in temperature to 10 K this broad line becomes narrow and shifts to a higher frequency. These data implicate dynamical processes and protein interaction with water as contributors to the quaternary structure dependence of the iron-histidine bond and thus reflect on the role of that bond in the energetics of cooperative ligand binding.
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