Secondary Structure of Poly-l-arginine and Its Derivatives

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Abstract
The conformations of poly-Nω,ω′-dicarbobenzyloxy-L-arginine, poly-Nω-carbobenzyloxy-L-arginine, and poly-L-arginine hydrobromide were studied by means of their infrared spectra (IR), X-ray photographs, optical rotatory dispersions (ORD), and circular dichroisms (CD). The presence of the α-helical conformation in poly-Nω,ω′-dicarbobenzyloxy-L-arginine and poly-Nω-carbobenzyloxy-L-arginine was confirmed by X-ray photographs and by their IR spectra. The ORD and the CD curves of these polymers were found to be characteristic of the α-helical conformation. It was also found that these polypeptides have a random coil conformation in trifluoroacetic acid. In the (2-chloroethanol : trifluoroacetic acid) solvent system, the typical helix-coil transition of poly-Nω-carbobenzyloxy-L-arginine was observed at 20–30% trifluoroacetic acid. The ORD and the CD curves of poly-L-arginine showed that the polymer took a random coil structure in the pH range from 2.7 to 11.0. However, when the pH is increased from 11.0 to 11.5, the spectra gradually change into the α-helix structure. Moreover, the polypeptide predominantly took the α-helix structure at pH 12.5. From the ORD and CD data, the polypeptide can be considered to have a right-handed α-helix conformation.