A Review on the Role of Acemannan in The Aloe -Based Nanostructures

In recent years^’ nanostructure materials (NSMs) have been of great interest as catalysts and other applications because of their unique textural and structural characteristics. Out of various biomaterials employed for NPs synthesis, plant extracts have attracted much attention due to their effectiveness, availability, and green characteristics. Due to the range of chemical constituents, currently, the incorporation of the Aloe chemical constituents into other substances makes the biosynthesis of NPs which are very necessary for the formation and applications of Aloe-based NPs. There are various roles of Aloe chemical constituents in the formation of Aloe-based NPs. The role of the functional groups in acemannan molecules in the formation of nanostructures has been discussed. For instance, acemannan in the AgNP has been described as acting as a reducing and stabilizing agent. It has been reported that capping agent molecules prevent nanoparticles from aggregation and oxidation to stabilize the NPs. The authors proposed the capping of reduced silver by acemannan as the possible chemistry involved in the formation of AgNP. The surrounding acemannan is a surfactant and inhibits the AgNP agglomeration. The enhanced antibacterial activity of AgNPs synthesized using A. vera extract was described as it is attributed to active components in the extract especially, acemannan as the main reason. In another way, the addition of A. vera in the nanofiber membranes (NFMs) can increase the antibacterial effect of the NFMs. Acemannan has various medicinal applications and when combined with the precursor of NPs, its medicinal potentials make the synthesized NPs have therapeutic activities.

Keywords: Nanostructures; Acemannan; Aloe; Role

In recent years^’ nanostructure materials (NSMs) have been of great interest because of their unique textural and structural characteristics [1]. Nanostructure materials can be classified according to dimensions less than 100 nm and they can be single or multi-phase polycrystals with grain sizes on the nanoscale [2]. The development of nanotechnology is a modern multidisciplinary science involving the fields of chemistry, physics, biology, and engineering, the production of nanoparticles (NPs), both in nature and by humans [3]. There are various chemical and physical methods to synthesize nanoparticles (NPs). Among them, the sol-gel process, chemical precipitation, chemical vapor deposition, hydrothermal, and microwave methods have been reported mostly [4]. However, these methods are not effective in many aspects. Therefore, currently, green synthesis, single-pot biomimetic, and/or biological methods of synthesis are preferred over chemical and physical methods due to their rapidity, eco-friendliness, nonpathogenic, and economical attributes. Besides, these biosynthesis methods exclude the use of high temperature, energy, pressure, and toxic chemicals [5]. Therefore, nowadays, biogenic or green synthesis of (NPs) using bacteria, fungi, actinomycetes, algae, and higher plants have emerged as potential nano factories [6-8] and their applications are based on the chemical constituents of these living things. The green synthesis of nanomaterials such as silver [9], zinc oxide [10], magnesium oxide [11], gold [12], cerium oxide [13], copper oxide [14], titanium dioxide [15], activated carbon [16], palladium [17] and tin oxide [18] has been conducted extensively in recent years. Out of various biomaterials employed for these purposes, plant extracts have attracted much attention due to their effectiveness, availability, and green characteristics [19, 20].

Aloe species can store water and important chemical constituents in their swollen and succulent leaves because of their ability to survive in conditions such as hot and dry, which makes them a unique source of phytochemicals [21]. The range of chemical constituents of the Aloe species can be used in preparing beauty and cosmetics, medicinal and pharmaceutical, personal care and toiletry products, and bittering agents in alcoholic drinks [22]. Currently, many researchers are focused on the incorporation of Aloe extracts into substances such as metal/metal oxides at the nanostructure [23]. This is due to the Aloe species having a variety of active constituents responsible for the target application. However, due to the synergetic effect, the identification of exact chemical components responsible for the synthesis and applications of nanoparticles has not been stated clearly in the works of literatures. Moreover, there is a lack of a comprehensive review that presents a general idea about the roles of single chemical constituents like acemannan in both formation and applications of Aloe-based NPs. In addition to that almost all kinds of literature, fabricated NPs from leaves of Aloe especially, A. vera. Herein; the review summarizes the recent update on these ideas somewhat

The Aloe acemannan is structurally unique which makes it a characteristic compound of Aloe species amongst other well-known plant mannans (which have distinct side chains or are unacetylated and insoluble) [24]. From the parts of the Aloe, gel and skin of Aloe are the main sources of acemannan, which has β-(1, 4) linkages and a variable degree of acylation. The dry matter of most Aloe species are polysaccharides constitute. Acemannan is a type of storage polysaccharide, an acetylated glucomannan, and it is located in the protoplasts of parenchyma cells that contain many polysaccharides in the cell wall matrix. Aloe acemannan variability depends greatly on the species and cultivation conditions [25]. The significant effect on the physical and biological characteristics of acemannan is due to the distribution of acetyl groups and galactosyl units in the main chain [26]. Acemannan, found in internal leaf Aloe gel, is a polysaccharide composed of β-(1, 4)-linked highly acetylated mannose, β-(1,4)-linked glucose, and α-(1,6)-linked galactose, Figure 1 [24, 27]. Acemannan found in A. vera gel has a backbone of β-(1, 4)-D-mannosyl residues acetylated at the C-2 and C-3 positions that exhibit a mannose monomer: acetyl ratio of approximately 1:1 and contains some side chains of main galactose attached to C-6 [28]. Being a carbohydrate, acemannan has the functional groups responsible for its reducing ability.

