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Nishihara S, Angata K, Aoki-Kinoshita KF, et al., editors. Glycoscience Protocols (GlycoPODv2) [Internet]. Saitama (JP): Japan Consortium for Glycobiology and Glycotechnology; 2021-.
Introduction
Sulfate groups linked to hydroxyl groups (O-sulfate groups) and amino groups (N-sulfate group) of sugar residues are essential for the activities of sulfated glycans. The removal of these sulfate groups (desulfation) is a powerful tool for analysis and modification of functions of these glycans. Among the chemical desulfation methods reported so far (1,2), the so-called solvolysis has been widely applied because of its convenience and relatively mild conditions, whereas pyridinium salt of sulfated polysaccharide is incubated in dimethylsulfoxide containing water or methanol (3). Since the solvolysis rate of N-sulfate is much greater than that of O-sulfate, specific N-desulfation of heparin/heparan sulfate can be achieved solvolytically at room temperature (ranged from 15°C to 30°C) (4). Among the O-sulfate groups, 6-O-sulfate (an ester of primary alcohol) is more reactive than other O-sulfates (esters of secondary alcohol). Preferentially 6-O-desulfated material is obtained by solvolysis followed by restoring N-sulfate (selective N-resulfation). Nevertheless, O-desulfation of positions other than O-6 occurs at a non-negligible level when intending to complete 6-O-desulfation, whereas 6-O-desulfation is incomplete under conditions where other sulfate groups remain unaffected. Additionally, the solvolytic conditions may cause cleavage of the glycosyl bonds due to pyridinium ion and water or methanol in the system, especially when applied to acid-labile polysaccharides.
Several silylating reagents increase the rate of desulfation when added to a solvolytic system using pyridine as the solvent without water and methanol (5). Among them, N,O-bis(trimethylsilyl)acetamide (BSA) and N-methyl-N-(trimethylsilyl)trifuluoroacetamide (MSTFA) extremely accelerate 6-O-desulfation in a rate comparable with enzymatic reactions (6), whereas they completely suppress O-desulfation at other positions and N-desulfation (5–9). Although both reagents are quite effective for 6-O-desulfation, MSTFA is recommended for the reaction at a high temperature (110°C) (Figure 1). Using these reagents, trimethylsilyl group is introduced to both free and O-sulfated hydroxyl groups and then removed by water during the subsequent dialysis or, if necessary, further by ammonium fluoride-mediated desilylation (10) to recover free hydroxyl groups (see Scheme below). In this system, the silylating reagent also reacts with water and removes residual moisture to prevent hydrolytic cleavage of the polysaccharide. Instead of the trimethylsilylating reagents, N-tert-butyldimethylsilyl-N-methyltrifluoroadetamide, a tert-butyldimethylsilylating reagent structurally related to MSTFA, is also referred to as a complete 6-O-desulfation reagent for heparin in a recent review (11).
In the case of glycosaminoglycans, the degree of the 6-O-desulfation can be monitored by enzymic digestion using heparinase (heparin lyase) mixture (for heparin/heparan sulfate) or chondroitinase mixture (for chondroitin sulfate/dermatan sulfate) followed by chromatography of the resulting unsaturated disaccharides carrying free or sulfated O-6 at the aminosugar residue (7,9). As for the polysaccharides other than glycosaminoglycan, the rate of the 6-O-desulfation is semiquantitatively estimated from the 13C-nuclear magnetic resonance (NMR) spectrum (8,9) or methylation analysis.
Other silylating reagents, such as 4-trimethylsilyloxypent-3-en-2-one (TPN) and chlorotrimethylsilane (CTMS or trimethylchlorosilane), promote N- and O-desulfation at any position (5). Nonspecific desulfation alternative to solvolysis can be performed by those silylating reagents instead of BSA or MSTFA at 80°C (12). The optimized conditions for the highest degree of desulfation and the lowest depolymerization using CTMS are established for algal polysaccharides (13).
Protocol
This chapter describes the method for complete 6-O-desulfation of heparin, the polysaccharide quite resistant to desulfation. The process comprises three steps, which are as follows:
- 1.
Preparation of pyridinium salt of sulfated polysaccharide (relevant to Step 1 in the Methods).
- 2.
Regioselective 6-O-desulfation of the pyridinium salt of the polysaccharide using MSTFA (Step 2).
- 3.
Removal of residual silyl groups survived dialysis after the desulfation step (Step 3).
When treating a smaller amount of polysaccharides (<10 mg), use a larger proportion of solvents and reagents (twice or more) for handling. Desulfation at other positions milder than the conventional solvolytic method can be performed using CTMS or TPN instead of MSTFA.
Materials
- 1.
