Dielectric resonator-based side-access probe for muscle fiber EPR study

We present a novel dielectric resonator (DR)-based resonant structure that accommodates aqueous sample capillaries in orientations that are either par allel (i.e., side-access) or perpendicular to the direction of an external (Zeeman) magnetic held, B-0. The resonant structure consists of two commerc ially available X-band DRs that are separated by a Rexolite spacer and reso nate in the fundamental TE01 delta mode. The separator between the DRs is u sed to tune the resonator to the desired frequency and, by appropriately dr illed sample holes, to provide access for longitudinal samples, notably cap illaries containing oriented, spin-labeled muscle fibers. In contrast to th e topologically similar cylindrical TE011 cavity, the DR-based structure ha s distinct microwave properties that favor its use for parallel orientation of lossy aqueous samples. For perpendicular orientation of a dilute (6.25 mu M) aqueous solution of IASL spin label, the S/N ratio was at least one o rder of magnitude better for the side-access DR-based structure than for a standard TE102 cavity, EPR spectra acquired for maleimide spin-labeled myos in filaments also revealed ca. 10 times better S/N ratio than those obtaine d with a standard TE102 cavity. For the side-access DR with sample capillar ies oriented either parallel or perpendicular to the external magnetic held , the Q- and filling factors are in good agreement with the theoretical est imates derived from the distribution of magnetic (H-1) and electric (E-1) c omponents. (C) 2000 Academic Press.