NMR data for RG-I isolated from Arabidopsis thaliana siliques

Pectin is generally divided into four distinct structural categories,
namely homogalacturonan, xylogalacturonan, rhamnogalacturonan I (RG-I),
and rhamnogalacturonan II. While much of the structural diversity of
homogalacturonan, xylogalacturonan, and rhamnogalacturonan II has been
elucidated, the structural features of RG-I are less well understood. In
this work, we employed multiple complementary analytical techniques to
present a detailed structural analysis of RG-I in the model system
Arabidopsis thaliana. Starting with highly purified RG-I from different
Arabidopsis tissues, we employed comparative linkage and NMR analysis
along with mass spectrometry analysis of enzymatically digested RG-I
oligosaccharides. Besides the presence of the canonical 5-arabinan,
4-galactan, 6-galactan, and arabino-galactan RG-I side chains of varying
lengths, we show that a large portion of the 6-galactan is terminated by
either 4-O-methyl β-GlcA residues or, to a smaller degree, β-GlcA that
lacks the Me-ether group. Importantly, O-acetylation of RG-I GalA residues
is a minor modification, while 10% of the backbone Rha residues are
3-O-acetylated, and most of the acetylated Rha additionally branches with
β-Gal side chain. Taken together, the combined results of these different
analytical techniques present the most comprehensive structural overview
of Arabidopsis thaliana RG-I to date.