Induced Conformational Changes of Synthetic Polymers

The Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka 567 Japan 

The photoresponsive polymer systems are among the earliest, in which the chain conformation is reversibly controlled by photo- 

Winnik (ed.). Photophysical and Photochemical Tools in Polymer Science, 269-291. 

It is well known that the chain conformation of polymers depends on solvent quality and temperature. In good solvents, polymers have an expanded conformation, while they shrink in bad solvents at low temperature. Polyelectrolytes change their conformation depending on pH and salt concentration. It is obviously a tedious method to change these conditions to regulate the chain conformation. If we could control the conformation reversibly by photoirradiation outside the sample, the method would be superior to the above method in the response time, reversibility and easy procedure. 

The reactions are named photoisomerizations. The isomerizations are always accompanied by certain physical and chemical property changes. 

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LIGHT-INDUCED CONFORMATIONAL CHANGES OF SYNTHETIC POLYMERS... 

Hoshino, Y, Nakamoto, M. and Miura, Y (2012) Control of protein-binding kinetics on synthetic polymer nanoparticles by tuning flexibility and inducing conformation changes of polymer chains,/Am Chem Soc, 134,15209-15212, doi 10.1021/ja306053s. 

Allosteric effects play a major role in biology, for instance in the conformational changes induced by the binding of an effector and regulating the activity of an enzyme [9.14a] they have also been studied in synthetic receptors [8.201, 8.211, 9.14b, c]. Similarly, cooperativity, a thermodynamically well defined process [9.15a], is displayed by a number of biological species as well as by abiotic ones [8.70a, 8.201, 9.15b-d] (see also Section 9.3.1, [9.64, 9.65]), in particular in organized media such as polymer solutions or gels [7.8cd, 8.290, 8.292]. 

In addition to the commonly used synthetic polymers such as pNIPAM and poly(N-vinylcaprolactam) (PVCL), elastin-like polymers were introduced as thermoresponsive polymers for affinity precipitation (Shimazu et al., 2003). An elastin-like polymer consists of the repeating penta-peptide, Val-Pro-Gly-Xaa-Gly (Xaa being any amino acid except proline). It undergoes reversible phase transition under temperature or salt concentration changes similar to pNIPAM (Shimazu et a/., 2003). An elastin-like polymer was used as a terminal tag for the facilitation of recombinant protein purification as attaching it to the hydrolase enzyme improves its purification. Purification, in excess of 1300-fold was achieved after two cycles of reversible changes of elastin-like polypeptide conformation precipitation induced by environmental changes (Shimazu et al., 2003). 

Several biopolymers and synthetic optically active polymers are known to exhibit an inversion of helicity (helix-helix transition) between right- and left-handed helical conformations when changing the external conditions, such as solvent, temperature, or by light irradiation. However, switching of the macromolecular helicity by chiral stimuli is rare, and can be used to sense the chirality of specific chiral guests. The helicity of optically active helical poly(phenylacetylene)s 67-69 can be switched by external chiral and achiral stimuli [123-126]. The first example of such a helix inversion induced by... 

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