A gradual disruption of tight side-chain packing: 2D H-1-NMR characterization of acid-induced unfolding of CHABII

Little is known about the mechanism of the transition between native protei ns and partially folded intermediates. Complete assignments of 2D H-1-NOESY spectra of CHABII at 5 degrees C, pH 6.3, 5.5, 4.6 and 4.0, reveal that lo wering of pH results in an extensive but gradual disappearance of NOEs, imp lying a gradual disruption of tight side-chain packing. Moreover, a tertiar y packing core is identified at 5 degrees C and pH 4.0, characterized by pe rsistent long-range NOEs, Thus, we suggest that severe disruption of tight side-chain packing of CHABII can occur at a stage where its secondary struc ture and tertiary topology remain highly native-like.