title: Nature and locations of the most slowly exchanging peptide NH protons in residues 1 to 19 of ribonuclease S.
abstract: The locations have been found of the eight most slowly exchanging peptide protons in residues 1 to 19 of ribonuclease S. The resonance lines of these eight protons are resolved by proton magnetic resonance at 360 MHz when either S-peptide (residues 1 to 19) or peptide 1-15 is bound to S-protein (residues 21 to 124). Other peptide protons have been removed by exchange in the sample preparation [( 1H]S-peptide is added to deuterated S-protein in D2O), and also by exchange-out of the less protected protons in residues 1 to 19. At pH 5.1, 0 degrees C, there is a 100-fold difference in rates of exchange between the eight most protected protons and the less protected protons of S-peptide. The highly protected protons are protected 10(4)-fold compared to free S-peptide. The protected protons have been identified by 1H nuclear magnetic resonance after denaturing ribonuclease S in greater than or equal to 3 M-urea-d4, D2O, pH 2.3, -4 degrees C, followed by comparing the chemical shifts of the remaining eight protons with the known -NH spectrum of the free peptide, which has been assigned from the two-dimensional homonuclear correlated spectrum and by comparison with earlier work. The eight highly protected NH protons are localized in one segment, residues 7 to 14. All eight protons are H-bonded: those of residues 7 to 13 are H-bonded within the 3-13 alpha-helix and that of residue 14 is H-bonded to the beta-sheet. The NH proton of residue 16, which also is H-bonded to the beta-sheet, is not one of the highly protected protons. Both the N atoms of the eight NH groups and also the O atoms of their CO acceptor groups are shielded from solvent in most cases, according to the molecular area calculations of Finney (1978).
mesh_terms: D000577:Amides / Q000032:analysis; D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010455:Peptides / Q000032:analysis; D011489:Protein Denaturation; D011522:Protons; D012260:Ribonucleases; D013696:Temperature; D014508:Urea
chemical_list: D000577:Amides; D010455:Peptides; D011522:Protons; D014508:Urea; D012260:Ribonucleases; C022542:ribonuclease S