17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+

The magnetic couplings of 17O in H217O coordinated to high spin Mn2+ in a frozen aqueous solution were determined using the complementary magnetic resonance techniques of pulsed and continuous wave (cw) ENDOR (electron nuclear double resonance), ESEEM (electron spin echo envelope modulation), and PFSEPR [pulse field sweep electron paramagnetic resonance (EPR)]. Several complications arise from the high electron spin multiplicity of the d5, Mn2+ ion and the high nuclear spin multiplicity, I=5/2, of the 17O nucleus. At the applied magnetic field strengths in 9 GHz EPR studies, the zero-field splitting of the S=5/2 Mn2+ ion in aqueous frozen solution is small relative to the electron spin Zeeman interaction so that the MS=±1/2,±3/2,±5/2 electron spin states all contribute to the ENDOR spectrum. This results in a complex spectrum in which the 17O ENDOR powder pattern arising from the MS=±1/2 manifolds are separately resolved but the powder patterns from the MS=±3/2,±5/2 manifolds overlap the multiple 1H ENDOR lines arising from all six MS manifolds [X. Tan, M. Bernardo, H. Thomann, and C. P. Scholes, J. Chem. Phys. 98, 5147 (1993)]. Given this complexity, a combination of complementary spectroscopic techniques and numerical simulations are used to deconvolute the overlapping spectra and to assign the spectral lines. The ENDOR spectra provided an experimental description of H217O hyperfine couplings to high spin Mn2+ in a frozen solution. The ESEEM results are consistent with the first-order assignments of the ENDOR lines and demonstrate the feasibility of ESEEM measurements of 17O ligand hyperfine couplings to Mn2+. Simulations of the 17O ENDOR hyperfine patterns of aqueous frozen solutions of Mn2+, especially those near 20 MHz, indicated an A-tensor anisotropy of A⊥=−6.5±0.5 MHz and A∥=−9.5±0.5 MHz, consistent with couplings observed by single crystal ENDOR of H217O ligated to Mn2+ doped in [LaMg(NO2)12⋅24(H2O)]. More detailed simulations of the ENDOR pattern below 10 MHz indicated the need for quadrupole couplings consistent with those measured by single crystal ENDOR and with those determined by gas phase measurements on H17OD. Simulations of the ENDOR spectra recorded by the cw and pulsed techniques have delineated important features of the techniques which must be taken into account for a quantitative analysis of the ENDOR amplitudes. It is expected that the general ENDOR conditions employed and the theory developed will be useful in frozen solution studies of 17O involved as a ligand to Mn2+ in enzymes.