THEORY OF DOUBLE-QUANTUM 2-DIMENSIONAL ELECTRON-SPIN-RESONANCE WITH APPLICATION TO DISTANCE MEASUREMENTS

A formulation is presented for calculating double quantum two dimensio nal electron spin resonance (DQ-2D ESR) spectra in the rigid limit tha t correspond to recent experimental DQ-2D ESR spectra obtained from a nitroxide biradical. The theory includes the dipolar interaction betwe en the nitroxide moieties as well as the fully asymmetric g and hyperf ine tensors and the angular geometry of the biradical. The effects of arbitrary purses (strong but not truly nonselective pulses) are includ ed by adapting the recently introduced split Hamiltonian theory for nu merical simulations. It is shown how arbitrary pulses in magnetic reso nance create ''forbidden'' coherence pathways, and their role in DQ-2D ESR is delineated. The high sensitivity of these DQ-2D ESR signals to the strength of the dipolar interaction is demonstrated and rationali zed in terms of the orientational selectivity of the ''forbidden'' pat hways. It is further shown that this selectivity also provides constra ints on the structural geometry (i.e., the orientations of the nitroxi de moieties) of the biradicals. The theory is applied to the recent do uble quantum modulation (DQM) experiment on an end-labeled poly-prolin e peptide biradical. A distance of 18.5 Angstrom between the ends is f ound for this biradical. A new two pulse double quantum experiment is proposed (by analogy to recent NMR experiments), and its feasibility f or the ESR case is theoretically explored. (C) 1997 American Institute of Physics.