Influence of Charge Block Length on Conformation and Cluster Formation of Atactic Peptide Polyampholytes

We investigate the effect of the charge block length on the chain conformation and phase behavior of atactic peptide polyampholytes (PAs) with equal numbers of cationic and anionic ionizable monomers. Atactic peptide PAs were produced by 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. Turbidity measurements show that 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 atactic 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.