Slow tert-butyl ester acidolysis and peptide 3(10)-helix to alpha-helix transition in HFIP solution

We have already shown by CD and NMR techniques that the terminally protected homo-octapeptides Z (and Ac)-[L-(alpha Me) Val](8)-OtBu undergo a slow and irreversible 3(10)-helix to alpha-helix transition when dissolved in 1, 1, 1,3,3,3-hexafluoroisopropanol. In the present work, we find by HPLC and CD that under the aforementioned experimental conditions, a slow acidolysis of the tertbutyl ester functionality does take place affording the corresponding octapeptide free acids. The results of our combined chromatographic and spectroscopic experiments are confirmed by a comparison with the properties of independently synthesized and chemically characterized authentic compounds.