The ferric form of the wild-type mouse neuroglobin (Ngb), a newly discovered heme protein which is primarily expressed in the brain of mammals, has been characterized by electron paramagnetic resonance (EPR) spectroscopy. The study reveals the simultaneous presence of two related structural forms in a wide range of pH values. The dominant low-spin form (>90%) with g-tensor principal values 3.15, 2.16 and 1.34 can be attributed to a His-Fe-III-His configuration. The high-spin form with g(perpendicular to) = 5.97 and g(parallel to) similar to 2, can be ascribed either to a hexacoordinated His-Fe-III-H2O form or to a pentacoordinated His-Fe-III. The high-spin to low-spin ratio is found to decrease with increasing pH values. (C) 2002 Elsevier Science B.V. All rights reserved.