| Abstract: | Pulsed electron paramagnetic resonance spectroscopy has been used to detect Mn(II)-ligand superhyperfine couplings in complexes with creatine kinase and in the Mn(II) metalloprotein concanavalin A. Electron spin-echo envelopes from Mn(II), bound in these complexes, are modulated by superhyperfine interactions between Mn(II) and nearby, weakly coupled nuclear spins. The characteristic frequencies of the modulations were obtained by Fourier transformation of the three-pulse, spin-echo envelopes. In transition-state analogue complexes of creatine kinase (enzyme-MnIIADP-anion-creatine), superhyperfine interactions from the directly coordinated nitrogen of the thiocyanate ligand give envelope modulations. The source of the modulations was confirmed by measurements with the 14N and 15N forms of thiocyanate. On the other hand, the nitrogen of coordinated nitrate, which is two bonds removed from the paramagnetic center, does not produce detectable modulations. In spectra for Mn(II) concanavalin A, envelope modulations are detected due to the nitrogen of the coordinated histidine residue. Complexes prepared in 2H2O give strong signals due to weakly coupled 2H. For Mn(II)-doped single crystals of sodium pyrophosphate, signals are observed in the frequency domain spectra that are due to coupling from 31P. Phosphorus signals from the ADP ligand in complexes with creatine kinase show approximately the same coupling constant but have a much broader line width. |
