Proton NMR studies of the aromatic residues in the basic pancreatic trypsin inhibitor (BPTI)

Abstract

The aromatic region of the 1H NMR spectrum of the basic pancreatic trypsin inhibitor (BPTI), which contains the resonances of four tyrosyl and four phenylalanyl residues, was investigated at 360 MHz and 270 MHz. With double resonance experiments, three pairs of coupled tyrosine 3,5- and 2,6-proton resonances could be identified, showing that the rotations of the aromatic rings of these tyrosines about the axis given by the C β-C γ bond are rapid on the NMR time scale. On the other hand, the globular protein conformation is clearly manifested in the chemical shifts of the aromatic protons of the tyrosines. With the available evidence, it appears unlikely that these conformation dependent chemical shifts could be caused by interactions with the ring current fields of neighboring aromatic residues, which emphasizes the importance of alternative mechanisms effective in producing conformation dependent chemical shifts in globular proteins.