DIVISIONO F IKDUSTRIAL AWD CELLULOSE CHEMISrRT, MCGILI.b-NIVERSIlY, CHEMISTRY DIVISION,P U L P AND PAPER RESEARCH INSTITUTE O F CANADA]
[ C o N r R I B U r I O N FROM rIIE
AND I I I B W O O U
The Constitution of a Hemicellulose from Milkweed (Asclepias syuiaca) Floss1 BY F.W.BARTHAND T. E. TIMELL RECEIVED JULY 7,1958 The general chemical composition of the floss and the stalk of the common milkweed (Asclepzas syuiaca, L.) has been determined. Partial hydrolysis of the floss gave galactose, glucose, mannose, xylose, galacturonic acid, 4-0-methyl-D-glucuacid)-D-xylopyranose and a triouronic acid. Alkaline extraction of ronic acid, 2-~-(4-~-methyl-~-glucop~ranosyluronic the floss yielded a hemicellulose composed of xylose and uronic acid residues. HydrolJ-sis of the fully methylated polysaccharide gave a mixture of 2-0- and 3-O-methyl-~-xylose,2,3-di-O-methyl-~-xylose, 2,3,4-tri-O-methyl-D-xplose and 2-~-(2,3,4-tri-~-methyl-~-glucopyranosy~uronic acid)-3-0-methyl-~-xylopyranose in a molar ratio of 0.8: 38.6: 1:2.3. The number-average degree of polymerization of the methylated hemicellulose was 97 and the corresponding value for the native polymer was 172. On the basis of these results it is suggested that the hemicellulose contains approximately 170 @-D-xylopyranose residues linked together by 1,4-glycosidic bonds and with, on the average, one branching point present per molecule. Every 14th anhydroxylose unit carries a single terminal side chain of 4-O-methyl-D-glucuronic acid attached by an a glycosidic bond to the 2-position of the xylose residues. The molecular-weight distribution contains one maximum.
Milkweeds are tall plants containing a milky tents between the two parts of the plant is considerjuice in all their parts which has given them their able. It had previously3 been shown that the name. The common milkweed (,4sclepias syriaca, weight-average degree of polymerization of the L.), which is native to eastern North America, is a cellulose present in the floss was 10,500. The corperennial and multiplies by seeds and creeping responding figure for the cellulose in the stalk was roots. The seeds have tufts of silky hair (the so- 9,300.11 called floss) and are enclosed by a pod. LThen the Partial acid hydrolysis of the floss gave the above latter opens, the seeds are spread by the wind. The neutral sugars in addition to a misture of uronic seed fibers consist of a single cell and have occa- acids, part of which was treated with methanolic sionally been used as a substitute for kapok. hydrogen chloride, reduced with lithium aluminum Previous investigations dealt with the molecular hydride1”13 and hydrolyzed to yield a mixture of properties of the cellulose component of the f l ~ s s . ~ galactose, .~ xylose and 4-0-methyl-~-glucosewhich This paper is concerned with the general chemical was resslved on a cocoanut charcoal column. The composition of the floss and the stalk and with the galactose was isolated and characterized by paper constitution of a hemicellulose isolated from the chromatography, the xylose crystallized and the 4~ - m e t h y l - ~ - g l u c owas s e identified through its crysformer. The chemical composition 01 the floss and the talline o ~ a z o n e . ~The ~ , main ~ ~ portion of the uronic stalk is presented in Table I. The difference in acids was partially resolved on a column of ion excellulose, uronic anhydride, acetyl and xylan con- change resin,15followed by resolution on a charcoal column to yield galacturonic acid, 4-O-methyl-DTABLE I glucuronic acid, 2-0-(4-O-methyl-a-~-glucopyranoCHEMICALCOMPOSITIOSOF THE FLOSSASD STALK OF THE syluronic acid)-D-xylopyranose and a triouronic COMMON hfILKWEEDa acid. The galacturonic acid was identified chroComponent Floss Stalk matographically after reduction to galactose. Rea-Cellulose4 39.6 52.6 duction of the ester glycoside of the 4-O-methyl-~Pentosan5 35.3 19.2 glucuronic acid with lithium aluminum hydride and 15.1 15.6 Lignine subsequent hydrolysis gave ~-O-methyl-D-glucose! 0.2 1.4 Ash’ characterized through its osazone. While this acid 6.1 3.7 Acetyl8 probably originated from the pentosan portion of Uronic anhydrideQ 5.6 10.2 the floss, the galacturonic acid was believed to 1.7 2.3 Galactan arise from pectic material. 