Electron Spin Resonance of the Oxygen Molecule in Blood

The oxygen molecules trapped by hemoglobins in blood are supposed to be nearly free electronically, or to have electron spin 1 each. Yet no magnetic spin resonance of blood is observed near 3.3 kG with 9.5 GHz microwave, as expected for a free spin of S=1. The spin of this molecule in a gaseous state is known to be strongly coupled to the molecular axis. For an oxyhemoglobin in which the oxygen molecule is trapped, the cylindrical symmetry around the molecular axis may be violated, generating coupling of the spin to the axes perpendicular to the molecular axis. Assuming the same compling strength of spin around the molecular axis but no coupling to other axes as in the gaseous state, and assuming completely quenched rotation, the magnetic resonance line of the oxygen molecules in oxyhemoglobin is expected to have a sharp peak at 12 kG when 9.5 GHz microwave is used. Possible magnetic spin resonance lines under similar conditions but with non-cylindrical spin field is also discussed.