Pectin–Cellulose
Interactions in the <i>Arabidopsis</i> Primary Cell Wall
from Two-Dimensional Magic-Angle-Spinning
Solid-State Nuclear Magnetic Resonance
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Abstract
The primary cell wall of higher plants consists of a
mixture of
polysaccharides whose spatial proximities and interactions with each
other are not well understood. We recently obtained the first two-dimensional
(2D) and three-dimensional high-resolution magic-angle-spinning <sup>13</sup>C solid-state nuclear magnetic resonance spectra of the uniformly <sup>13</sup>C-labeled primary cell wall of <i>Arabidopsis thaliana</i>, which allowed us to assign the majority of <sup>13</sup>C resonances
of the three major classes of polysaccharides: cellulose, hemicellulose,
and pectins. In this work, we measured the intensity buildup of <sup>13</sup>C–<sup>13</sup>C cross-peaks in a series of 2D <sup>13</sup>C correlation spectra to obtain semiquantitative information
about the spatial proximities between different polysaccharides. Comparison
of 2D spectra measured at different spin diffusion mixing times identified
intermolecular pectin–cellulose cross-peaks as well as interior
cellulose–surface cellulose cross-peaks. The intensity buildup
time constants are only modestly longer for cellulose–pectin
cross-peaks than for interior cellulose–surface cellulose cross-peaks,
indicating that pectins come into direct contact with the cellulose
microfibrils. Approximately 25–50% of the cellulose chains
exhibit close contact with pectins. The <sup>13</sup>C magnetization
of the wall polysaccharides is not fully equilibrated by 1.5 s, indicating
that pectins and cellulose are not homogeneously mixed on the molecular
level. We also assigned the <sup>13</sup>C signals of cell wall proteins,
identifying common residues such as Pro, Hyp, Tyr, and Ala. The chemical
shifts indicate significant coil and sheet conformations in these
structural proteins. Interestingly, few cross- peaks were observed
between the proteins and the polysaccharides. Taken together, these
data indicate that the three major types of polysaccharides in the
primary wall of <i>Arabidopsis</i> form a single cohesive
network, while structural proteins form a relatively separate domain