Block Sequence Effects on Phase Behavior, Oligomerization, and Conformation of Racemic Polyampholyte Peptides

Polyampholytes (PAs) are polymers containing both positively and negatively ionizable groups along their backbone. Using solid phase synthesis, we synthesized a set of 5 PA peptides consisting of 16 glutamic acid and 16 lysine residues arranged in increasing blocks sizes from 1 residue (alternating sequence) to 16 residues (diblock sequence). Our previous study on PA peptides consisting of all L-chiral residues demonstrated sequence-dependent secondary structure formation; thus, complicating determination of eletrostatic contribution to PA conformation and phase behavior. To minimize hydrogen bonding, we redesigned our PAs to consist of randomly ordered D,L-chiral residues instead. From FTIR and CD measurements, no secondary structure was detected. Similar to L-chiral PAs, increasing block size increased phase separation; however, liquid-liquid phase separation was observed instead of liquid-solid phase separation. We also observed increasing oligomerization of the PA peptides with increasing block size. Through SAXS, we detail the effect of block size on the conformation of PA assemblies and how block size be used to tune inter- and intra-molecular electrostatic interactions.