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Massa, A. Arabinogalactan (AG) and Hyaluronic Acid (HA). Encyclopedia. Available online: (accessed on 29 November 2023).
Massa A. Arabinogalactan (AG) and Hyaluronic Acid (HA). Encyclopedia. Available at: Accessed November 29, 2023.
Massa, Antonio. "Arabinogalactan (AG) and Hyaluronic Acid (HA)" Encyclopedia, (accessed November 29, 2023).
Massa, A.(2021, December 03). Arabinogalactan (AG) and Hyaluronic Acid (HA). In Encyclopedia.
Massa, Antonio. "Arabinogalactan (AG) and Hyaluronic Acid (HA)." Encyclopedia. Web. 03 December, 2021.
Arabinogalactan (AG) and Hyaluronic Acid (HA)

The properties of mixtures of two polysaccharides, arabinogalactan (AG) and hyaluronic acid (HA), were investigated in solution by the measurement of diffusion coefficients D of water protons by DOSY (Diffusion Ordered SpectroscopY), by the determination of viscosity and by the investigation of the affinity of a small molecule molecular probe versus AG/HA mixtures in the presence of bovine submaxillary mucin (BSM) by 1HNMR spectroscopy.

polysaccharides hyaluronic acid-arabinogalactan mixtures diffusion ordered spectroscopY (DOSY)

1. Introduction

Hydrogels formed by chemically cross-linked hyaluronic acid (HA) are widely used in the formulation of artificial tears for the treatment of dry eye syndrome [1][2][3][4][5][6][7]. Natural HA is a linear anionic polysaccharide consisting of alternating units of N-acetyl-D-glucosamine and sodium-D-glucuronate groups (Figure 1). With a molecular weight that can reach several millions of Dalton, HA is involved in numerous biological functions [8][9][10][11][12]. It is soluble in water and has a high degree of functionalization and charge density. Its solutions show high viscosity; in solution, it usually arranges in a 3D structure characterized by intramolecular hydrogen bonding [13][14]. This peculiarity also drives its physical–chemical interactions with other molecules. All these properties, as well as its biodegradability and immune neutrality, make HA an optimal biomaterial for wound healing applications and tissue engineering [14][15], in particular, in the regeneration of cartilage [16] and teeth structure [17].
Figure 1. Repetitive unit of hyaluronic acid (HA) and a structural fragment of arabinogalactan (AG).
In order to overcome the issues related to chemical cross-linking and modifications of natural HA [5][6][7][8], the development of new formulations, formed by mixtures of different polysaccharides, has been proposed [18][19]. In particular, enhanced mucoadhesive properties have been reported for mixtures of (HA) and tamarind-seed polysaccharide (TSP), a cellulose-like polysaccharide with a high degree of branched glycosyl substitution [18][19]. This mixture is a candidate for new formulations of artificial eye drops. A new artificial eye drop formed by the combination of hyaluronic acid (HA) with another polysaccharide, arabinogalactan (AG), has been recently proposed by Silvani and co-workers [20]. Notably, this mixture synergistically decreases the xanthine oxidoreductase (XOR) activity, inhibiting UA (uric acid) and ROS (reactive oxygen species) formation, and therefore it may contribute to the treatment of the dry eye syndrome, reducing irritation and related pathological conditions. AG is a natural polysaccharide formed of arabinose and galactose in a ratio of 1:6 and a molecular weight ranging between 10,000 and 120,000 Da [21]. AG is mostly extracted from the Larch tree (Larix occidentalis L. decidua) but it is abundant in a large variety of plants. Due to the broad array of species, the available AGs show a very wide range of biological properties and documented activities such as the improvement of vascular permeability [22], the support of digestive health by improving intestinal microflora [23][24][25], the enhancement of the immune function [21][22] and a significant in vitro stimulation of dermal fibroblast activity and proliferation [26]. Besides all these properties, AG is also extensively used for treating skin burns and wound-healing in middle and South America [26]. The AG wound-healing property is probably due to the combination of its anti-inflammatory [27], cicatrizing [28] and antimicrobial effects [29]. AG has been approved by the FDA for human consumption in large quantities [30], thus it represents an effective natural alternative to be used for both wound-healing and the reduction of ROS in different matrixes. AG has a high morphological freedom and it has flexible branches with exposed hydroxyl groups (lateral groups) (Figure 1); this peculiarity may play a key role in the interaction with other molecules and polysaccharides.
Nevertheless, to our knowledge, articles reporting physical–chemical properties of mixtures of AG and HA are still lacking. In this context, as part of our interest in the analysis of biomolecules [31], the aim of the present study is the investigation of mixtures of arabinogalactan (AG) and hyaluronic acid (HA) by viscosity measurements and NMR spectroscopy. This investigation can be very useful in highlighting new properties for the development of new formulations of the two polysaccharides for pharmaceuticals and cosmetic applications. The determination of possible synergic effects between different polysaccharides in solution is often challenging and there is a need for simple and reliable methods that can be used to highlight interactions at molecular level. For this purpose, solution rheology and Nuclear Magnetic Resonance (NMR) are particularly useful since the analysis is very simple and reliable, while no sample derivatization or pre-treatment is required [7][12][18][19]. As water is the main component of polysaccharide-based eye drops, studying the behavior of solutions at macroscopic and microscopic levels might show correlations between different polysaccharides, or provide information on their physicochemical properties [7][12][18][19]. In particular, DOSY (Diffusion Ordered SpectroscopY) determines the diffusion coefficient of H2O among other molecules and, therefore, this measure can be correlated to the water incorporated in the polymers and to the interactions between polymers of different natures [12]. Additional information about interactions between polysaccharides can be obtained using small molecule molecular probes, as reported in a recent investigation about the properties of solutions of HA and TSP [18][19]. In particular, the formation of stable supramolecular aggregates and enhanced mucoadhesive properties of polysaccharide mixtures have been correlated, by NMR spectroscopic methods in a four-component solution, to their affinity with the anti-inflammatory drug diclofenac sodium salt (DS, Figure 2), a small molecule molecular probe, with respect to bovine submaxillary mucin (BSM) [18][19]. In this case, the interaction between the two polysaccharides was sufficiently effective to perturb the known interaction between DS and mucin and, consequently, the NMR signals of the molecular probe itself.
Figure 2. Diclofenac sodium salt.

