Amino acids are organic compounds containing basic amino groups and acidic carboxyl groups, in which the hydrogen atoms on the carbon atoms of carboxylic acids are replaced by amino groups. Similar to hydroxy acids, amino acids can be divided into α-, β-, γ-amino acids according to the different positions of the amino group on the carbon chain, but the amino acids obtained after protein hydrolysis are all α-amino acids. And there are only twenty kinds of α-amino acids, which are the basic unit of protein. Polyamino acids are polymers made by the homopolymerization of the same kind of amino acids or copolymerization of different amino acids.

Figure 1. General amino acids structure.

Applications:

Polyamino acid is a polymer compound with good biodegradability, biocompatibility and regular secondary structure, which is widely used in the field of biomedicine, such as drug controlled release, gene transmission, tissue engineering, antibacterial materials, bioadhesion and Biological separation. The most widely studied polyamino acids are synthetic polyamino acids formed by linking a-amino acids through skin bonds.

• Drug delivery carrier: Polyamino acids have good biocompatibility and biodegradability, and the degradation products are natural amino acids and their derivatives without toxic and side effects and can be used in in vivo stents and drug gene delivery vehicles. For example, direct bonding of hydrophobic drugs to the side chain of homopolyamino acid poly(L-glutamic acid) can improve the water solubility and stability of the drug in the blood. The carboxyl group of the side chain of poly(L-glutamic acid) is often used to bond a variety of hydrophobic anticancer drugs, such as Doxorubicin, Cyclophosphamide, Melphalan, Mitomycin C, Paclitaxel, and Camptothecin. The bonding drug has a good effect on many cancers and even some examples of paclitaxel resistance. In addition, doxorubicin (DOX) is bonded to the side chain of PEG-polyaspartic acid (PEG-PAsp) to make the bonded material have a hydrophobic structure, which can self-assemble in water to form nanoparticles, making it possible to obtain an EPR effect and increase the targeting of drugs.

Figure 2. The chemical structure of PEG-PAsp.

• Tissue engineering: With its good biocompatibility and biodegradability, polyamino acids have not only obtained great applications in the fields of drugs and gene carriers but also are being widely researched and applied in tissue engineering, antibacterial materials and other biomedical fields. Polyamino acids are highly hydrated and have a porous structure, so they can simulate the performance of extracellular matrix to a certain extent (for example, support cells and material transport), so they are widely used in the field of tissue engineering. For example, amphiphilic block polyamino acids (composed of hydrophilic polylysine or polyglutamic acid and hydrophobic polyleucine) can be directly dissolved in water to form water micelles. This water micelle system can be applied to tissue engineering of the central nervous system.
• Tumor imaging: Non-invasive tumor imaging has great potential for early detection and treatment of human diseases. For example, synthetic PEG-ICG-CPNPs based on pegylated indocyanine green-labeled lysine calcium phosphate nanoparticles can be used for tumor imaging. Infrared tumor imaging experiments show that the fluorescence at the tumor site is gradually weakened over time, confirming the therapeutic effect of polymer nanoparticles.

Figure 3. PEG-ICG-CPNPs for fluorescence imaging of isolated organ tissues during tumor imaging.

References:

1. Bae Y, Nishiyama N, Kataoka K.(2007) "In vivo antitumor activity of the folate-conjugated pH-Sensitive polymeric micelle selectively releasing adriamycin in the intracellular acidic compartments." Bioconjugate Chemistry. 18(4):1131-1139.
2. Xiao C, Tian H, Zhuang X, Chen X, Jing X.(2009) "Recent developments in intelligent biomedical polymers." Science in China Series B-Chemistry. 52(2):117-130.