Determination of absolute interproton distances by variable temperature two-dimensional cross-relaxation spectroscopy

We propose to use the temperature dependence of laboratory frame cross-relaxation rate to deduce the absolute value of interproton distances in solutions. For rigid molecules undergoing isotropic random motion, the cross relaxation rate becomes zero at a characteristic temperature T₀, while at a higher temperature T_max it reaches its maximum. At T_max the correlation time is precisely defined and the interproton distance can be calculated directly from the cross-relaxation rate. The applicability of the method is tested on a small model compound, cyclo-(L-Pro-Gly). For the geminal proton pair of glycine, the measured distance of 1.89 ± 0.011 Å disagrees with the model value beyond the experimental error, indicating that the accepted model parameters need to be reevaluated.