On the temperature-induced coil to globule transition of poly-N-isopropylacrylamide in dilute aqueous solutions

Poly-N-isopropylacrylamide (PNIPAM) is a chemical isomer of poly-leucine, h aving the polar peptide group in the side-chain rather than in the backbone . It has been demonstrated experimentally that PNIPAM dissolved in aqueous solution undergoes a collapse transition from coil to globule on increasing temperature above the theta-point. By a careful reviewing of existing expe rimental data, we emphasize that such coil to globule collapse has to be co nsidered an intramolecular first-order transition, analogous to the cold re naturation of small globular proteins. The main theoretical approaches to t he coil to globule collapse in homopolymers are discussed briefly, and a cr itical comparison between the existing models is performed. We point out th at, as a general result, the coil to globule collapse is expected to be a f irst-order transition for rigid and semi-rigid macromolecules. Finally, tak ing advantage of the analogy between the coil to globule collapse of PNIPAM and the cold renaturation of small globular proteins, we try to clarify so me important and intriguing aspects of protein thermodynamics. This leads t o the conclusion that the amphiphilic nature of polypeptide chain plays the fundamental role for the existence of two temperature-induced conformation al transitions. (C) 2000 Elsevier Science B.V. All rights reserved.