AMPHIPHILIC POLYMERS DESIGNED FOR BIOMEDICAL APPLICATIONS
4.2 CHEMICAL STRUCTURES OF AMPHIPHILIC POLYMERS BASED ON POLYSACCHARIDES
4.2.1 SIDE-CHAIN AMPHIPHILIC POLYMERS
The synthesis of the polymers with side chains can be obtained by chem- ical modification of a hydrophilic polymer for attachment of hydrophobic or amphiphilic pendent segments. Pendants can be short or long (grafted polymers), and hydrophobes can belong to known classes of lipophilic compounds, such as long alkyls, steroids, or can become hydrophobic by changing external conditions, for example, poloxamers which become hydrophobic by raising temperature. Polymers with charges (polyelectro- lytes) can have charges and hydrophobes located on different side chains, or located on the same side chains. In the latter case, polymers are called polysoaps or polysurfmers, because the side chains are actually surfac- tants bound to the polymer backbone through their charged groups (head- attached) or hydrophobes (tail-attached).
In the following, the amphiphilic polysaccharide structures will be classified as a function of the chemical structure of the attached side groups: (1) bile acid and (2) quaternary ammonium groups. Dextran was chosen for exemplification of the chemical structures of the synthesized amphiphilic polymers.
4.2.1.1 AMPHIPHILIC POLYMERS WITH BILE ACID SIDE GROUPS Bile acids are natural amphiphilic compounds with an important contri- bution to biological processes, such as emulsification and membrane transport of cholesterol (CH), vitamins, retinol, β-carotene (Small, 1971).
Rigid steroid skeleton and facial amphiphilicity (Fig. 4.1) lead to forma- tion of micelles and other supramolecular structures with specific proper- ties. Their hydroxyl and carboxylic acid groups can be easy chemically modified; therefore, bile acids have been often used as building materials for various low molecular or polymeric structures (Cunningham and Zhu, 2016; Durand, 2007; Zhu and Nichifor, 2002). Bile acids were covalently bound to many polysaccharides, such as heparin (Bae et al., 2013; Hwang and Lee, 2016), chitosan (Cadete et al., 2012; Park et al., 2014), carboxy- methyl curdlan (Yan et al., 2015), starch (Yang et al., 2014), or hyal- uronic acid (Wei et al., 2015), and the resulting amphiphilic derivatives found different biomedical application, especially as micellar carriers for hydrophobic drug delivery (Cadete et al., 2012; Park et al., 2014; Yan et al., 2015).
FIGURE 4.1 Chemical structure of bile acids.
In the most of the abovementioned reports, the bile acids were attached to a polysaccharide through their COOH groups. Nichifor and Carpov (1999) prepared two classes of polysaccharide derivatives with the bile acid connected to polysaccharide OH groups either at C24 carboxylic group (DM-BA(m)) or at C3 OH group (DM-CACOONa(m)) (Nichifor et al., 2004a) (Fig. 4.2). The direct attachment of a bile acid by esterifica- tion between polysaccharide OH groups and bile acid carboxylic groups was easily achieved in the presence of N,N-dicyclohexylcarbodiimide (DCC) as a coupling agent and N,N-dimethylaminopyridine (NMAP) as a catalyst, in a mixture N-methylformamide/DMF/dichloromethane 10/9/1 v/v/v. Several natural bile acids were used (BA = CA, DCA, and GCA), as well as a cholic acid derivative, N-(3α,7α,12α–trihydroxy-5-β-cholan- 24-carboxy)-ε-aminocaproic acid) (in this case X in Figs. 4.1 and 4.2 is NH─(CH2)5─CO). The degree of substitution (DS) = m ≤6 mol%, as at higher bile acid content the water solubility is lost. A similar synthetic
procedure was used for the preparation of polymers DM-CACOONa(m), by dextran reaction with a cholic acid derivative, 3-(succinoyloxy)cholic acid trichloroethyl ester, in the presence of DCC and NMAP. The attachment of this derivative was followed by a selective elimination of the trichlo- roethyl groups, which afforded amphiphilic polymers with bile acid side groups connected to polymer backbone at their C3 positions and carrying free carboxylic groups at C24 positions. The presence of anionic carboxylic groups at the end of the side chains endowed these polymers with a poly- electrolyte character and a better water solubility than that of non-charged DM-BA polymers, which was preserved until a DS ≤25 mol%.
FIGURE 4.2 Chemical structures of amphiphilic dextrans with bile acid side groups. D means dextran, M is dextran molar mass, in kDa, and m is substitution degree, in mol%.
m + n = 100.
