Double quantum two-dimensional Fourier transform electron spin resonance: Distance measurements

The first double quantum two-dimensional Fourier transform electron spin resonance (2D-FT ESR) experiments are reported. Extension of 2D-FT ESR to enable detection of the ΔMs = ±2transition enhances the capability of ESR in studying structural properties (i.e. distances in bilabeled molecules). Å distance of 21 Å was found for a poly-proline peptide, spin labeled at both ends, in agreement with earlier measurements using fluorescence energy transfer.