(450c) Coil and Helical Conformations of Poly(Ethylene Oxide) and Poly(Ethylene Imine) in Solution

Poly(ethylene glycol) (PEG) is a simple polymer with repeating units [- C - C - O -]. The carbon atoms are hydrophobic; the oxygen atoms are hydrophilic and participate in hydrogen bonds. PEG is soluble in aqueous and organic solvents, and is widely used in commercial products. In water, the PEG molecule forms a coil in solution. Poly(ethylene imine) (PEI) has repeating [- C - C - NH -], where the nitrogen atom can both accept and donate hydrogen bonds. We have studied the conformation of PEG and PEI in isobutyric acid. We had previously shown that PEG changes conformation from coil to helix in isobutyric acid. Other solvents that also lead to PEG helix formation are n-propanoic acid and isopentanoic acid. Solvents that do not cause PEG helix formation are n-butanol, isobutanol, and acetic acid. PEI also forms helices in isobutyric acid. PEG in isobutyric acid reverts from helix to coil when the temperature in increased to about 43 deg C, and the PEI helix unfolds at about 53 deg C. We hypothesize that the PEG and PEI helices are stabilized by water of hydration on the polymer molecule, and we tested this by (a) altering the hydrogen bonding by adding urea, and (b) removing trace water from PEG in isobutyric acid by adding molecular sieve. Both the urea and the molecular sieve cause the PEG helices to revert to coils, indicating that the water of hydration plays a key role in the folding into helices.