Nitrosyl hemoglobin: EPR components at low temperatures.

The EPR spectrum of nitrosyl hemoglobin has been studied from 7.5 K to 104 K. It is composed of at least three components (A, B and C) which have a different dependence on temperature and power level. The A component decreases with increasing temperature. The B component disappears at around 30 K and is replaced by C. Relaxation of A follows the Orbach mechanism with an energy of 28 cm-1. This behavior can be attributed to phonon induced changes in the orientation of NO with respect to the heme plane.