Poly(vinylpyrrolidone) bottlebrush and comb polymers using denatured albumin backbones for potential DNA delivery

Bottlebrush polymers are widely proposed for a large variety of applications, from soft materials to photonic and optoelectronic applications. For medical applications, it is desirable to have a degradable backbone, such as denatured albumin. To generate true bottlebrush polymers, the polymer must be attached to the backbone, here denatured bovine serum albumin (BSA), at high density. The aim of this work was to identify conditions that help generate poly(vinylpyrrolidone) (PVP) grafted albumin with the highest possible polymer grafting density. BSA was denatured at 70°C and pH 4.5 in a urea solution, then the solution was adjusted to pH values from 4.0 to 6.2 and reacted with ethylenediamine (EDA) to yield highly cationised (positively charged) BSA (cBSA). MALDI-ToF analysis revealed that the highest degree of amine functionalisation can be achieved when the reaction is carried out at a pH value between 5 and 6. PVP with 38 repeating units was prepared using a carboxylic acid functionalised RAFT agent. The polymer was reacted with cBSA and the highest number of attached PVP chains, 77 PVP chains, was obtained when the system was cationised at pH 6 and the polymer conjugation was carried out for 24 h. During this process, 114 amino functionalities remained unreacted. It can be concluded that the PVP-grafted cBSA obtained was not a true polymer brush but rather a comb polymer. However, the resulting structures were found to condense siRNA due to the abundance of positively charged amino groups.