MAS NMR structure refinement of uniformly 13C enriched chlorophyll a/water aggregates with 2D dipolar correlation spectroscopy

2D MAS dipolar correlation spectroscopy of uniformly 13C enriched chlorophyll a/water aggregates was performed using broadband RFDR to promote exchange of coherence through homonuclear dipolar couplings. It is shown experimentally that incorporation of TPPI and coherence pathway selection in the RFDR pulse scheme yields virtually pure 2D absorption lineshapes. From a series of 2D correlation spectra collected at two spinning speeds with different mixing times of ∼ 1 and ∼ 10 ms all 13C resonances of the chlorophyll a molecule were assigned. The spectra with the longer mixing times reveal through-space intermolecular polarization transfer. This demonstrates that structural information can be obtained from uniformly 13C enriched samples, which paves the way for a full structural characterization of amorphous solids.