2. Diffusion Coefficients of Water in AG/HA Solutions

Water proton diffusion coefficients D determined by DOSY (Diffusion Ordered SpectroscopY) have been recently utilized by Wende et al. [12] in the characterization of crosslinked HA hydrogels in 90/10 H2O/D2O solution. Therefore, we thought to investigate the properties of AG/HA mixtures at different ratios, measuring D of water protons in H2O/D2O at 90/10 ratio by DOSY using a 400 MHz spectrometer at 25.0 ± 0.1 °C (Table 1). In the preliminary experiments, we soon found that the value of reference D (Entry 1) was in accordance with that reported by Wende [12]. Then, we proceeded to analyse solutions of the two polysaccharides. The value of D at 1/1 ratio AG/HA (Entry 2) was close to that of reference D of water protons. However, considering the standard errors, a sharp comparison cannot be performed. A further increase of AG concentration shows, however, a clear trend, indicating a progressive decrease in the mobility of water (Table 1, Entries 3–5).
Table 1. Diffusion coefficients D of water protons in D2O/H2O = 10/90.
Entry Mixture (Total Conc. Polysaccharides) Diffusion Coefficient H2O
(×10−9 m2/s) a
1 Reference in D2O/H2O (10/90) 2.07 ± 0.01
2 AG/HA 1:1 (3.0 mg/mL) 2.09 ± 0.02
3 AG/HA 2:1 (4.5 mg/mL) 2.07 ± 0.02
4 AG/HA 3:1 (6.0 mg/mL) 2.04 ± 0.02
5 AG/HA 4:1 (7.5 mg/mL) 1.99 ± 0.01
a Experiments performed on the same samples were run in duplicates.
As observed by Wende and co-workers [12], mono-component exponential fitting was found to give a good fit for most of the samples and used on all measurements to avoid overfitting. Relatively high standard errors were detected [12], comparable to those obtained by us, but, to the best of our knowledge, an investigation about the effect on D accuracy of H2O/D2O at different ratios has never been reported.
Therefore, we decided to determine diffusion coefficients D by DOSY using a 400 MHz spectrometer of H2O (0.1%) in large excess of D2O (99.9%) of solutions of AG and HA, at the same ratios and concentrations, for comparison. Solutions of solely AG or HA were also analyzed (Table 2). The value of reference D of water protons was significantly lower (Table 2, entry 1) than that previously determined (Table 1, entry 1). The presence of a larger amount of D2O and rapid proton/deuterium exchange would explain the reduced water mobility and therefore the reduced value of D. Since the diffusion coefficient is an average measure of bound and unbound water and the H2O/HDO ratio, the reduced mobility of water should be referred mainly to as HDO, which is the main species when an excess of D2O is present. The mass effect and increased hydrogen-bond strength cause the observed decrease of D with respect to those reported in Table 1 [32].
Table 2. Diffusion coefficients D of water protons in 99.9% D2O.
Entry Mixture (Total Conc. Carbohydrate) Diffusion Coefficient H2O
(×10−9 m2/s) a
1 Reference in D2O 99.9% 1.703 ± 0.007
2 AG (3.0 mg/mL) 1.666 ± 0.005
3 HA (3.0 mg/mL) 1.658 ± 0.005
4 AG/HA 1:1 (3.0 mg/mL) 1.675 ± 0.006
5 AG/HA 2:1 (4.5 mg/mL) 1.666 ± 0.009
6 AG/HA 3:1 (6.0 mg/mL) 1.656 ± 0.005
7 AG/HA 4:1 (7.5 mg/mL) 1.648 ± 0.005
a Experiments performed on the same samples were run in duplicates.
D of water protons (0.1%) in D2O (99.9%) at 25.0 ± 0.1 °C was also determined for comparison at 600 MHz spectrometer obtaining a value of 1.714 × 10−9 m2/s, which is in accord with the one, within the experimental error, found at 400 MHz spectrometer (Table 2, entry 1). Encouraged by these preliminary results, we proceeded with an analysis of mixtures of AG/HA at different ratios. For all the tested mixtures, a lower value of D of water protons was detected with respect to reference D of water (Entry 1), indicating a progressive reduced mobility of water at the increase of AG concentration. A slight increase of water protons D value was detected at the ratio AG/HA = 1/1 (3.0 mg/mL total concentration, Table 2, entry 4) with respect to solutions of solely AG or HA at the same concentration (Table 2, entries 2 and 3), probably indicating that the slightly increased mobility of water could be due to the interactions between the two polysaccharides, and the formed aggregates tend to exclude water molecules which are more mobile. Then, a further increase of the AG/HA ratio and concentration (Table 2), leads to a progressive reduced mobility of water protons as previously determined (Table 1). In all the cases lower standard errors were detected (see Table 1 for comparison). The two series of data in Table 1 and Table 2 clearly indicate that the mobility of water is reduced increasing the concentration of AG. Probably, water molecules are entrapped and complexed by carboxylate and hydroxyl groups mainly via hydrogen bonds and van der Waals interactions, causing a macroscopic retainment of a higher amount of water and a progressive decrease in its mobility. These aspects can be of high importance because they can lead to new formulations for pharmaceuticals and cosmetics applications that show better hydrating power and higher retention of water increasing of AG concentration.
However, the decrease in the diffusion coefficient observed at the increase of the total concentration of AG could be ascribed to eventual increase of viscosity. Viscosity greatly affects diffusion of molecules and the diffusion coefficient D determined by DOSY experiments [14][33][34]. Therefore, the determination of viscosity of D2O solutions of the mixture of polysaccharides was performed for a better comprehension of the phenomena herein highlighted (see next section).

