Design, Synthesis and Characterization of a Series of Self-assembling Polypeptides
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A series of polypeptides with well-defined sequences, (Asp3Phe1)n, (Asp2Phe1)n, (Asp1Phe1)n, (Asp1Phe2)n, and (Asp1Phe3)n, containing the hydrophilic amino acid, aspartic acid (Asp) and the hydrophobic amino acid, phenylalanine (Phe), were synthesized. Their behaviour in aqueous solution was investigated by performing fluorescence quenching and non-radiative energy transfer (NRET) experiments which were complemented by dynamic (DLS) and static (SLS) light scattering. The photophysical properties of the polypeptides were dependent on their Phe content. An increase in the Phe content led to an increase in the extinction coefficient, fluorescence quantum yield, and fluorescence average lifetime of the polypeptides. Circular dichroism experiments revealed that except for the (Asp1Phe3)n polypeptide, which adopts an alpha-helical conformation in aqueous solution, the other polypeptides did not adopt any known conformation in solution. The fluorescence quenching studies performed using molecular pyrene physically bound to the polypeptide via hydrophobic interactions resulted in protective quenching for the (Asp1Phe1)n, (Asp1Phe2)n, and (Asp1Phe3)n polypeptides, with some pyrenes having a lifetime of ~300 ns. Evidence for protective quenching was also observed in the polypeptides richer in Phe, when pyrene was covalently attached onto the polypeptides. However pyrene was found to be fully exposed to the quencher solution for the more hydrophilic polypeptides. The presence of NRET between a naphthalene labeled polypeptide and a pyrene labeled polypeptide for the (Asp1Phe2)n and (Asp1Phe3)n polypeptides and its absence for the (Asp3Phe1)n, (Asp2Phe1)n, and (Asp1Phe1)n polypeptides led to the conclusion that those polypeptides richer in Phe generate interpolymeric aggregates which provide protection to a hydrophobic cargo like molecular pyrene whereas the hydrophilic polypeptides exist predominantly as unimolecular micelles. These results were confirmed by DLS and SLS experiments.