Abstract:

In this work, we investigate the effect of the charge-block length on the chain conformation and phase behavior of peptide polyampholytes (PAs) with equal numbers of cationic and anionic ionizable monomers. Peptide PAs were produced using solid-phase peptide synthesis (SPPS), which enables precise control of the molecular weight, monomer sequence, and chain tacticity. Using circular dichroism (CD) and Fourier-transform infrared (FTIR) spectroscopy, we observe that the atactic peptide PAs show no evidence of secondary structure formation regardless of block length, whereas isotactic peptide PAs exhibit block-length-dependent secondary structures. Turbidity measurements show that the isotactic peptide PAs phase separate with increasing block length. In contrast, the atactic peptide PAs remain in solution over a wide-range of salt concentrations.  Small-angle X-ray scattering (SAXS) measurements reveal that the chain conformation of peptide PAs is compressed compared to expectations for neutral systems. Interestingly, we find that the atactic peptide PAs form block-length-dependent multichain clusters in solution. These results are consistent with a recent theory predicting that PAs have a block-length- and concentration-dependent threshold for phase separation.