Progress towards a DNA-based model system for the study of electron spin-spin interactions

A liposome-encapsulated spin trap for the detection of nitric oxide

Electron paramagnetic resonance (EPR) is one of the few methods that allows for the unambiguous detection of nitric oxide (NO). However, the dithiocarbamate-iron spin traps employed with this method inhibit the activity of nitric oxide synthase and catalyze NO production from nitrite. These disadvantages limit EPR's application to biological NO detection. We present a liposome-encapsulated spin-trap (LEST) method for the capture and in situ detection of NO by EPR. The method shows a linear response for [NO]≥4µM and can detect [NO]≥40nM in a 500µL sample (≥20 pmol). The kinetics of NO production can be followed in real time over minutes to hours. LEST does not inhibit the activity of inducible nitric oxide synthase or nitrate reductase and shows minimal abiotic NO production in the presence of nitrite and NADH. Nitrate reductase-like activity is detected in cell lysates of the coccolithophore Emiliania huxleyi and is elevated in virus-infected culture. This method shows particular promise for NO detection in cell lysates and crude preparations of NO-producing tissues.

Structure and dynamics of a DNA-based model system for the study of electron spin-spin interactions

We report on the structure and dynamics of a model system for measuring long-range distances in biological macromolecules by saturation-recovery EPR. Four DNA duplexes that incorporate a paramagnetic dysprosium ion (Dy(III)) and a nitroxide spin-label were examined by electron paramagnetic resonance (EPR), circular dichroism (CD), and ultra-violet absorbance (UV) spectroscopy. Dy(III) is chelated by the modified base deoxythymidine-EDTA, (dT-EDTA). Electron spin-spin interactions between the Dy(III) ion and the nitroxide radical are observed at distances as great as approximately 5.3 nm. A slight change in the conformation of those nucleotides lying between the EDTA(Dy(III)) complex and the nitroxide spin-label results in a "stiffening" of the DNA helix on the EPR time scale. Changes in conformation and helix dynamics are due to the binding of the EDTA(Dy(III)) complex to the phosphodiester backbone of the complementary strand. Molecular mechanics calculations indicate that binding occurs in the 5' direction on the complementary strand, at a position 3 or 4 phosphates distant from the dT-EDTA(Dy(III))*dA base pair.