Self-assembly Of PH-sensitive Poly(Styrene-co-4-vinylpyridine) & Casein Hydrolysis

There are two subjects in this thesis:1. Self-assembly of pH-sensitive random copolymers: Poly(styrene-co-4-vinyl -pyridine)Recently, the random copolymer was widely studied because of simple synthesis, low cost and the various morphologies of self-assembled nanoparticles. A series of pH-sensitive random copolymers poly(styrene-co-4-vinylpyridine) (PSVP) with molecular weight about 10,000 and 4-vinylpyridine molar contents of 19-53% were conveniently synthesized by free radical polymerization. The copolymers experience the formation, swell and dissociation of multi-chain nanoparticles when the pH of the aqueous solutions/dispersions changes from 5.1 to 1.0. The nanoparticles have hydrodynamic diameters around 100 nm, a spherical shape, and a relatively uniform structure at pH range of about 5-3 and a multi-core structure at lower pH. The random distribution of the building units causes some of the hydrophilic units, protonated 4-vinylpyridine groups, to be trapped inside the nanoparticles. So, the hydrophobicity of the nanoparticles is tunable by changing the 4-vinylpyridine content in the copolymers. For the copolymers with higher 4-vinylpyridine molar content, the pH range in which the multi-chain nanoparticles form shifts to higher value, the multi-chain nanoparticles dissociate and the copolymers form single-chain hydrophobic domains at low pH.The D_h of the nanoparticles increases with the initial concentration of the copolymer in DMF. However, the D_h of the nanoparticles have little change with the final concentration of the copolymer when the initial concentration was fixed. The nanoparticles were stable after storage of 1 month. There is little effect of 1 % residual DMF on the nanoparticles. The size of the nanoparticles formed from THF solution was larger than that from DMF solution. Because of the shielding effect of NaCl, the diameter of the nanoparticles increases with the salt concentration and pH of the solution in the pH range of 2.5-4.2.2. Casein hydrolysisThe amino acid sequences of four milk caseins are known, so the multiple polypeptides can be prepared simultaneously by trypsin site-specific hydrolysis. However, there are many hydrolysis sites in the caseins, and a lot of polypeptides with different molecular weights will be thereby produced, which induces many problems for purification. The effects of hydrolysis time, trypsin and casein concentration on the hydrolysates were investigated. Most caseins were completely hydrolyzed after 0.5h hydrolysis with the casein concentration of 20 mg/mL and trypsin concentration of 0.04 mg/mL. The higher molecular weight of the polypeptides will be hydrolyzed to the lower molecular weight of polypeptides with the increase of hydrolysis time. The hydrolysate can be separated into several fractions by HC1, TCA or ammonium sulfate. The fractions separated by tuning pH of the solution with HC1 can be used for further purification with HPLC.