40.2 53.0 Clucan The methoxyl content and equivalent weight of 3.8 1.7 Mannan the main uronic acid fraction corresponded to that Xi1 1.5 Araban of a nionomethylated aldobiouronic acid contain27.3 10.6 XylanLo ing a pentose and a hexuronic acid residue. Its infrared spectrum was identical to that of an authena All values in per cent. of extractive-free material. tic sample of 2-O-(&-O-methyl-a-D-glucopyranosyl(1) Paper presented before t h e Division of Cellulose Chemistry at uronic acid) -D-xylopyranose.’6 Reduction of the t h e 134th Meeting of t h e American Chemical Society in Chicago, Ill., methyl glycoside with lithium aluminum hydride, September, 1955. followed by hydrolysis, yielded I-O-methyl-D-glu(2) T . I. The frequency distribution contained one maximum. Summarizing the above evidence, a siniplified structure can now be proposed for the hemicellulose present in milkweed floss. This polysaccharide evidently contains approximately 170 p-D-xylopyrmose residues linked together by l,.l-glycosidic bonds and with, on the average, one branching point present per molecule through either or both of positions 2 and 3. Every fourteenth anhydroxylose unit carries a single, terminal side chain of 4O-nieth!-l-D-glucuroriic acid, attached by an cy-glycosidic bond to the 2-position of the xylose residues. The acetyl content of the floss is fairly high and from what is now known about similar hemicelluloses from wood?' 4 2 it appears probable that most of the hydroxyl groups in the native material arc esterified with acetic acid. The hemicellulose is polymolecular arid the lower D.P. limit is the same as that observed earlier for a methyl glucuronoxylaii from white birch,lgwhereas the average D.P. is somewhat lower The constitution of the hemicellulose of milkweed floss is similar, albeit not identical, to that of several x!-lan polysaccharides from other sources. Hemicelluloses containing branched chain molecules include those from American beech,43corn cob,44esparto grass4aand wheat s t r a ~ . ~ ~A, large ~ ~ . num~' ber of xylans have been found to contain single side chains of l-O-methyl-D-glucuronic acid, with one invariably linked a-1,2. In this respect, as well as in the absence of any L-arabofuranose residues, the present pol?-saccharide resemb!es inost closely the linear methyl glucuronoxylans present in many deciduous woods, such as Europwri beech, 3 4 yellow birch, white birch, white elm26or sugar The number of xylose residues per acid group in these hemicelluloses has been found to vary from 7 for elm to 11 for white birch. The corresponding figure for the present pol?-saccharide, namely 14,indicates the presence of less acid side chains. Kapok. which is also n seed hair, seems to contain a methyl glucuronovylan with twice as many 4-O-xnethyl-D-glucuronic acid residues?" AS the 1ic.niicellulose from the related milkweed floss.
Methylation of another sample of hemicellulose to a slightly lower methoxyl content (35%) and subsequent separation of the methyl derivatives obtained on irxthanolysis and hydrolysis indicated the same amount of non-reducing end-groups. The molecular weight, as tleterniined by osmometry, W:LS nlso exactly the same, thus corroborating the a b o \ ~results. ,\ttempts t o determine the molecular tveight of the native polysaccharide by osmotic pressure measurements with the acetyl d e r i v a t i ~ failed e ~ ~ ~be~~ 1 z cause of the incomplete solubility of the latter. The z intrinsic viscosity of the hemicellulose was estiN + y - z = 170 mated in cupriethylenediamine" as 0.812 dl./g. r 1 Vsing the relationship developed earlier for a simj- P - ~ - S y l p1 - [ X - n - X y l p 11~-313-~-X>-lp 1 -44p-~-Xylp 1-31 I3 2 or 3 1ar polysaccharide, this would correspond to a Pn value 01 172. For an estiniation of its polymolecularity, the hemicellulose was fractionally precipi4 - 0 - r \ I e - ~ ~ - ( : J . pn tated from an alkaline solution by gradual addition The similarity in cliernic:il coniposition between inall, E. I,. Hirat a n d I
