Hydrogen Exchange of Individual Amide Protons in the F Helix of Cyanometmyoglobin†

Hydrogen exchange of the individual amide protons of alanine-90 (F5), glutamine-91 (F6), serine-92 (F7), and histidine-93 (F8) residues in cyanometmyoglobin of sperm whale has been studied by 1H nuclear magnetic resonance spectroscopy at 360 MHz. The amide proton resonances of F5, F6, and F7 have been assigned by use of the selective nuclear Overhauser effect between the consecutive amide protons. At pH 6.8, and in the temperature range of 5-20 °C, these protons show a 104-fold retardation compared to the rates in free peptides. Apparent activation enthalpies for hydrogen exchange of F5, F6, and F8 protons are 18.5 ± 0.4, 9.5 ± 0.3, and 18.5 ± 0.3 kcal/mol, respectively. Some implications of these results on the nature of the opening processes involved in hydrogen exchange are considered.