Such a 3-D space-filling organization of the aggregated Chl a from MAS NMR would match existing models inferred from electron microscopy and low-resolution X-ray powder diffraction, while a micellar model based on neutron diffraction and antiparallel stacking observed in solution can be discarded.Īn improved 2D 13C– 13C CP 3 MAS NMR correlation experiment with mixing by true 1H spin diffusion is presented. Probably the only way this can be realized with the sheets is by forming bilayers with interpenetration of elongated tails. In line with the microcrystalline order observed for the rings, the long T 1's, and absence of conformational shifts for the 13C in the phytyl tails, it is proposed that the Chl a form a rigid 3-D space-filling structure. Evidence is found for the presence of neutral structural water molecules forming a hydrogen-bonded network to stabilize Chl a sheets. A doubling of a small subset of the carbon resonances, in the 7-methyl region of the molecule, provides evidence for two marginally different well-defined molecular environments. The shift constraints and long-range 1H– 13C intermolecular correlations reveal a 2-D stacking homologous to the molecular arrangement in crystalline solid ethyl-chlorophyllide a. Second, proton chemical shifts are obtained from 1H– 13C heteronuclear dipolar correlation spectroscopy in high magnetic field. ![]() First, high-field (14.1 T) 2-D MAS NMR homonuclear ( 13C– 13C) dipolar correlation spectra provide a complete assignment of the carbon chemical shifts. Long-range 1H– 13C correlations are used in conjunction with carbon and proton aggregation shifts to establish the stacking of the chlorophyll a (Chl a) molecules. ![]() To probe intermolecular contacts, d max can be set to ∼4.2 Å by choosing an LG-CP contact time of 2 ms. An effective maximum transfer range d max can be determined experimentally from the detection of a gradually decreasing series of intramolecular correlations with the 13C along the molecular skeleton. Magic angle spinning (MAS) NMR with Lee–Goldburg cross-polarization (LG-CP) is used to promote long-range heteronuclear transfer of magnetization and to constrain a structural model for uniformly labeled chlorophyll a/H 2O.
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