title: pH dependence of the high-resolution proton nuclear magnetic resonance spectrum of the lac repressor headpiece.
abstract: The pH dependence of the N-terminal 51 amino acid headpiece (HP) of the lac repressor has been followed by using 1H NMR spectroscopy to monitor the chemical shifts of resolved aromatic and methyl resonances. The NMR evidence reveals the folded HP domain to be stable from pH 1 to 10 at 23 degrees C. All observed resonances shift toward their expected random-coil positions above pH 10, which suggests that a general unfolding occurs. The four tyrosine rings reflect a combination of unfolding and titration in the order 7 greater than 17, 12 greater than 47. This pH-induced unfolding is completely reversible. In addition, strikingly similar pH behavior for selected tyrosine and methyl resonances at acid pH values suggests that clusters of various tyrosine, methyl, and carboxyl side chains exist in the native structure.
mesh_terms: D000596:Amino Acids; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D012097:Repressor Proteins; D014157:Transcription Factors; D014443:Tyrosine
chemical_list: D000596:Amino Acids; D012097:Repressor Proteins; D014157:Transcription Factors; D014443:Tyrosine; D006639:Histidine