Alternating vs Random Amphiphilic Polydisulfides: Aggregation, Enzyme Activity Inhibition and Redox-responsive Guest Release

Abstract

This paper reports synthesis an alternating copolymer (ACPs) with bio-reducible amphiphilic polydisulfide backbone and highlights the impact of the alternating monomer-connectivity on the self-assembly, morphology, chain-exchange dynamics, drug-release kinetics and enzyme-activity inhibition. Condensation polymerization between hydrophobic 1,10-bis(pyridin-2-yldisulfaneyl)decane and hydrophilic 2,3-mercapto succinic acid (1.04:1.00 ratio) generates the amphiphilic ACP P1 (Mw = 8450 gm/mole, Ð =1.3), that exhibits self-assembly in water, leading to the formation of ultra-thin (height < 5.0 nm) entangled fibrillar network. In contrast a structurally similar amphiphilic random copolymer P2 shows truncated irregular disc-like morphology under the same condition. It is postulated that due to perfect alternating sequence of the hydrophobic and hydrophilic segments in P1, it’s immiscibility-driven aggregation in water leads to a pleated structure, which further assembles to the observed long fibrillar structures, similar to crystallization-driven self-assembly. In fact, wide-angle X-ray diffraction analysis lyophilized P1 sample shows sharp peaks indicating its crystalline nature (% crystallinity ~ 37%), which are completely missing for P2. Effect of such distinct self-assembly on the chain-exchange dynamics was probed by fluorescence resonance energy transfer (FRET) using 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO) and 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) as the FRET- donor and acceptor, respectively. DiI and DiO entrapped solutions of P1, when mixed, no prominent FRET appeared even after 24h. In sharp contrast, for P2, intense FRET emission noticed and the FRET ratio (~ 0.9) reached saturation in ~ 15h, indicating much enhanced kinetic stability of P1 aggregates. Glutathione-induced release of encapsulated Nile red showed much slower kinetics for P1 compared to that of P2, corroborating with the observed slow chain-exchange dynamics of the highly stable alternating copolymer assembly. Further, the well-ordered assembly of P1 showed excellent surface functional group display (zeta potential -32 mV compared to -14 mV for P2) which resulted in effective recognition of α-chymotrypsin (Cht) protein surface by electrostatic interaction. Consequently, P1 could significantly (> 70 %) suppress the enzymatic activity of Cht, while in presence of P2, the enzyme was still active in > 70% efficacy.