Thermodynamic parameters for the helix–coil thermal transition of ribonuclease‐S‐peptide and derivatives from ¹H‐nmr data

Abstract

Abstract Values for the thermodynamic quantities, Δ H ° = 11.8 ± 2.0 Kcal/mole and Δ S ° = 43.6 ± 6.0 e.u., of the 3‐13 helix–coil equilibrium of isolated S‐peptide (19 residue N‐terminal fragment of ribonuclease A) in aqueous solution (3 m M , 1 M NaCl, pD 5.4) have been determined from a joint analysis of the Thr 3γ, Ala 6β, Phe 8 meta , and Phe 8 para 1 H chemical shift vs temperature curves (−7 to 80°C) in several aqueous–trifluorethanol mixtures. Chemical shifts in the coil and in the helix have been determined for up to 16 protons belonging to the 3‐13 fragment. Thermodynamic parameters have also been determined for C‐peptide (13 residue fragment) and a number of S‐peptide derivatives. From the variation of the values of the thermodynamic parameters at p D 2.5, 5.4, and 8.0, a quantitation of the two helix‐stabilizing side‐chain interactions can be made: (1) Δ(Δ H °) ≃ 5 Kcal/mole and Δ(Δ S °) ≃ 18 e.u. for the salt bridge Glu 2 − … Arg 10 + and (2) Δ(Δ H °) ≃ 3 Kcal/mole and Δ(Δ S °) = 9 e.u. for the one in which the His 12 + imidazolium group is involved, presumably a partial stacking with the Phe 8 side chain.