Amberlite IR-120B (Rohm and Haas, PA, H+ form converted from Na+ form), AG 50W × 8 (Bio-Rad CA, available in H+ form) (Note 1), or their pyridinium form (converted from H+ form) (Note 2)
- 2.
Pyridine (reagent or analytical grade) (Note 3)
- 3.
N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) or N,O-bis(trimethylsilyl)acetamide (BSA) (Sigma-Aldrich, MO, or Supelco (a branch of Sigma-Aldrich) distributed by Merck, Germany, or also manufactured by Wako Pure Chemical and Tokyo Chemical Industry, Japan) (Note 4)
- 4.
Methanol (reagent or analytical grade)
- 5.
Sodium hydroxide (reagent grade)
- 6.
Ammonium fluoride (reagent grade)
Instrument
- 1.
Dialysis membrane UC36-32-100 (Viscase, IL) or equivalent (Molecular weight cut off 12000–14000)
- 2.
Glass filter funnel
- 3.
Oil bath or heating mantle equipped with a magnetic stirrer and a stirrer bar
- 4.
Freeze-dry equipment
- 5.
Methanol-durable plastic tube or bottle for desilylation
Methods
- 1.
Protocol for the preparation of pyridinium salt of sulfated polysaccharide (Notes 1 and 2).
- a.
Dissolve 200 mg of sulfated polysaccharide in water (20 mL or more when the solution is too viscous to flow) and cool below 4°C.
- b.
Apply the solution to the H+ form of Amberlite IR120 column (wet volume of ~15 mL, 2 × 5 cm) or pyridinium form of Amberlite IR120 column (wet volume of ~30 mL, 2 × 10 cm or 3 × 5 cm).
- c.
Elute the column with water (~50–100 mL).
- d.
Neutralize the effluent with pyridine (<pH 6). (Skip this step when ion exchanger of pyridinium form is applied).
- e.
Freeze-dry the effluent.
- 2.
Protocol for complete 6-O-desulfation of heparin pyridinium salt (Note 5).
CAUTION: Use sufficiently dried glassware.- a.
Soak the pyridinium salt of heparin (~200 mg) obtained from protocol 1 in dry pyridine (20 mL).
- b.
Add MSTFA (4 mL) (Note 4).
- c.
Heat and stir at 110°C for 2 h. (Note 4).
- d.
Pour the reaction mixture into crushed ice (20 g) and add methanol (5 mL).
- e.
Dialyze against water.
- f.
Add several drops of 1 M NaOH to <pH 9 (Note 6).
- g.
Dialyze against water.
- h.
Freeze-dry the dialyzed solution.
- 3.
Protocol for the removal of the residual trimethylsilyl group in the desulfated material (Note 7)
- a.
Soak the freeze-dried material obtained from protocol 2 in 50 mL of 0.5 M methanolic ammonium fluoride.
- b.
Incubate the mixture at 50°C for 3 h in a plastic tube or bottle (Note 8) equipped with an open tube at the top.
- c.
Filter the mixture with a glass filter funnel immediately and wash the precipitate with methanol thrice.
- d.
Dissolve the precipitate in water and add several drops of 1 M NaOH to <pH 9.
- e.
Dialyze against water.
- f.
Freeze-dry the dialyzed solution.
Notes
- 1.
Other strong cation exchangers carrying sulfonate groups, such as Dowex 50W × 8 can also be applied, whereas weak cation exchangers containing carboxymethyl or phosphate group are not suitable. Since Amberite IR 120B is distributed in Na+ form, the newly purchased resin should be washed slowly with at least three volumes of 1 M HCl and then three volumes of water to convert into H+ form. Although AG 50W × 8 are available in H+ form, the colored substance should be removed by washing with water just before use.
- 2.
For especially acid-labile samples, the use of the pyridinium form of the ion exchanger is recommended, since the free acid form of sulfated polysaccharides eluted from the cation exchanger of H+ form is highly acidic. In this case, a larger amount (twice or more) of the exchanger may be needed. The pyridinium form of the ion exchanger is prepared from the H+ form (30 mL in column) by neutralizing (eluting) with 10% pyridine (50 mL) followed by washing with water.
- 3.
To remove and avoid moisture, add ~10 g of KOH (or NaOH)/500 mL directly to the freshly opened bottle of pyridine and keep it for more than a day at room temperature before use.
- 4.
In several cases, the reaction with N,O-bis(trimethylsilyl)acetamide (BSA) at 80°C is sufficient for complete 6-O-desulfation. Nevertheless, some polysaccharides, such as heparin and highly sulfated algal polysaccharide, are resistant to desulfation and require higher reaction temperature, which, however, causes side reactions (8). Desulfation rate by MSTFA is similar to that by BSA, whereas MSTFA does not cause the side reaction even at 110°C required for the complete 6-O-desulfation of heparin (8,9). Both MSTFA and BSA should be free from other silylating reagents, such as CTMS and trimethylsilyl imidazole, which may cause desulfation of other positions (5).
