Magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) spectroscopies at various applied magnetic fields (0-6 T) and temperatures (2.0-31 K) have been used to investigate the electronic properties of the visible (Q(0-0) or alpha band) region of oxy- and nitrosylferrohemoglobin (HbNO). OxyHb, a d(6) (S = 0) diamagnet, exhibits the expected pseudo-first derivative MCD and pseudo-second derivative MLD temperature-independent features centered at 574 nm. HbNO, a d(7) (S = 1/2) paramagnet, also exhibits a temperature-independent pseudo-first derivative MCD spectrum, but centered at 571 nm. So far as we are aware, this behavior is unprecedented in the MCD spectra of paramagetic iron-porphyrins, which are expected to be dominated by temperature-dependent C-0 terms. The HbNO MCD spectrum does, however, demonstrate limited field-dependent saturation magnetization behavior and the MLD spectrum is currently below the detection limit. In addition, an MCD signal from reoxygenated venous blood is reported and compared with MCD signals from oxy- and HbNO derivatives. Finally, a combination of MCD and MLD spectroscopies has been used to estimate the orbital angular momentum (M-L) value of the a band excited state of oxyHb as 4.2 (+/-0.7). (C) 2002 Elsevier Science (USA). All rights reserved.