Poly(acrylic acid)-block-poly(L-valine):: Evaluation of β-sheet formation and its stability using circular dichroism technique

Secondary structure formation in four novel hybrid poly(acrylic acid)-b-poly(L-valine) (PAA-b-PLVAL) block copolymers, that is, PAA(40)-PLVAL(100), PAA(80)-PLVAL(100), PAA(80)-PLVAL(80), and PAA(80)-PLVAL(60), was investigated by circular dichroism. The formation of stable and well-defined beta-sheet structure in the PLVAL hydrophobic domains was observed for all the copolymers. At pH 5, PAA(80)-PLVAL(60) With the lowest PLVAL/PAA molar ratio possessed the lowest P-sheet content of 12%, and it increased to 62% for PAA(40)-PLVAL(100) system. The beta-sheet formation in the block copolymers was controlled by both random PAA-PLVAL hydrogen bonds at low pH and electrostatic repulsive forces on the PAA segment at high pH; hence, the P-sheet structure was most stable at intermediate pH. The length of PAA segments was critical in the beta-sheet solubilization and in providing sufficient shielding of the hydrophobic core from denaturing agents such as urea.