Various biocompatible polymeric systems were designed by covalently conjugating specific functionalities to poly(amino acid) derivatives, such as pH sensitivity or mucoadhesion.
In chapter 2, new pH-sensitive graft copolymer based on poly(2-hydroxyethyl aspartamide) (PHEA) were prepared by attaching various cationic monomers and a hydrophobic segment on poly(succinimide). The molecular structure and degree of substitution of the polymers was confirmed by $^1H-NMR$. Their pH-induced phase transition behavior, especially solubility in aqueous medium, was investigated by light transmittance measurements. Phase transition of each copolymer occurred at a vicinity of the $pK_a$ value of the cationic groups, and their insoluble pH ranges were broadened as the feed amount of pH sensitive moieties was increased. Depending on the cationic grafts having different $pK_a$ values, it could be tuned the pH ranges where the copolymer became insoluble. Copolymers PHEA-g-$C_{18}$-PY, PHEA-g-$C_{18}$-IM, and PHEA-g-C_{18}-BU exhibited phase separations in solutions at the pH ranges of 4~6, 6~8, and 9~12, respectively. These polymers have unique feature of which their pH sensitivity profiles are identified to three regimes. Under low pH conditions (below $pK_a$), the polymer solution is transparent. At medium pH (around $pK_a$), a polymer precipitation occurred in solution. At pH > $pK_a$, a polymer solution is gradually dissolved again. Each of these pH regimes was characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) and zeta-potential measurement for the study of their physicochemical properties.
In chapter 3, a novel controlled release drug delivery system for pH triggered release of active ingredients onto fabrics and skin is described. This system comprises microparticles prepared from PHEA-g-$C_{18}$-IM50 and PHEA-g-$C_{18}$-BU70 copolymers which can contain a wide range of active ingredients. These polymeric encapsulation materials have been specifically designed to have a function that the desired pHs trigger the breakdown of the particles and release the encapsulated active ingredients. In each case, target pH values are pH 5.4, a natural skin pH, for cosmetic drug carrier and ph 7.0, a rinsing water pH, for laundry detergent.
These carrier systems contained pH sensitive polyaspartamide derivatives have many possible applications such as cosmetic systems employing triggered release of active materials by skin pH and detergents having fabric softening effect.
In chapter 4, a new type of thiolated polyaspartamide was synthesized and characterized with a surface plasmon resonance (SPR) biosensor. L-cysteine grafted poly(2-hydroxyethyl aspartamide) (PHEA)s was obtained by grafting S-(Trityl)-L-cysteine to poly(succinimide) and subsequently deprotecting Trityl groups. The degree of substitution was confirmed by $1^H-NMR$, and elemental analysis. The amount of free thiol groups immobilized on the PHEA was determined by colorimetric reaction of thiols with Ellman’s reagent. The stability of thiol groups at different pH levels was also reported. Mucoadhesive interaction was determined by pig mucin immobilized SPR biosensor. It is concluded that cysteine grafted PHEA enhances the interaction with mucin but that the amount of free thiol groups among the grafted thiols determines the mucoadhesive properties.
본 연구는 폴리 아미노산 유도체에 다양한 기능기를 부가함으로써 pH 민감성, 점막 부착성과 같은 기능성을 가지는 생분해성 고분자 물질에 관한 것이다. 폴리 아미노산 유도체의 일종인 poly(2-hydroxyethyl aspartamide)는 아스파틱산을 단량체로 구성되며, poly(succinimide) (PSI)로부터 간단한 aminolysis 반응으로 얻을 수 있다. PSI의 succinimide ring은 아민 및 티올기와 쉽게 반응하므로, 다양한 기능기를 쉽게 부가할 수 있다.
PSI에 양이온성 기능기와 소수성 기능기를 부가함으로써 pH 민감성 고분자를 합성하였고, pH에 따른 상전이 현상을 광투과도를 이용하여 측정하였다. 각 고분자의 상전이는 부가된 양이온 기능기의 $pK_a$ 값 부근에서 관찰되었으며, 상전이 영역은 pH 민감성 작용기의 부가량이 많을수록 넓어졌다. PHEA-g-$C_{18}$-PY, PHEA-g-$C_{18}$-IM, PHEA-g-$C_{18}$-BU 고분자는 각각 pH 4~6, 6~8, 9~12 영역에서 상전이를 나타내었다. 이들 고분자는 상전이 영역이 뚜렷한 3개의 영역으로 구분되는 특징을 가졌다. pKa 보다 낮은 pH 조건에서는 고분자 수용액이 투명했고, $pK_a$ 부근에서는 침전이 발생했으며, pKa보다 높은 pH에서는 다시 녹아 투명해졌다. 이들 pH 영역에서 TEM, DLS, zeta potential을 측정하여 물리화학적 성질 및 구조 변이를 연구하였다.
PHEA-g-$C_{18}$-IM과 PHEA-g-$C_{18}$-BU 고분자를 이용하여 pH에 따라 약물을 방출하는 약물 전달체를 제조하고, 그 성질을 연구하였다. PHEA-g-$C_{18}$10-BU50은 세탁세제의 방출 조절제로서 활용가능성을 알아보기 위해, 소금 입자에 코팅처리하여 pH 7, 11에서 각각 용해성을 평가하였다. PHEA-g-$C_{18}$10-IM50은 pH 6~8 영역에서 녹지 않고, 피부 pH인 5.4에서 녹으므로, 화장품용 활성물질 전달체로서 활용 가능성을 pH 5.4, 7.0에서 각각 관찰하였다. 그 결과, 두 고분자는 목표 pH에서 용해되어 포집 물질이 방출됨을 확인하였다.
다음으로, S-(Trityl)-L-cysteine을 PSI에 부가한 후 Trityl기를 deprotection하여시스틴이 부가된 PHEA를 제조하였고, 점막부착성을 surface plasmon resonance (SPR)을 이용하여 평가하였다. PHEA에 부가된 thiol기는 pH 5.0에서는 안정했지만, pH 7.4에서는 불안정하여, disulfide 결합을 형성하였다. 점막부착성 평가 결과, 시스틴이 부가된 PHEA가 부가되지 않은 PHEA에 비해 최고 6배 더 높은 값을 나타내었고, 그 점막부착성은 고분자내에 존재하는 free thiol group의 양에 비례하는 것으로 밝혀졌다. 따라서, thiol기가 부가된 점막부착성 고분자에서 free thiol group의 양이 점막부착성을 좌우한다.
