2D13C-13C MAS NMR correlation spectroscopy with mixing by true 1H spin diffusion reveals long-range intermolecular distance restraints in ultra high magnetic field

An improved 2D C-13-C-13 CP3 MAS NMR correlation experiment with mixing by true H-1 spin diffusion is presented. With CP3, correlations can be detected over a much longer range than with direct H-1-C-13 or C-13-C-13 dipolar recoupling. The experiment employs a H-1 spin diffusion mixing period tau(m) sandwiched between two cross-polarization periods. An optimized CP3 sequence for measuring polarization transfer on a length scale between 0.3 and 1.0 mn using short mixing times of 0.1 ms < τ(m) < 1 ms is presented. For such a short tau(m), cross talk from residual transverse magnetization of the donating nuclear species after a CP can be suppressed by extended phase cycling. The utility of the experiment for genuine structure determination is demonstrated using a self-aggregated Chl a/H2O sample. The number of intramolecular cross-peaks increases for longer mixing times and this obscures the intermolecular transfer events. Hence, the experiment will be useful for short mixing times only. For a short tau(m) = 0.1 ms, intermolecular correlations are detected between the ends of phytyl tails and ring carbons of neighboring Chl a molecules in the aggregate. In this way the model for the structure, with stacks of Chl a that are arranged back to back with interdigitating phytyl chains stretched between two bilayers, is validated. (C) 2002 Elsevier Science (USA).