A two-dimensional (magnetic field and concentration) electron paramagneticresonance method for analysis of multispecies complex equilibrium systems.Information content of EPR spectra

A two-dimensional simulation method has been developed for the interpretati on of electron paramagnetic resonance (EPR) spectra consisting of a multitu de of strongly overlapping signal components. The set of EPR spectra for co mplex equilibrium systems is analyzed simultaneously as a function of metal and ligand concentrations and pH. The formation constants of the various s pecies are adjusted together with the magnetic parameters of the component EPR spectra. At most 10 EPR-active and 5 EPR-silent species can be involved to simulate a maximum of 36 experimental spectra, while the number of adju sted parameters is at most 100. Statistical parameters are suggested to giv e the confidence intervals for parameter estimation and to distinguish alte rnative speciation models. The efficiency of the program is demonstrated fo r the copper(II)-L-asparagine system, in which 10 species, including 3 pair s of isomers, are characterized with magnetic parameters and formation cons tants. On the basis of the magnetic parameters, a structural assignment is made for the detected species. The two-dimensional approach can also supply the formation constant of the EPR-silent species, as demonstrated for the copper(II)-glycyl-L-serine system.