A new Q-band EPR probe for quantitative studies of even electron metalloproteins

Existing Q-band (35 GHz) EPR spectrometers employ cylindrical cavities for more intense microwave magnetic fields B-1, but are so constructed that only one orientation between the external field B and B-1 is allowed, namely the B perpendicular to B-1 orientation, thus limiting the use of the spectrometer to measurements on Kramers spin systems (odd electron systems). re have designed and built a Q-band microwave probe to detect EPR signals in even electron systems, which operates in the range 2 K less than or equal to T less than or equal to 300 K for studies of metalloprotein samples. The cylindrical microwave cavity operates in the TE011 mode with cylindrical wall coupling to the waveguide, thus allowing all orientations of the external magnetic field B relative to the microwave held B-1. Such orientations allow observation of EPR transitions in non-Kramers ions (even electron) which are either forbidden or significantly weaker for B perpendicular to B-1. Rotation of the external magnetic field also permits easy differentiation between spin systems from even and odd electron oxidation states. The cavity consists of a metallic helix and thin metallic end walls mounted on epoxy supports, which allows efficient penetration of the modulation field. The first quantitative EPR measurements from a metalloprotein (Hemerythrin) at 35 GHz with B-1 parallel to B are presented. (C) 1999 Academic Press.