3. Determination of Viscosity on D2O 99.9% Solutions

The viscosity was determined on D2O solutions of AG/HA mixtures (Figure 1 and Table 3), because D2O and H2O have different viscosity [35]. The trends of viscosity versus the shear rate for the solely AG, HA and their mixtures is shown in Figure 3. Viscosity values at three different shear rates are listed in Table 3. First of all, the analysis of the curves highlighted an interesting phenomenon, a progressive decrease of the viscosity increasing the total concentration of AG at any shear rate (Figure 3). The correlation of the data in Table 2 with the viscosity measurements (Figure 3) shows lower diffusion coefficients at lower viscosity. As reported in the literature, the increase of the molecular weight and concentration of hyaluronic acid in polymer solutions leads to the reinforcement of the three-dimensional network of the polymer, and consequently an increased viscosity [14]. Favored by HA-AG interactions, the decrease of viscosity and the decrease of water mobility observed at the increase of AG concentration can be ascribed to reduction of molecular entanglements and intermolecular interactions between HA chains, and to the strengthening interactions with water. These outcomes have a great practical appeal because an efficient manipulation of the rheological behavior of HA solutions can be achieved when combined with AG. In perspective, these properties can allow a wider use of the two polysaccharides not only in eye-drops preparations, but also as a mix for cosmetics and medical device applications, when decreased viscosity combined with increased hydrating power are required.
Figure 3. Viscosity vs. the shear rate for polysaccharide solutions in 99.9% D2O.
Table 3. Viscosity values for three different shear rates.
Sample η at 0.1 s−1
η at 1 s−1
η at 10 s−1
HA 3 mg/mL 250.14 243.69 186.97
AG 3 mg/mL 22.90 2.73 1.48
AG/HA 1:1 3 mg/mL 86.69 38.87 34.04
AG/HA 2:1 4.5 mg/mL 53.91 34.43 31.66
AG/HA 3:1 6 mg/mL 44.60 26.93 25.48
AG/HA 4:1 7.5 mg/mL 39.81 24.29 23.46
The analysis of the viscosity curve of HA solution shows two trends:
1. Shear rate < 1 s−1 Newtonian behavior. The entanglements untangle and entangle again, thus viscosity is not affected by the shear rate.
2. Shear rate > 1 s−1 pseudoplastic behavior. The entanglements untangle and the chains orient themselves toward the shear direction, the gel deconstructs, the layers become thinner, and the viscosity drops down.


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