Two-dimensional NMR spectroscopic studies of fenugreek (Trigonella foenum-graecum L.) galactomannan without chemical fragmentation

Fenugreek galactomannan, as compared to other gums particularly of guar and locust bean, is less exploited in the food industry. With regard to the structure of fenugreek galactomannan, though reports on galactose substitutional pattern are available, supporting data by NMR correlation studies are scanty. The present investigation deals with the observations made by 2D NMR spectroscopy of fenugreek galactomannan. Sequential extraction of fenugreek flour with cold water, hot water and alkali resulted in different fractions. Purification of alkali extracted polysaccharide B by copper complex precipitation and GPC (TSK gel) fetched two fractions, viz. Fra. 1 and Fra. 2 with a gal:man ratio of 1:1.04 and 1:1.12, respectively. Aspergillus niger beta -D-mannanase had no effect on the hydrolysis of Fra. 1 and Fra. 2. indicating high substitution and interference by galactose stubs. All the assignments of H-1- and C-13- signals could be achieved and verified through various H-1-H-1 and H-1-C-13 correlation experiments, viz. DQF-COSY, TOCSY and HSQC. HMBC experiments resulted in correlations of H-1(gal alpha)/C-6(man), H-4(man)/C-1(man) and H-6(man)/C-1(gal). The assignment of anomeric protons and confirmation of (1 --> 4) linked mannan back bone with (1 --> 6) attachment of gal clarified the structure [4)[alpha -gal-(1 --> 6)-beta -man-(1 -->](n). (C) 2001 Elsevier Science Ltd. All rights reserved.