A high field NMR study of the products ensuing from konjak glucomannan C(6)-oxidation followed by enzymatic C(5)-epimerization.
Konjak glucomannan (KGM) is a water-soluble linear copolymer of (1-->4) linked beta-D-mannopyranosyl and beta-D-glucopyranosyl units. It has been selectively C6-oxidized by a 2,2,6,6-tetramethylpiperidin-1-oxy mediated reaction to obtain the corresponding uronan. Oxidized KGM has been treated with three different C-5 epimerases, AlgE4, AlgE6, and AlgE1, to obtain uronans with a various content of alpha-L-gulopyranuronate residues, namely, KGME4, KGME6, and KGME1. By use of 1D selective and 2D NMR techniques, a full assignment of the high field (600 MHz) NMR spectra of the purified native KGM and of the oxidized and epimerized derivatives has been obtained. Since in the anomeric region of the (1)H NMR spectrum of native KGM, diads sensitivity is present, the glucose-glucose, glucose-mannose, mannose-mannose, and mannose-glucose distribution has been obtained. In the (13)C spectrum of oxidized KGM, due to the presence of triad sensitivity on the C-4 resonance of glucuronic and mannuronic units, a better sequential investigation has been possible. As a result the average length of mannuronic blocks, N(M) is obtained. When AlgE4, AlgE6, and AlgE1 enzymes are used for the epimerization of oxidized KGM, the reaction products differ significantly both in the proportion and in the distribution of the mannuronic and guluronic residues. In epimerized KGM derivatives, a careful deconvolution of (1)H spectra allows the measurement of the degree of epimerization. In the case of KGME1 and KGME6, the average blocks length, N(G), of the guluronic blocks introduced in the polysaccharidic chain with the epimerization has also been calculated. Due to the shortness of mannuronic blocks in the oxidized KGM before the epimerization, N(G) in the epimerized compounds is also very low.
Crescenzi
Vittorio
V
Department of Chemistry, University of Rome La Sapienza, Rome, Italy.
Skjåk-Braek
Gudmund
G
Dentini
Mariella
M
Masci
Giancarlo
G
Bernalda
Maria Scala
MS
Risica
Daniela
D
Capitani
Donatella
D
Mannina
Luisa
L
Segre
Anna Laura
AL
D009682:Magnetic Resonance Spectroscopy; D008351:Mannans / Q000737:chemistry* / Q000378:metabolism*; D009844:Oligosaccharides / Q000032:analysis / Q000737:chemistry; D010084:Oxidation-Reduction; D019998:Racemases and Epimerases / Q000378:metabolism*; D013237:Stereoisomerism
D008351:Mannans; D009844:Oligosaccharides; C022901:(1-6)-alpha-glucomannan; D019998:Racemases and Epimerases