Pulsed and continuous wave electron nuclear double resonance patterns of aquo protons coordinated in frozen solution to high spin MN2+

Abstract

Listen

Translate article

For the water protons that coordinate to Mn2+, the frozen solution ENDOR (electron nuclear double resonance) spectra are made complex by the anisotropic electron–proton hyperfine interaction and by multiple contributions of the electron spin 5/2 manifold. A spin 5/2 Mn2+ ion having magnetic quantum numbers Ms=±1/2, ±3/2, ±5/2 and small zero-field splittings has overlapping electron spin EPR transitions. Proton hyperfine couplings to each of these electron spin states have yielded overlapping ENDOR patterns whose interpretation is nontrivial, even in so simple a system as Mn2+ ion having hexaaquo coordination. We have experimentally obtained and theoretically explained these proton ENDOR patterns and in so doing have laid the foundation for interpreting and sorting out frozen solution ENDOR patterns in more complex (enzyme) environments. Pulsed and cw ENDOR experiments showed features of metal-coordinated water protons occurring not only within a few MHz of the free proton frequency (as will happen for an electron spin 1/2 system) but extending over a range of up to 35 MHz. The EPR line of the Mn2+ S=5/2 manifold was broadened by zero-field splitting for hundreds of Gauss away from g=2.00, and the relative intensity of different ENDOR features reflected couplings to differing Ms spin states at varying fields across this EPR line. An expression was derived to show the dependence of proton ENDOR frequencies on the electron spin quantum number, Ms, upon the principal values of the intrinsic proton hyperfine tensor, upon the direction of the magnetic field, and upon the free proton frequency. This expression provided the starting point for powder simulations of the overall ENDOR pattern. These ENDOR powder pattern simulations were not elementary extensions of first-order theory as often applied to single-crystallike ENDOR spectra obtained at frozen solution EPR extrema. These simulations addressed the interpretation and the conditions for obtaining consistent hyperfine information from nuclei hyperfine coupled to Mn2+ where all Ms levels of the Mn2+ ion can furnish contributions to ENDOR and EPR. The aquo protons yielded an isotropic coupling As=0.8 MHz and a dipolar coupling Ad=3.4 MHz, where A∥=2Ad+As (=7.6 MHz) and A⊥=As−Ad (=−2.6 MHz). Such couplings are in agreement with those obtained by proton ENDOR of [Mn(H2O)6]2+ in single crystals [R. DeBeer, W. DeBoer, C. A. Van’t Hof, and D. Van Ormondt, Acta Cryst. B29, 1473 (1973)].