Ion Specificity Of Macromolecules In Crowded Environments

In this thesis, we have investigated the ion specificity of macromolecules in crowded environments by employing thermos-sensitive polymer and polyzwitterions as model systems. The main results are as follows:(1) The thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) has been employed as a model system to study the specific ion effects on the solubility of macromolecules in crowded environments of dextran and polyethylene glycol (PEG). Our study demonstrates that crowding agents can interact with either anions or PNIPAM chains. The chaotropic anion SCN- interacts with dextran but does not interact with PEG. Both Cl- and CH3COO-do not interact with dextran and PEG. On the other hand, dextran can interact with PNIPAM as hydrogen-bond donors, whereas PEG interacts with PNIPAM as a hydrogen-bond acceptor. The salting-in effect exerted by SCN" on PNIPAM is weakened in the crowded environment of dextran but is strengthened in the crowded environment of PEG due to the distinct anion-crowder interactions. In parallel, the salting-out effect generated by Cl- and CH3COO- on PNIPAM is weakened by the crowding of dextran but is strengthened by the crowding of PEG because of the different macromolecule-crowder interactions. Our study reveals that the ion specificity of macromolecules is altered significantly changing from dilute solutions to crowded environments.(2) We have investigated the specific ion effects on the hydrolysis of N-succinyl-Ala-Ala-Pro-Phe p-nitroanilide by a-chymotrypsin decorated with polyzwitterions (CP) in crowded environments. Our study shows that the stability of CP is higher than the bare a-chymotrypsin (CT) in both dilute solutions and crowded environments. In dilute solutions, the binding affinity of CP to the peptide is higher than CT. However, the binding affinity of CP is smaller than CT as the environment changes from dilute solutions to crowded conditions. In crowded environments, the binding affinity of CP to the peptide can be tuned by the dissolved ions. The binding affinity of CP in the presence of different types of ions increases following the series Cl-< Ac-< SCN-. Thus, SCN- can be employed to produce a higher affinity of CP than CT in crowded environments.