High Field and Multi-Frequency EPR in Single Crystals of Sperm Whale Met-Myoglobin: Determination of the Axial Zero-Field Splitting Constant and Frequency Dependence of the Linewidth

Abstract

Electron paramagnetic resonance (EPR) measurements for frequencies from 28.5 GHz to 608.1 GHz and magnetic field up to 14 T have been performed on single crystals of sperm whale high-spin ( S =5/2) met-myoglobin. The EPR resonance field along the c * -axis deviates from the g =5.71 straight line at high frequencies. The axial zero-field splitting constant ( D ) of the met-myoglobin sample is evaluated to be 9.47±0.05 cm -1 by analyzing the resonance fields with the S =5/2 spin Hamiltonian including the D term. The angular dependence of EPR spectra in the a b plane has been also investigated at high frequencies. Two kinds of EPR spectra are observed corresponding to two kinds of different heme sites in the unit cell. A notable change in the linewidth of the spectrum along the c *-axis occurs above 350 GHz, suggesting that the dominant relaxation process changes around 350 GHz. The origins of the linewidth are discussed.