title: 1H-n.m.r. study of the folding of ribonuclease 12-(beta-(3-pyridyl)-L-Ala) S-peptide (1-14).
abstract: The 1H-n.m.r. spectra (360 MHz) of 12-(beta-(3-pyridyl)-L-Ala) ribonuclease S-peptide (1-14), a tetradecapeptide incorporating (beta-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) in equilibrium coil equilibrium, and the corresponding values for the thermodynamic quantities delta H degrees and delta S degrees determined. Helix populations at 0 degrees C have been measured as a function of pD, showing their dependence on two apparent pKa's at approximately 3.3 and 5.5, with a maximum at pD approximately 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 chi 81 180 degrees, chi 82 100 degrees, chi 121-60 and chi 122 80.
mesh_terms: D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D010446:Peptide Fragments; D011487:Protein Conformation; D012260:Ribonucleases; D013816:Thermodynamics
chemical_list: D010446:Peptide Fragments; C052256:ribonuclease S-peptide (1-14), 12-(beta-(3-pyridyl)-Ala)-; D012260:Ribonucleases