1H-n.m.r. study of the folding of ribonuclease 12-(β-(3-pyridyl)-l-Ala) S-peptide (1-14)

The 1H‐n.m.r. spectra (360 MHz) of 12‐(β‐(3‐pyridyl)‐l‐Ala) ribonuclease S‐peptide (1–14), a tetradecapeptide incorporating (β‐3‐pyridyl‐l‐Ala) instead of His at position 12, have been assigned. The shift vs. temperature dependence has been analyzed at three different pD's in terms of a two‐state helix (3–13) ± coil equilibrium, and the corresponding values for the thermodynamic quantities ΔH° and ΔS° determined. Helix populations at 0°C have been measured as a function of pD, showing their dependence on two apparent pKa's at ˜ 3.3 and 5.5, with a maximum at pD ˜ 4.2. All the obtained results show that the new peptide has very similar folding properties to those shown by S‐peptide and particularly to those of C‐peptide. The 3–13 helix formed is stabilized by two interactions: a salt‐bridge Glu 2‐. Arg 10+ and a partial stacking between the aromatic rings of residues Phe 8 and His 12. Calculations involving ring current shifts and potential energies validate the possible existence of this latter interaction, which must present a local geometry defined by χ1X8 180°, χ2X8 100°, χ112 – 60 and χ212 80.