Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide)

Principles of protein folding--a perspective from simple exact models

General principles of protein structure, stability, and folding kinetics have recently been explored in computer simulations of simple exact lattice models. These models represent protein chains at a rudimentary level, but they involve few parameters, approximations, or implicit biases, and they allow complete explorations of conformational and sequence spaces. Such simulations have resulted in testable predictions that are sometimes unanticipated: The folding code is mainly binary and delocalized throughout the amino acid sequence. The secondary and tertiary structures of a protein are specified mainly by the sequence of polar and nonpolar monomers. More specific interactions may refine the structure, rather than dominate the folding code. Simple exact models can account for the properties that characterize protein folding: two-state cooperativity, secondary and tertiary structures, and multistage folding kinetics--fast hydrophobic collapse followed by slower annealing. These studies suggest the possibility of creating "foldable" chain molecules other than proteins. The encoding of a unique compact chain conformation may not require amino acids; it may require only the ability to synthesize specific monomer sequences in which at least one monomer type is solvent-averse.

Temperature-dependent absorption/desorption behavior of lower critical solution temperature (LCST) polymers on various substrates

We have been studying adsorption and retention (resistance to desorption) behavior of temperature sensitive LCST polymers on different substrates as a function of temperature. According to our studies with Poly 64 (a copolymer of 60% (mol) NIPAAm and 40% (mol) NnBAAm, LCST = 8.5 degrees C in water), the copolymer retention depends on the rinse temperature. When the rinse temperature is above the LCST, the polymer adheres well to most surfaces. On the contrary, at rinse temperatures below the LCST, most of the adsorbed polymer is easily rinsed off. These studies are relevant to our work on the thermally reversible adsorption of LCST polymers conjugated to peptides and proteins, such as affinity ligands, for uses in immunoassays and affinity separations. The interaction between the LCST polymer and most hydrophobic polymer surfaces is mainly due to hydrophobic interactions, and the critical surface tension (gamma c) and the solubility parameter (delta) of the solid polymer substrate are the most important factors which influence the LCST polymer adsorption and retention. The critical surface tension appears to correlate best with the LCST polymer adsorption levels on different substrates, while the solubility parameter correlates best with the retention of the adsorbed polymer. According to our preliminary study, n-butyl groups probably interact more strongly with the substrates than isopropyl groups because of the greater hydrophobic surface area of the former groups.

Thermoreactive water-soluble polymers, nonionic surfactants, and hydrogels as reagents in biotechnology

Thermoprecipitating polymers such as poly (N-isopropylacrylamide), poly(N-vinyl caprolactam), and some ethylene oxide-containing surfactants appear to be suitable for developing new separation systems to complement traditional precipitation, chromatography, and extraction of biological molecules. The nature of thermally induced phase separation of polymers and nonionic surfactants is discussed and examples are given. Covalent coupling of an enzyme to a thermoprecipitable polymer results in a biocatalyst which combines the qualities of soluble and immobilized enzymes. Biocatalysts of this type can be separated from reaction media by precipitation after temperature increase. The use of thermoprecipitating polymer-protein conjugates in immunoassays overcomes one of the shortcomings of traditional methods with solid sorbent-linked antigen or antibody-diffusional limitations. Thermoreactive hydrogels produced by crosslinking of thermoprecipitating polymers can be successfully used for concentrating macromolecules or microbe-rich slurries. Alternate volume changes of hydrogels on heating and cooling produce a "hydraulic pump" which can enhance the productivity of an immobilized biocatalyst. Hydrogels could be used to control reaction or diffusion rates by a thermal feedback mechanism.

Synthesis and characterization of a soluble, temperature-sensitive polymer-conjugated enzyme

The enzyme, alkaline phosphatase, has been conjugated to a temperature-sensitive polymer which exhibits a lower critical solution temperature (LCST). A series of copolymers containing different molar ratios of N-isopropylacrylamide(NIPAAm) and N-acryloxysuccinimide(NAS) were synthesized and then conjugated to the enzyme. These polymer-enzyme conjugates precipitate and flocculate in aqueous solution above the LCST, and redissolve when cooled below that temperature. The kinetics of the conjugated enzymes have been characterized as a function of temperature and compared to free enzyme. The effect of the conjugation degree between polymer and enzyme on the activity of the conjugated enzymes was also investigated.