Trifluoromethanesulphonic acid as an α-helix promoting agent for basic poly(α-amino acids)

Abstract

Poly-(L-arginine), (Arg) n, usually adopts an extended conformation in aqueous solution. In concentrated aqueous solutions of methane sulphonic acid (MSA) between 2 and 6 mol/l, however, the c.d. spectrum of the α-helix occurs. This α-helix promoting effect of MSA was not observed for poly-( l-lysine), (Lys) n, poly-( l-ornithine), (Orn) n and poly-(N-δ- trimethyl- l-ornithine ) , (Me 3Orn) n, and poly-(N,ϵ- trimethyl- l-lysine ) (Me 3Lys) n. But, if using aqueous solutions of trifluoromethanesulphonicacid (TFMSA), the basic polyamino acids (BPAA) mentioned above attain the α-helical conformation at concentrations below 0.5 mol TFMSA/l with the exception of (Me 3Orn) n. The α-helix promoting effect of TFMSA increases in the following order: (Orn) n < (Lys) n < (Me 3Lys) n < (Arg) n . The specific ellipticity |Φ| 331 of (Arg) n as a function of TFMSA concentration passes a sharp maximum in contrast to a very broad one in MSA solutions. It was found that values of the van't Hoff transition enthalpy ΔH νH obtained for (Arg) n in TFMSA and MSA are between 21 and 35 kcal depending on concentration and ≈ 12 kcal for (Lys) n in TFMSA solution.