There are different kinds of chromatographic techniques and spectroscopic analyses that are used to determine the structure of acemannan. Chromatographic techniques such as high-performance liquid chromatography (HPLC), gas chromatography (GC), and high-performance gel permeation chromatography (HPGPC) are mostly applicable. Homogeneity and molecular weight are mostly measured by HPLC, HPGPC [29, 30], and SEC [31, 32] technologies. After being completely hydrolyzed by trifluoroacetic acid, the hydrolysate is separated and analyzed by HPLC, GC, or GC-MS [33]. Detection of functional groups is commonly carried out by IR or FT-IR. In order to determine the composition of the main chain and branched chain, methylation analysis combined with GC-MS is an effective method to determine the linkage types of glycosyl residues [34]. Nuclear magnetic resonance (NMR) spectra, including 1H, 13C were widely used to determine the abnormal structure, position, and linkage sequence of glycosyl residues [35]. In order to determine the conformational characteristics of the solution at 540 nm by semi-quantitative estimation by UV, circular dichroism (CD) spectra can directly analyze the conformational structure, usually by characterizing the Congo red polysaccharide complexes [36]. Moreover, recent research shows that A. vera polysaccharide can be determined by the use of size exclusion chromatography (SEC)–multi-angle laser light scattering (MALS)–differential refractive index (DRI) [37].

There are various roles of Aloe phytochemicals in the formation of Aloe-based NPs. However, roles such as reducing, capping, and stabilizing agents are very important in the characterizations and applications of Aloe-based NPs. These three properties are interrelated to one another. If the formed NPs are reduced or capped to precursor, then it stays stable. The stable NPs can be applied to the target applications. Reducing agents have the role of driving electrons from the solution to the ions (usually metallic ones) to form atoms. In other words, they reduce the salts into the reduced form, which is usually insoluble [38]. There is the presence of a -OH group in most phytochemicals obtained from Aloe spp. and this -OH served as a reducing agent, converting metal ions into metal/metal oxide NPs. Also, carbonyl functional groups are present in the phytochemical of Aloe spp. play a significant role in NPs fabrication [23]. The role of the functional groups in acemannan molecules in the formation of AgNP has been described as acting as a reducing and stabilizing agent [39]. It has been reported that capping agent molecules prevent nanoparticles from aggregation and oxidation to stabilize the NPs [40]. Aloe species have phytochemicals and/or functional groups responsible for capping agents [41, 42]. The appearance of prominent bands indicated the surface association of O–H bearing carbohydrates. The authors proposed the capping of reduced silver by acemannan as the possible chemistry involved in the formation of AgNP as represented in Figure 2. The surrounding acemannan is a surfactant and inhibits the AgNP agglomeration.

The influence of additional particles of Aloe phytochemicals attached to the nanoparticle can change its overall properties [23]. The enhanced antibacterial activity of AgNPs synthesized using A. vera extract was described as it is attributed to active components in the extract especially, acemannan as the main reason [41]. In another study, the addition of A. vera in the nanofiber membranes (NFMs) can increase the antibacterial effect of the NFMs. This might be due to the presence of substances such as acemannan, and other constituents in A. vera, resulting in its better antimicrobial activity [43]. In literature, acemannan has been mentioned as one of the responsible polysaccharides in the wound healing potential of insulin-loaded nanoemulsion with A. vera gel in diabetic rats [44]. The β-(1, 4)-glycosidic bond configuration of acemannan is an important consideration in terms of the therapeutic effects of A. vera gel since humans cannot enzymatically break down these bonds [45]. Acemannan has various medicinal applications and when combined with precursor of NPs, its medicinal potentials make the synthesized NPs to have therapeutic activities.

Currently, the incorporation of the Aloe chemical constituents into other substances makes the biosynthesis of NPs which are very necessary for the formation and applications of Aloe-based NPs. Due to the reason the unique nature of Aloe plants, Aloe-based nanoparticles are very important in a broad area of study. Although the activities of Aloe phytochemicals are discussed synergistically in NPs, active constituents like Aloe acemannan have a great role in the formation and applications of Aloe-based nanostructure particles. This is because in nanobiotechnology plants serve as incredibly rich sources of naturally synthesized chemical compounds that are an environmentally acceptable, readily available, inexpensive, and renewable source of materials. This means nanoparticle synthesis using the Aloe plant provides a simple, eco-friendly, and efficient route. However, in most kinds of literature, only the leaf gel of A. vera has been studied. Therefore, two points are recommendable in the future perspectives. The first point is using other parts of the plant such as leaf latex, flower, root, etc. to synthesize NPs, due to the Aloe plants are rich in phytochemicals in other parts of the plant other than leaf gel. Secondly, there are hundreds of species in the genus Aloe; hence limited A. vera is not recommendable. Using other Aloe species has two advantages; comparative study and as the result to get more effective species among the genus. Another idea for the future perspective is to test phytochemicals' role separately based on their identity which may use for the formation of NPs in various applications such as medicine, food,environmental protection, material preparations, etc

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