First Observation of the Molten Globule State of a Single Homopolymer Chain

In a recent laser light-scattering study on the “globule-to-coil” transition (melting) of a single poly(N-isopropylacrylamide) (PNIPAM) linear chain in water (the solution is in the stable one-phase region at all temperatures), we observed that at the initial melting stage the ratio of the hydrodynamic radius to the radius of gyration underwent an unexpected maximum $\left(\sim 1.61\right)$ which is even higher than $(5/3{)}^{1/2}$ predicted for a rigid and uniform globule, indicating a molten globule. This finding suggests that just as with a protein molecule the melting of a homopolymer globule is not an all-or-none process and the molten state is a general phenomenon for macromolecules in solution.