Direct Analysis of Backbone–Backbone Hydrogen Bond Formation in Protein Folding Transition States

Here we investigate the role of backbone–backbone hydrogen bonding interactions in stabilizing the protein folding transition states of two model protein systems, the B1 domain of protein L (ProtL) and the P22 Arc repressor. A backbone modified analogue of ProtL containing an amide-to-ester bond substitution between residues 105 and 106 was prepared by total chemical synthesis, and the thermodynamic and kinetic parameters associated with its folding reaction were evaluated. Utlimately, these parameters were used in a Φ-value analysis to determine if the native backbone–backbone hydrogen bonding interaction perturbed in this analogue (i.e. a hydrogen bond in the first β-turn of ProtLʹs β–β–α–β–β fold) was formed in the transition state of ProtLʹs folding reaction. Also determined were the kinetic parameters associated with the folding reactions of two Arc repressor analogues, each containing an amide-to-ester bond substitution in the backbone of their polypeptide chains. These parameters were used together with previously established thermodynamic parameters for the folding of these analogues in Φ-value analyses to determine if the native backbone–backbone hydrogen bonding interactions perturbed in these analogues (i.e. a hydrogen bond at the end of the intersubunit β-sheet interface and hydrogen bonds at the beginning of the second α-helix in Arc repressorʹs β–α–α structure) were formed in the transition state of Arc repressorʹs folding reaction. Our results reveal that backbone–backbone hydrogen bonding interactions are formed in the β-turn and α-helical transition state structures of ProtL and Arc repressor, respectively; and they were not formed in the intersubunit β-sheet interface of Arc repressor, a region of Arc repressorʹs polypeptide chain previously shown to have other non-native-like conformations in Arcʹs protein folding transition state.