Table of Contents:
- Oxidation
- Acetylation and Acylation
The derivatives curdlan with phosphoric groups (mono- or dibasic groups) were synthesized by reaction of curdlan with phosphorous acid in molten urea (Suflet et al. 201 la) or phosphoric acid in presence of urea/DMSO (Chen et al. 2009).
The structure of monobasic curdlan phosphate (PCurd) was investigated by FTIR and NMR spectroscopy and the substitution degree was calculated from
FIGURE 10.5 FTIR spectra of monobasic curdlan phosphate compared with native curdlan.
electrochemical titration. In Figure Ю.5 the PCurd FTIR spectrum are presented, compared with native curdlan. In the FTIR spectrum of PCurd several new bands appeared: a band at 24 Ю cm-1 corresponding to the P-FI bond, another one at 1216 cm4 corresponding to the P-0 bond, a shoulder at 1054 cm-1 attributable to the P-OH bond, and a band at 836 cnr1 corresponding to the P-O-C bonds (Pretsch et al. 2010; Suflet et al. 2011). The NMR data provide evidence for the formation of phosphorylated curdlan. In the 'H NMR spectrum (Figure 10.6) of PCurd, two peaks at 6.17 and 7.84 ppm are observed which correspond to a doublet with a coupling constant JH_P of 668 Hz. This value is characteristic for the P-H bond.
Fully water-soluble PCurd with M„. of 178,000 g mol-1 was obtained with a DS up to 1, with a dissociation constant pK0 = 2.79-2.81 and intrinsic viscosity [p] = 81.93 and 0.69 dL g_1 in pure water, and at infinite ionic strength, respectively (Suflet et al. 2011).
The dibasic curdlan phosphate was synthetized by reaction with phosphorus acid in DMSO and urea. This derivative with only 0.056-0.075 substitution degree exhibit antitumour activities in vitro and relatively strong inhibition ratios against Sarcoma 180 (S-180) tumour cells in vivo (Chen et al. 2009).
FIGURE 10.6 'H NMR spectrum of monobasic curdlan phosphate in D20 (Suflet et al. 2011).
Regioselective oxidation of C6 primary hydroxyls from AGU units of polysaccharides was performed using the 2,2,6,6-tetramethylpiperidine- 1-oxyl radical (TEMPO) system to obtain l,3-(3-polyglucoglucuronic acid. This reaction opened a new horizon for polysaccharide chemistry by improving their viscosity and solubility in water, thus expanding applications in food, cosmetic, textile, and even medical and pharmaceutical fields. Any w'ater-insoluble polysaccharide like chitins, regenerated celluloses become water-soluble by the TEMPO-mediated oxidation through partial or complete conversion of the C6 primary hydroxyls to carboxylate groups. The TEMPO-mediated oxidation system was applied to (3-glucans in order to prepare water-soluble (1 ->3)-(3-linked polyglucuronic acids. Tamura and co-workers (Tamura et al. 2009; Tang et al. 2018) obtained an oxidized curdlan derivative using the TEMPO/NaBr/NaCIO system at room temperature and basic medium (pH = 10). Unfortunately, during the oxidation process there is a significant depolymerization so a high degradation of the polysaccharide chain of the curdlan from 6,790 to 86.
In 2010, Tamura et al reported the regioselective oxidation of primary OH groups of curdlan to carboxylate groups with lower levels of depolymerization using a 4-acetamido-TEMPO/NaClO/NaClO, system in water at pH 4.7 (Tamura et al. 2010). In Table 10.4 the main properties of oxidized curdlan by the new TEMPO- mediated system are presented, compared with TEMPO/NaBr/NaCIO.
TABLE 10.4
Main Characteristics of the Oxidized Curdlan
Sample | pH | M. | Solubility in Water | DS | Crystallinity Index | References |
Curdlan |
| insoluble | - | 0.28 | Tamura et al. (2009) | |
Oxidized Curdlan by: TEMPO/NaCIO/ NaCIO | 10 | 17.000(86) | soluble | 0.28-1.00 | 0.00 | Tamura et al. (2009) |
4-acetamide- TEMPO /NaCIO/NaCIO, | 4.7 |
| soluble | 0.95 | Tamura et al. (2010); Watanabe et al. (2014) |
Similar to CMCurd, oxidized curdlan by TEMPO system can be used as both reducing and stabilizing agents for the green synthesis of silver (Yan et al. 2013) and gold (Yan et al. 2015a) nanoparticles or drug retention system (Yan et al. 2015b).
The 4-acetamido-TEMPO-oxidized curdlan was hydrophobically modified by reaction with deoxycholic acid to attain novel amphiphilic curdlan derivatives for the preparation of nano-carriers for antitumour drug like doxorubicin (Yan et al. 2015b).
Acetylation/acylation of polysaccharides is the introduction of acetyl (-CO-CH,) acyl (-СО-alkyl; -СО-aryl) groups in the polysaccharide molecules. A series of ester derivatives of curdlan with varying alkyl chain lengths (C2-C12) were synthesized by the heterogeneous reaction using trifluoroacetic anhydride (Figure Ю.7) (Marubayashi
FIGURE 10.7 Syntheses of curdlan esters. Each acyl group is labeled with the corresponding carbon number (Marubayashi et al. 2014).
et al. 2014). The reactions yield was relatively high (>70%). The curdlan esters having two up to six carbon atoms in the alkyl chain are in a crystalline form and those with eight, ten, and twelve atoms are amorphous polymers.
The glass transition temperature (T) was evidenced only for derivatives with C2-C4 alkyl radicals (171°C for CDAc (C2), 170°C for CDPr (C3). and 74°C CDBu). Also, CDAc (C2) and CDPr (C3) possessed the melting temperature (Tm) higher than 200°C (287 and 213°C, respectively). The longer ester groups gave the lower Tg (170 -* 50°C) and Tm (290 -* 170°C) (Marubayashi et al. 2014). These results showed that the properties of curdlan esters can be adjusted by changing the length of the alkyl ester groups. Moreover, these esters with a higher thermal stability compared to the native curdlan can be used as thermoplastic materials.
Acetylated derivatives of curdlan were also synthesized in alkaline medium w'ith different amounts of acetylated reagent (acetic anhydrides), in order to prepare derivatives with variable DS (0.71-1.04) (Chen et al. 2014). These derivatives exhibited a stronger antioxidant abilities on scavenging DPPH (l,l-diphenyl-2-picrylhydrazyl) radical, and inhibitory effects in P-carotene-linoleic acid systems compared with the native polysaccharide although the molecular weight of these derivatives decreased due to the slight degradation of the polysaccharide chain during the acetylation reaction.
Two kinds of regioselective substituted curdlan hetero esters, 2.4-di-O-ace- tyl-6-O-propionyl-curdlan (CD24Ac6Pr) and 2,4-di-0-propionyl-6-0-acetyl-curd- lan (CD24Pr6Ac) were also synthesized (Chien et al. 2017; Chien and Iwata 2018). The reactions occurring by protecting the C6 primary hydroxyl group were followed by the acylation of the secondary hydroxyl groups at C2 and C4. The position of ester groups on secondary hydroxyl group plays a decisive role in the melting behaviour and crystal structure of these curdlan esters. Also, the mechanical properties of curdlan esters are affected by the substitution position of curdlan esters but can be controlled by the adjustment on its molecular structures.
