Dosimetry of 60Co and 192Ir gamma-irradiated agarose gels by proton relaxation time measurement and NMR imaging, in a 0-100 Gy dose range.

Localized irradiation of the skin and subcutaneous tissues with large single doses of gamma rays can induce immediate effects characterized by erythema, desquamation, and necrosis. Correlations between the evolution of the lesions and dosimetry studies have to be established by biophysical methods. NMR studies of the effects of an irradiated Fricke solution might be a means of controlling the delivered irradiation doses. After exposition to ionizing radiations, ferrous ions are transformed into ferric ions. Both are paramagnetic ions, and proton spin-lattice relaxation is accelerated depending on the oxidation reaction. In this study, solution of ammonium ferrous sulfate in an acid environment was incorporated into a gelling substance made with agarose, so that T1 weighted image contrast could be used to detect ferric ion formation. Experiments with 192Ir and 60Co gamma rays with doses in the 0 to 100 Gy range were conducted with Fe2+ concentrations of 0.5, 1, 1.5, and 2 mM in a gelling substance containing 4% agarose. A relationship was established between the amount of Fe3+ created and the spin-lattice proton relaxation rate, which led to a straightforward dose-effect relation. The use of such high doses allowed us to reproduce realistic conditions of accidental overexposure. A linear relationship was obtained between the doses absorbed and the NMR parameters measured (T1 and relative image intensity).

Two-dimensional 1H NMR study of recombinant insect defensin A in water: resonance assignments, secondary structure and global folding.

A 500 MHz 2D 1H NMR study of recombinant insect defensin A is reported. This defense protein of 40 residues contains 3 disulfide bridges, is positively charged and exhibits antibacterial properties. 2D NMR maps of recombinant defensin A were fully assigned and secondary structure elements were localized. The set of NOE connectivities, 3JNH-alpha H coupling constants as well as 1H/2H exchange rates and delta delta/delta T temperature coefficients of NH protons strongly support the existence of an alpha-helix (residues 14-24) and of an antiparallel beta-sheet (residues 27-40). Models of the backbone folding were generated by using the DISMAN program and energy refined by using the AMBER program. This was done on the basis of: (i) 133 selected NOEs, (ii) 21 dihedral restraints from 3JNH-alpha H coupling constants, (iii) 12 hydrogen bonds mostly deduced from 1H/2H exchange rates or temperature coefficients, in addition to 9 initial disulfide bridge covalent constraints. The two secondary structure elements and the two bends connecting them involve approximately 70% of the total number of residues, which impose some stability in the C-terminal part of the molecule. The remaining N-terminal fragment forms a less well defined loop. This spatial organization, in which a beta-sheet is linked to an alpha-helix by two disulfide bridges and to a large loop by a third disulfide bridge, is rather similar to that found in scorpion charybdotoxin and seems to be partly present in several invertebrate toxins.