Folding and Unfolding of Polymer Helices in Solution

We have previously shown that poly(ethylene glycol) (PEG) assumes a helical conformation in isobutyric acid. We now show that the formation of helices by PEG in isobutyric acid requires the presence of a trace amount of water: We can make the helices coil and uncoil by adding or removing trace water to/from the solvent. We also show that the similar polymer poly(ethylene imine) (PEI) forms helices in isobutyric acid and that PEG forms helices in isopentanoic and n -propanoic acids but not in isobutanol or n -butanol. PEI ( M n = 21.8 kg/mol) forms only helices in isobutyric acid, whereas PEG ( M n = 21.0 kg/mol) forms a mixture of helices and coils. Helical PEI ( M n = 21.8 kg/mol) shows a helix-to-coil transition when the temperature is increased to about 50 °C, while PEG ( M n = 21.0 kg/mol) shows a helix-to-coil transition at 40−45 °C. When the trace water in the solution is D 2 O, the PEG ( M n = 21.0 kg/mol) helix-to-coil transition moves to a higher temperature of about 56 °C, perhaps due to stronger hydrogen bonding.