Effect of polypeptide sequence on polypeptide self-assembly

This study represents a unique example where the self-assembly of five amphiphilic polypeptides was monitored as a function of their hydrophilic-to-lipophilic balance (HLB). The HLB was tuned by preparing a series of polypeptides with aspartic acid (Asp) and phenylalanine (Phe) according to the sequence (Asp(x)Phe(y)) where x and y vary from 1 to 3. The self-assembly of the (Asp(x)Phe(y))(n) polypeptides with M(w) values ranging from 8 to 12 K was studied in solution by fluorescence quenching and nonradiative energy transfer (NRET) fluorescence experiments. Dynamic (DLS) and static (SLS) light scattering studies were used to complement the results obtained. The fluorescence quenching experiments conducted with the chromophore pyrene used as a free probe, physically bound to the polypeptides via hydrophobic interactions, demonstrated that the chromophore was less exposed to the solution for the sequences having a higher hydrophobic character. Protective quenching was also observed for those polypeptides randomly labeled with pyrene where the phenylalanine content was high, whereas pyrene was found to be fully exposed to the quencher for the polypeptides having more hydrophilic sequences. NRET used to probe interpolymeric association was not observed between a naphthalene and a pyrene labeled polypeptide for the polypeptide sequences which were richer in aspartic acid, whereas energy transfer took place with the more hydrophobic polypeptides. This observation led to the conclusion that those sequences with a higher content of aspartic acid essentially generate unimolecular polymeric micelles, whereas those with a higher content of phenylalanine generate polymeric aggregates which offer protection from the solvent to their hydrophobic cargo. The presence of these polymeric aggregates was also confirmed by DLS and SLS studies which suggest that species 100-200 nm in diameter are present in solution.