Abstract: The ESR spectrum characteristic of imidazoline nitroxide radicals as pH spin probes were studied.A detailed investigation of the geometrical structure, electronic structure and spin density distribution of pH spin probes was made with AM1 quantum chemistry calculations. The a_N of pH spin probes was calculated, it is consistent with the experimental value within the experimental errors. The relationship curve of pk_a. with electron density was obtained. The application of imidazoline nitroxide radicals as the pH probe in examining the PH of the organism was also discussed.

Key words: Imidazoline nitroxide radicals, pH spin probes, Quantum chemistry AM1