Minor glucans from Laetiporus sulphureus basidiocarps

Abstract

We have continued research on basidiocarps of Laetiporus sulphureus (Bull.:Fr.) Murr grown by implantation under Irkutsk Oblast conditions [1–3]. Alkaline-soluble polysaccharides (ASPS) of this species were found to contain glucans, the dominant component of which was laetiglucan I, a linear -(1 3)-glucan [3]. Herein we study the structures of minor glucans from L. sulphureus. Basidiocarps of L. sulphureus (80 g) were processed as before in order to isolate ASPS [3]. Separation of the ASPS complex produced fractions LSA-1a (11.62 g, laetiglucan I), LSA-1b (1.80 g), and LSA-2 (0.99 g) [3]. LSA-1b contained two components, LSA-1b-1 and LSA-1b-2, which were isolated by preparative gel chromatography (Sephadex G-200, 3 80 cm, NaOH eluent, 0.01 M). LSA-1b (1.5 g) afforded LSA-1b-1 (1.06 g) and LSA-1b-2 (180 mg). LSA-1b-1. MW 180 kDa. IR and 13C NMR spectra and methylation agreed with those of laetiglucan I [3]. LSA-1b-2. MW 150 kDa, [ ]D –21° (c 0.25, 0.1 M NaOH). IR spectrum ( , cm –1): 3289, 2933, 1645, 1342, 1140, 1021, 922, 890 ( -bond), 819. Only glucose was observed after total hydrolysis. Periodate oxidation consumed 0.266 mol of IO4 – per anhydro unit and released 0.131 mol of HCOOH. The hydrolysate after reduction and Smith degradation of the polysaccharide oxidized by periodate contained glycerine and glucose in a 1:6.7 mole ratio. Glycerine was released from terminal glucose units. The presence of glucose indicated that (1 3)-bonds stable to periodate were present. Hydrolysis of the acetylated derivative of LSA-1b-2 oxidized by CrO3 did not produce hexoses. This was possible if its anomeric centers had the -configuration. Methylation of LSA-1b-2 produced a hydrolysate with permethylate 2,3,4,6-tetra-O-Me-Glcp; 2,4,6-tri-O-Me-Glcp; and 2,4-di-O-Me-Glcp in a 1:5.69:1.02 ratio. This indicated that the structure of LSA-1b-2 was a polysaccharide, the main chain of which consisted of (1 3)-bound glucose units substituted 14.7% at the C-6 position with single glucose units. 13C NMR data conformed that LSA-1b-2 was an O-6 branched -(1 3)-glucan (Table 1) [4–6]. LSA-2 was demineralized and deproteinated before being studied. This produced fraction LSA-2 (82% yield per LSA-2 mass). Low-molecular-weight impurities were removed from LSA-2 over a column of Molselect G-25 (3 20 cm, water eluent) and then over Sephadex G-100 (2 60 cm, water eluent). The resulting LSA-2 (534 mg) was a homogeneous polysaccharide. LSA-2 . MW 110 kDa, [ ]D +211° (c 0.18, 0.1 M NaOH). IR spectrum ( , cm –1): 3447, 2932, 1382, 1274, 1155, 1048, 1018, 917, 841 ( -bond), 763, 611. The polymer consisted only of glucose and did not undergo periodate oxidation. The hydrolysate after oxidation of its peracetate by CrO3 contained glucose. The hydrolysate after methylation of LSA-2 contained 2,4,6-tri-O-Me-Glcp as the main product and traces of 2,3,4,6-tetra-O-Me-Glcp. The 13C NMR spectrum of LSA-2 showed six resonances. The position of the resonance for C-1 (100.51 ppm) corresponded with the -configuration of the glucoses whereas the shift of the resonance of C-3 to weak field was due to glycosylation of this position, indicating a (1 3)-type bond (Table 1). The data enable LSA-2 to be identified as a linear -(1 3)-glucan [7, 8]. The studied polysaccharides LSA-1b-2 and LSA-2 were called laetiglucans II and III and were the type of glucans first isolated from L. sulphureus basidiocarps.