Improved sensitivity for imaging spin trapped hydroxyl radical at 250 MHz

Abstract

Radicals, including hydroxyl, superoxide, and nitric oxide, play key signaling roles in vivo. Reaction of these free radicals with a spin trap affords more stable paramagnetic nitroxides, but concentrations in vivo still are so low that detection by electron paramagnetic resonance (EPR) is challenging. Three innovative enabling technologies have been combined to substantially improve sensitivity for imaging spin-trapped radicals at 250 MHz. 1) Spin-trapped adducts of BMPO have lifetimes that are long enough to make imaging by EPR at 250 MHz feasible. 2) The signal-to-noise ratio of rapid-scan EPR is substantially higher than for conventional continuous-wave EPR. 3) An improved algorithm permits image reconstruction with a spectral dimension that encompasses the full 50 G spectrum of the BMPO-OH spin adduct without requiring the wide sweeps that would be needed for filtered backprojection. A 2D spectral-spatial image is shown for a phantom containing ca. 5 μM BMPO-OH.

Keywords: EPR imaging; hydroxyl radicals; image reconstruction; rapid-scan EPR; spin-trapping.

Figure 1

Zero-gradient spectrum of BMPO-OH at 250 MHz (blue) and simulation obtained with the parameters in Table 1 (red). Harmonics of the scan frequency up to fifth order were subtracted from the data.

Figure 2

2-D spectral-spatial image of BMPO-OH in the phantom sample (A), a spectral slice through the image (B) and the error function (C) that was used in the spectral slice fitting routine to distinguish regions that contain BMPO-OH from noise-containing baseline regions.

Scheme 1

Spin trapping of hydroxyl radical with DMPO and BMPO.