- 5.
Degree of 6-O-desulfation can be controlled by reaction temperature or reaction time, but the amount of the silylating reagent should be conserved. In this case, the lowered reaction temperature and the shortened reaction time may have different selectivity to the 6-O-sulfate groups to give different NMR spectral patterns (unpublished result).
- 6.
Step 2f in pyridinium salt conversion into sodium salt is required when the product contains sulfate group unreacted or that linked to other than O-6. Without these steps, degradation due to autohydrolysis may occur during storage.
- 7.
This protocol is required when the dialysate from desulfation (Step 2g) is quite turbid or contains insoluble material due to the trimethylsilyl group surviving the dialysis.
- 8.
Use a methanol-durable plastic container since the fluorine-containing compound is corrosive to glassware.
References
- 1.
- Usov AI. Polysaccharides of the red algae. Adv Carbohyr Chem Biochem. 2011;65:115–217. [PubMed: 21763512] [CrossRef]
- 2.
- Takano R. Desulfation of sulfated carbohydrate. Trends Glycosci Glycotechn TIGG. 2002;14:343–351. [CrossRef]
- 3.
- Nagasawa K, Inoue Y, Kamata T. Solvolytic desulfation of glycosaminoglycuronan sulfates with dimethyl sulfoxide containing water or methanol. Carbohydr Res. 1977;58:47–55. [PubMed: 144018] [CrossRef]
- 4.
- Inoue Y, Nagasawa K. Selective N-desulfation of heparin with dimethyl sulfoxide containing water or methanol. Carbohydr Res. 1976;46:87–95. [PubMed: 1248016] [CrossRef]
- 5.
- Takano R, Kanda T, Hayashi K, Yoshida K, Hara S. Desulfation of sulfated carbohydrates mediated by silylating reagents. J Carbohydr Chem. 1995;14:885–888. [CrossRef]
- 6.
- Takano R, Matsuo M, Kamei-Hayashi K, Hara S, Hirase S. A novel regioselective desulfation method specific to carbohydrate 6-sulfate using silylating reagent. Biosci Biotech Biochem. 1992;56:1577–1580. [CrossRef]
- 7.
- Matsuo M, Takano R, Kamei-Hayashi K, Hara S. A novel regioselective desulfation of polysaccharide sulfates: Specific 6-O-desulfation with N,O-bis(trimethylsilyl)acetamide. Carbohydr Res. 1993;241:209–215. [PubMed: 8472253] [CrossRef]
- 8.
- Takano R, Ye Z, Ta T-V, Hayashi K, Kariya Y, Hara S. Specific 6-O-desulfation of heparin. Carbohydr Lett. 1998;3:71-77 (Harwood Academic Publishers, ISSN 10735070).
- 9.
- Kariya Y, Kyogashima M, Suzuki K, Isomura T, Sakamoto T, Horie K, Ishihara M, Takano R, Kamei K, Hara S. Preparation of completely 6-O-desulfated heparin and its ability to enhance activity of basic fibroblast growth factor. J Biol Chem. 2000;275(34):25949–58. [PubMed: 10837484] [CrossRef]
- 10.
- Zhang W, Robbins MJ. Removal of silyl protecting groups from hydroxyl functions with ammonium fluoride in methanol. Tetrahedron Lett. 1992;33:1177–1180. [CrossRef]
- 11.
- Palhares LCGF, London JA, Kozlowski AM, Esposito E, Chavante SF, Ni M, Yates A. Chemical modification of glycosaminoglycan polysaccharides. Molecules. 2021;26:5211–5233. [PMC free article: PMC8434129] [PubMed: 34500644] [CrossRef]
- 12.
- Thanh TTT, Yuguchi Y, Mimura M, Yasunaga H, Takano R, Urakawa H, Kajiwara K. Molecular characteristics and gelling properties of the carrageenan family, 1. Preparation of novel carrageenans and their dilute solution properties. Macromol Chem Phys. 2002;203:15–23. [CrossRef]
- 13.
- Kolender AA, Matulewicz MC. Desulfation of sulfated galactans with chlorotrimethylsilane. Characterization of beta-carrageenan by 1H NMR spectroscopy. Carbohydr Res. 2004;339:1619–1629. [PubMed: 15183736] [CrossRef]
Footnotes
The authors declare no competing or financial interests.
Figures
Figure 1:
Scheme for silylating reagent-mediated desulfation.