4.2.1.2 AMPHIPHILIC POLYMERS WITH QUATERNARY AMMONIUM SIDE GROUPS
Hydrophilic and biocompatible polysaccharides with quaternary ammo- nium groups have found numerous pharmaceutical, industrial, and envi- ronmental applications as additives for paper, textile, food, cosmetics, flocculants, or antibacterial agents (Belalia et al., 2008). Their amphiphilic
analogs display similar properties and applications, but also additional features due to the surface activity and ability to self-organize in micelle- like aggregates. The simultaneous presence of ammonium and hydro- phobic groups enhances polymer biological activity; for example, stronger interactions with some components of skin and hair provide an improved delivery of drugs from their topical formulations (Ballarin et al., 2011).
In order to prepare new amphiphilic polysaccharides with quaternary ammonium pendent groups and variable charge and hydrophobe content, Nichifor et al. (2010) developed a synthetic procedure, which avoids the intermediate synthesis of the quaternization reagent. According to this proce- dure, a neutral polysaccharide (dextran, pullulan, and hydroxyethyl cellu- lose) was treated with an equimolar mixture of epichlorohydrin (ECH) and a tertiary amine (R1R2R3N), where R1, R2, R3, are identical or different, and at least one of the R substituents is an alkyl chain with 2–16 carbons, or a benzyl (Bz) group. The procedure can be applied to various water-soluble or water- swellable polysaccharides in linear or cross-linked forms. The procedure provided several classes of cationic amphiphilic polymers, the self-assem- bling properties, and applications of which could be tuned by variation of DS (5–90 mol%), the length or structure of one R substituent, presence of a cross-linker and its nature. The general chemical structure of various classes of dextran carrying pendent N-(2-hydroxypropyl)-N,N-dimethyl-N-(R1/ R2)-ammonium chloride groups is depicted in Figure 4.3 and details about the chemical composition of each class is presented in Table 4.1.
FIGURE 4.3 General chemical structure of amphiphilic polymers with pendent quaternary ammonium groups. D is dextran, M is dextran molar mass, in kDa, QR refers to the quaternary ammonium group with an R1/R2 substituent, m + n + p + r = 100.
TABLE 4.1Chemical Composition of Cationic Dextran Derivatives with General Chemical Structure Presented in Figure 4.3. R1R2Cross-linker structureZSubstitution degree, mol% mnp Linear monopolar cationic dextran DM-QR(m) R1 = R2 = C2, C4, C8, C12, C16, or Bz–CHO5–9000 Monopolar cationic cross-linked dextranDM-QR(m)-ECH R1 = R2 = C2, C4, C8, C12, C16, or BzCHO25–3010–200 Bipolar cationic cross-linked dextranDM-QR1(m)R2(p)-ECH C2C12, C16CHO35–5010–2010–25 End-modified cationic dextran Z-DM-QR (m) R1 = R2 = C2, C8, Benzyl–C12, C18≤ 30–– End-modified cross-linked cationic dextran Z-DM-QR (m)-DVS R1 = R2 = BzC18≤ 2010–20–
Polymers DM-QR(m) are water soluble with DS up to 90 mol% and variable hydrophobicity given by the length of R substituent (C2 –C16).
The water solubility of polymers with R = C2─C8 or benzyl could be preserved until DS = 70–90 mol%, but was limited to DS = 30 mol%
when R = C12–C16.
Quaternization reaction performed on polysaccharide particles previ- ously cross-linked with epichlorohydrin (ECH) afforded cationic amphi- philic hydrogels. The hydrophilic/lipophilic balance (HLB) of these hydrogels was varied in two ways: (1) by changing the hydrophobicity of one substituent R in a single step quaternized gels, that is, monopolar gels DM-QR(m)-ECH, where R = C2–C16 or Bz (Nichifor et al., 2001a; Nichifor et al., 2001b); (2) by changing the ratio between DS1 and DS2 in two step quaternized gels, designed as bipolar gels DM-QR1(m)R2(p)-ECH, where R1= C2 and R2 = C12 or C16 (Mocanu and Nichifor, 2014). The synthesis of bipolar gels offered the opportunity to combine an appropriate HLB with a high cationic group content required by some potential applications, for example, as bile acid sequestrants. The swelling capacity of the hydrogels, the water amount retained (WR) by the gel at equilibrium, was influenced by the content in amino groups, hydrophobicity of the R1/R2 substituents, and the amount of ECH used in crosslinking step.