Avian pancreatic polypeptide fragments refold to native aPP conformation when combined in solution: A CD and VCD study

An equimolar mixture of avion pancreatic polypeptide (aPP) fragments aPP(1-11)-NH₂ and Ac-aPP(12-36) had an electronic circular dichroism (ECD) spectrum that was similar to that of whole aPP in H₂O and even more so in 30% (v/v) trifluoroethanol (TFE) in 15 mM Na₂HPO ₄, but was different from the sum of the spectra of the individual fragments. The vibrational circular dichroism (VCD) spectrum of the combined fragments in 30% (v/v) TFE in 15 mM Na₂HPO₄ in D ₂O was also similar to that of the intact aPP and unlike the sum of the VCD spectra of the fragments. The interaction of these fragments is thus sufficient to support the conformation of whole aPP. This study demonstrates that VCD, in combination with ECD, is useful for the study of protein-protein interactions.