Estimation of the circular dichroism exhibited by statistical coils of poly(L‐alanine) and unionized poly(L‐lysine) in water

Abstract

AbstractThe circular dichroism of Ac‐(Ala)x‐OMe and H‐Lys‐(Lys)x‐OH with x = 1, 2, 3, and 4 has been measured in aqueous solutions. The oligomers with x = 4 show similar circular dichroism spectra in water when the lysyl amino groups are protonated, and they respond in similar fashion to heating and to sodium perchlorate. Both oligomers at 15°C exhibit a positive circular dichroism band at 217–218 nm, which is eliminated by the isothermal addition of 4 M sodium perchlorate or by heating. The positive circular dichroism of the lysine oligomer is also eliminated when the pH is elevated to deprotonate the amino groups. Positive circular dichroism is still observed for Ac‐(Ala)4‐OMe at elevated pH. Circular dichroism spectra have been estimated for poly(L‐alanine) and poly(L‐lysine) as statistical coils under the above conditions, based on the trends established with the oligomers. Poly(L‐lysine) and poly(L‐alanine) are predicted to exhibit similar circular dichroism behavior in aqueous solution so long as the lysyl amino groups are protonated. The circular dichroism of the statistical coil of poly(L‐lysine), but not poly(L‐alanine), is predicted to change when the pH is elevated sufficiently to deprotonate the lysyl amino groups. These results suggest that the unionized lysyl side chains participate in interactions that are not available to poly(L‐alanine). Hydrophobic interactions may occur between the unionized lysyl side chains. Protonation of the lysyl amino groups is proposed to disrupt these interactions, causing poly(L‐alanine) and protonated poly(L‐lysine) to have similar circular dichroism properties.