Institute, Department of Chemistry, Polytechnic Institute of
S J nopsis
We have studied the helix coil transition for poly-7-ethyl-bglutamate (PELG) in the mixed solvent system: trifluoroacetic acid-trifluoroethanol by high rpsolution nuclear magnetic resonance spectroscopy. At 2 5 O , the transition occum over a range df qolvent compositions between 6 2 4 4 % trifluoroacetic acid. In addition, we have assigned T-values to the various types of protons in PELG.
It is well known that some proteins have helical conformations which are, in turn, quite important for biological activity. Studies of molecular conformations of synthetic polypeptides are significant since they serve as protein model compounds. Synthetic polypeptides exhibit a range of conformations such a$ helices, "random coils," and associated structures. Transitions among these are observed under various conditions and many experiments hhve been carried out on helix-coil transitions using mixed solv e n t ~. ~-~ Most of these studies depend upon optical rotation measurements. We wish to present preliminary data on another method of investigating helix-coil transitions; namely, high resolution nuclear magnetic resonance spectroscopy (NMR). To date, we note only one referebce to nuclear magnetic resonance structural studies on polypeptides.6 Bovey and his colleagues observed that poly-y-benzyl-cglutamate showed many peaks in NMR spectra in trifluoroacetic acid ("random coil" solvent), but in trichloroethylene (helix supporting solvent) no NMR spectral peaks attributable to polymer could be seen.
We employed poly-yethyl-L-glutamate* (PELG) using a 15% solution at 25' in all experiments. Figure 1A shows an NMR spectrum of PELG in pure trifluoroacetic acid. The assignments of peaks are recorded in Table I. A comparison with Bovey's assignments are included. I n the helixforming solvent, chloroform, the solution was very viscous. As a result it was difficult to obtain reliable data.
In other experiments we found trifluoroethanol to be an excellent helix * We wish to thank Professor C. H. Bamford for this polymer sample.