Self-Assembly of Pseudo Isocyanine Chloride in the Presence of Attractive Polyethylene Glycol Crowders

The present work analyzes the impact of a synthetic model crowder on the self-assembly of a probe in aqueous solution. Pseudo isocyanine chloride (PIC) serves as a probe exhibiting self-assembly to fiber-like aggregates and thus representing a widespread phenomenon in biological cells. Ethylene glycol (EG) or polyethylene glycol (PEG) is used as a model crowder. The specific feature of this type of crowder is an attractive interaction to the PIC cation, thereby retarding the self-assembly of PIC instead of promoting it. First, the self-assembly of PIC is analyzed in the presence of EG. The results are interpreted by means of preferential adsorption of EG on PIC cations and by means of a model process in which nucleation initiates chain growth via monomer addition. Supplemented by molecular dynamics simulations on PIC in aqueous solutions of EG, these experiments provide a microscopic understanding of the nature of the interaction between PIC and EG and their effect on the PIC self-assembly. Experiments are then extended to several PEG samples, differing in their degree of polymerization. The inhibiting effect of PEG crowders is found to be quantified by the number density of the monomeric EG units in the PEG chains rather than the number density of PEG chains, discarding significant volume exclusion effects of PEG-based crowders on the PIC self-assembly. The work thus unravels as a particular feature the impact of attractive crowder-probe interactions on the self-assembly of a probe. Results are not only highly relevant for biological systems but also support future developments in sensor technology based on the insight gained with this responsive synthetic model system.