Magnetic resonance Raman activity: Inverse Faraday effect in hemoglobin

Abstract

A complete polarization study of human oxy- and carbonmonoxyhemoglobin (carbonylhemoglobin) A and S is reported for backscattered light in the resonance Raman light-scattering situation with excitations in the long-wavelength region, λ=5682 Å and 5815 Å, and in the short-wavelength region, λ=4579 Å and 4880 Å, as comparison. All four polarization components of the scattered light with respect to the two conditions of linear and circular polarization of the incident light were measured. These were: (1) parallel; (2) perpendicular; (3) corotating; and (4) contrarotating.

This method has been used to characterize the three invariants of the nonsymmetric Raman tensor for randomly oriented molecules. These invariants are based on a model in which the scattered light is dependent on only an induced electric dipole. With no higher moments involved in the scattering process, the amplitude of any band measured under the four conditions should satisfy the relation: (1)+(2)=(3)+(4).

The experiments reported here demonstrate that although this relation is satisfied for short-wavelength (4579 Å and 4880 Å) excitation, it does not apply for long-wavelength (5682 Å and 5815 Å) excitation, for which (1)+(2)<(3)+(4) holds.

There are larged-d metal-electron influences on the long, but not the short, wavelength absorption region of hemoproteins. A large metal-electron magnetic dipole moment can thus be induced with circularly polarized light of long wavelength, owing to the availability of a transition involving charge transfer interaction coupling between metal orbitals and the porphyrin π^* orbitals.

The increase in light scattering is due to aninverse Faraday effect, resulting in magnetic resonance Raman activity. Owing to large crystal field influences and Jahn-Teller instability, the critical zero-field splitting energy is also large. The incident circularly polarized radiation at λ=5700 Å (ω=17,544 cm⁻¹) is approximately the critical cubic splitting parameter, △ _C , the calculated value for which is 18,500 cm⁻¹. This incident light equalizes or mixes the energy levels of the¹ A ₁ and⁵ T ₂ states; i.e., it populates magnetic substates. An induced electric dipole-magnetic dipole transition, besides an induced electric dipole transition, is then permitted for circularly polarized light at the Larmor precession frequency, but not permitted for linearly polarized light, as thed-d metal transition is electric dipole disallowed.

The hemoglobin S solution and gel scatter longwavelength light in a similar fashion; i.e., a large induced magnetic dipole moment is implicated for the circularly polarized conditions. In addition, there are bands at 750 and 1050 cm⁻¹ for the hemoglobin S gel not present in dissolved hemoglobin S nor in hemoglobin A. The presence of these bands is attributed to changes in theB _1g mode of porphyrin ring vibrations.