Biodegradable Amphiphilic Polyaspartic Acid Derivatives And Their Ph Sensitivity

Stimuli-responsive polymers have been extensively investigated and used as smart biomaterials such as drug delivery systems(DDS).The phase transition or phase separation of polymer is induced by a change in various conditions such as temperature,pH,electric field and solvent composition.However,the application of polymers to DDS is difficult because most stimuli-responsive polymers are not biodegradable and some are toxic.Polymeric biodegradability and biocompatibility are the most important factors for the application of polymers to drug delivery systems.Poly(amino acid)s is a kind of biodegradable materials in vivo.They are useful in pharmaceutical and biomedical areas.The precursor polymer,polysuccinimide(PSI),was synthesized by the thermal polycondensation of L-aspartic acid.Poly(α,β-L-aspartate)(PAsp) was prepared by hydrolysis of PSI.And PAsp was partially esterified to afford an amphiphilic biodegradable polyanion,poly(sodium aspartate-co-propyl aspartate)(PAsp-Na/ PAsp-P).The synthesized polyanion could be assembled into the nano-scaled aggregates in aqueous medium.The aggregate morphologies were studied by scanning electron microscopy(SEM) and transmission electron microscopy(TEM) as a function of pH.It was demonstrated that micellization of this random copolymer occurred with stimulus of pH changes to form various morphological micelles.The copolymer existed as precipitate at low pH(pH＜2).When pH increased to 4,the polymers were associated into spherical micelles with the core of poly(propyl aspartate)(PAsp-P) hydrophobic units and shell of some negatively charged poly(sodium aspartate)(PAsp-Na) units.At higher pH(pH＞5),toroidal nanostructures of the micelles were formed because rigid polyamide chains directly assemble into the large hollow spheres.The circular dichroism(CD) study showed that the conformation underwent a transition betweenα-helix and random coil at pH 3-7.The cooperative transitions were regulated by the degree of ionization of carboxylic side chains.When they were protonated(neutralized),the molecular backbone was in favor of the regular helical structure;when deprotonated(ionized), the electrostatic repulsions among side chains destabilized the intramolecular hydrogen bonds,thus randomizing the regular conformation.Some kinds of novel biodegradable polymers,poly(α,β-alkoxy-L-aspartates), were successfully synthesized by nucleophilic substitution in polysuccinimide. Biocompatible polysuccinimide derivatives conjugated with alkoxy side chains were obtained by using sodium alkoxide(n=1-4),sodium isopropoxide and sodium tertbutoxide as nucleophilic agents.Due to the steric hindrance effect and electronegative effect of the nucleophilic agent,poly(alkoxy-L-aspartate) with differentαandβ-amide unit compositions could be obtained.The contents ofα- andβ-amide unit were determined by ¹H NMR spectroscopy and ¹³C NMR spectroscopy.The highest content ofα-polyaspartate was 60%,obtained in poly(α,β-butoxy-L- aspartate).The lowest content ofα-polyaspartate was 30%, obtained in poly(α,β-tertbutoxy-L-aspartate) and poly(sodium aspartate) (PAsp-Na).Both steric and electronegative contributions were important factors that determine over all reactivity via either pathway.On ring-opening modes of the site selectivity,the stronger nucleophilicity of the nucleophilic agents,compared among methoxy,ethoxy,isopropoxy and tert-butoxy anions,would induce higherβ-amide content in the polymer.The steric hindrance effect of the nucleophilic agents such as methoxy,ethoxy,n-propoxy and n-butoxy anions leads to an increase in the extent ofαpeptide bonds in the reaction product.Amphiphilic polymer PAsp-Na/PAsp-R can be obtained by partially hydrolysis of poly(α,β-alkoxy-L-aspartates).Hydrolysis process of poly(α,β-butoxy-L-aspartates) (PAsp-B) was studied by UV spectroscopy and ¹H NMR.The results showed that the degradation of PAsp-B includes the rapid elimination of butyl side chains and the moderate cleavage ofpeptide linkages in backbone as well as that the hydrolytic properties of PAsp-B were controlled by hydrolytic environments such as temperature and pH value.The hydrolysis rate of PAsp-B in aqueous solutions decreased in the order of pH 12＞pH 4＞pH 7 and increase with increasing the environmental temperature(for example,the hydrolysis rate of PAsp-B is higher at 37℃than that at 25℃).