NITRIC-OXIDE DIFFUSION-COEFFICIENTS IN SOLUTIONS, PROTEINS AND MEMBRANES DETERMINED BY PHOSPHORESCENCE

The reactivity of nitric oxide under a given condition is a complex fu nction of its diffusivity and the concentration of reacting partners. Quenching by NO of luminescence from Ru and Pd chelates of mesoporphyr in M, two molecules which exhibit phosphorescence at room temperature, was utilized to evaluate the gas concentration and apparent diffusion coefficients. The properties of Ru-mesoporphyrin, a dye not previousl y employed as a probe for O-2 or NO, were determined and the assay was verified and used to quantify NO produced by decomposition of nitroso cysteine. The pseudo-second order quenching constants were obtained fr om Stern-Volmer plots measured under various conditions and used to ca lculate diffusion coefficients for nitric oxide in solutions, proteins and membranes. The diffusion coefficients were greater at 37 than at 25 degrees C and, at a given temperature, smaller in proteins and memb ranes than in water. The conclusion is that NO and O-2 closely resembl e each other in diffusivity but that NO is slightly less lipophillic, resulting in somewhat faster apparent diffusion in protein and slower diffusivity in lipid, relative to O-2. Taking a mean diffusion coeffic ient for NO of 10(-7) cm(2)s(-1), then within 10 s the mean path is 10 (-3) cm, or less than the diameter of a single cell. However, at low N O and O-2 concentrations, the halflife of NO will be considerably long er than 10 s, and consequently the path of NO diffusion much greater.