This review discusses the properties of complexes formed by proteins with polyelectrolytes (PPC) and two polyelectrolyte molecules of opposite charge (PEC). The most highly charged polymers with ionic groups in each monomer unit are considered in this paper. There are all reasons to regard PEC as macromolecular compounds produced as a result of equilibrium reactions with inherent permanent exchange of polyions in water-salt solutions. They combine two properties that might appear at first sight to be mutually exclusive, i.e. rather high stability and lability. Introduction of bioaffinity ligands endows PEC with the recognition capacity sufficient for the purposes of bioseparation and bioanalysis. Antibody-PEC conjugates were successfully used in the immunoassay combining the advantages of both homogeneous and heterogeneous assays and for modeling of chaperone action. The unique properties of polyelectrolyte complexes in combination with bioaffinity ligands makes them promising for the development of highly efficient means of protein isolation, new immunoassay procedures and creation of reversibly soluble biocatalysts.

To optimize polycation-based gene delivery agents, the influence of molecular characteristics of the polycations on physicochemical properties of polycation/DNA complexes and their relationships to cellular gene transfer need to be understood. With this aim, we have synthesized a series of model polymers based on quaternized poly(4-vinylpyridine)s (CnPVP-beta) with the same DP of 1600 but differing by the number n of methylene groups in N-alkyl ester substituents from 1 to 6 and/or by degree of alkylation beta from 25% to 95%. For polycations CnPVP-95, the efficiency of transfection of a plasmid vector expressing a secreted form of alkaline phosphatase started to be detectable at n = 4, noticeable at n = 5, and again undetectable at n = 6. A decrease in beta of active C5PVP-95 from 95% to 65% resulted in a further noticeable increase of activity with a 9-fold increase for C5PVP-65. This finding was attributed to the proton sponge mechanism due to protonation of non-alkylated pyridine moieties of CnPVP-beta/DNA complexes in slightly acidic media that was supported by the fluorescence quenching assay. The data demonstrate the advantages of partial alkylation of tertiary polyamines with medium-length alkyl agents for preparation of efficient nonviral gene delivery vectors.