Lifetimes of complexes of the antibiotic X-537A (lasalocid A) with physiological cations in methanol. Determination by NMR using a dissociative shift reagent.

Using a magnetic resonance method we have determined the lifetimes of the following complexes of X-537A (HX) in methanol solution at 25 degrees C, BaX+, 132 microseconds; SrX+, 34 microseconds; CaX+, 24 microseconds, and KX, 15 microseconds, each with an estimated uncertainty of 30%. For NaX and LiX the lifetimes are too short to measure by the present method. The lifetime found for BaX+ is considerably shorter than the result obtained by Patel and Shen using a different method. The present method depends upon the broadening of the resonance lines of certain protons in X- due to reactions such as X- + Ba2+ = BaX+, when the resonance of the proton in the "free" X- has been shifted from its normal frequency by Pr(III) which acts via the exchange PrX3 in equilibrium PrX2+ + X-. To determine the lifetimes of interest the equations for a three-state spin system were derived; under the relevant conditions the final equations take the form of the Swift-Connick equations for a two-state system, with the characteristics of one of the states dependent upon the PrX3 concentration. This dependence is used to extract the life-times from the data in a novel and simple way.