MAGNETIZATION EXCHANGE NETWORK EDITING - MATHEMATICAL PRINCIPLES AND EXPERIMENTAL DEMONSTRATION

We have examined analytically the effects of different NMR magnetizati on exchange network editing (MENE) procedures, starting from equilibri um or nonequilibrium conditions, on generalized systems that undergo m agnetization exchange (cross-relaxation and/or chemical exchange). Lar ger errors in derived exchange rate constants are found in experiments that start from nonequilibrium states than in those that start from e quilibrium. On the other hand, editing experiments that decompose the dynamic matrix into two or more block diagonal submatrices generally y ield more accurate exchange rate constants. Theoretical arguments, bac ked up by experimental results, demonstrate that MENE experiments lead to increased rates of relaxation for the edited cross and diagonal pe aks. We describe a new class of heteronuclear editing experiment in wh ich the system starts from a nonequilibrium state and is edited during the mixing period. In an experimental application of this approach to the spectral analysis of a small protein, recombinant human ubiquitin (M(r) 8565) labeled uniformly with N-15 and C-13, We demonstrate how the cross-relaxation network of the protein can be decomposed simultan eously into subnetworks of N-15-bound protons, aliphatic C-13-bound pr otons, aromatic C-13-bound protons, and (C-12/N-14/O)-bound protons, S uch a decomposition permits the measurement of slower magnetization ex change rates, including those that are masked in conventional cross-re laxation experiments.