In this study, synthesis and characterization of biodegradable poly(aspartic acid) was carried out with or without grafted long alkyl groups of different chain length for preparing self-aggregates. Effect of grafted long alkyl groups on self-aggregation behavior was investigated using various experimental methods. The self-aggregates incorporated with methotrexate by physical loading and chemical conjugation was characterized, and the interaction of modified poly(aspartic acid) with lipid bilayer was also studied by Langmuir-Blodgett method.
Poly(aspartic acid) grafted with long alkyl groups was synthesized from acid-catalyzed polycondensation of L-aspartic acid to give poly(succinimide), followed by hydrolysis and aminolysis by alkylamine. Poly(aspartic acid) grafted with alkyl group formed self-aggregates from milky suspension by the sonication method. The alkyl chain length and the grafting amount of alkyl groups influenced the properties of hydrophobic microdomains such as hydrophobicity, polarization, and aggregation number. The overall aggregate structure and properties such as aggregate size, CAC, stability, and chain conformation were directly dependent upon the property of hydrophobic microdomains. The more grafting amount of alkyl groups and longer alkyl chain length induced the more stable and compact self-aggregates. Extended chain conformation was forced to have compact structure by strong association of grafted alkyl groups as the degree of substitution increased. One self-aggregate was composed of several hydrophobic microdomains and the number of hydrophobic microdomains was linearly dependent upon the degree of substitution. Poly(aspartic acid) grafted with octadecyl chain existed as only self-aggregate in room temperature, whereas that with hexadecyl and dodecyl chains behaved like polymeric surfactant due to higher chain flexibility.
Self-aggregates based on poly(2-hydroxyethyl aspartamide) grafted with octadecyl groups was used for methotrexate delivery. Methotrexate was successfully incorporated into hydrophobic microdomains of self-aggregates. Partition constant of micellar phase to water increased linearly as the degree of substitution increased. However, premature large release of methotrexate was found due to rapid transformation of entrapped methotrexate into water-soluble salt form by continual contact with buffer medium. Chemically conjugated methotrexate to the polymer was released much slower than physical entrapment, and the polymer conjugated with methotrexate formed micelle-like structure due to inherent hydrophobic property of methotrexate. Increased hydrophobicity of self-aggregates by grafting alkyl group induced much slower release of methotrexate and lower critical aggregation concentration. The methotrexate release was accelerated in pH 10.0 due to rapid hydrolysis of ester linkages, while the release was suppressed at pH 5.0
Surface pressure/area isotherms of polypeptides modified with long alkyl chains and their miscibility with DPPC were investigated. The polypeptides modified with alkyl groups formed stable monolayers due to their hairy-rod structure and limiting area increased as the degree of substitution increased. When mixture monolayers with DPPC were formed, miscibility between modified polypeptides and DPPC was found to exist in all composition from analyzing isotherms of the mixture. In line with the result of miscibility at the air-water interface, the modified polypeptides was successfully inserted into lipid bilayer and contributed to the stability of liposomal suspension.
