NMR studies of structural phase transitions in random copolymers

Nuclear magnetic resonance (NMR) has been used in conjunction with circular dichroism (CD) and intrinsic viscosity (IV) measurements to probe the phase diagram of poly(L-glutamic acid-ran-L-leucine) (PEL). IV measurements indicate that PEL in aqueous salt solution undergoes a pH-mediated collapsing transition whereby its size decreases by a factor of 2.6 as the pH is decreased from 6.82 to 4.92. CD measurements indicate a pH-mediated helix-coil transition as the pH is increased from less than 5 to greater than 7. NMR relaxation times and nuclear Overhauser effects were obtained as a function of temperature for various phases of the PEL system and analyzed according to various models for polymer dynamics. The results of these analyses are discussed in the context of freezing transitions predicted by contemporary theories of random copolymers