Through Bonds or Contacts? Mapping Protein Vibrational Energy Transfer Using Non-canonical Amino Acids.

Vibrational energy transfer (VET) is essential for protein function. It is responsible for the efficient dissipation of excess energy after enzymatic reactions and photochemical processes, and has been linked to pathways of allosteric signal transduction. While it is understood that VET occurs via the backbone as well as via non-covalent contacts, little is known about the competition of these two transport channels, which determines the pathways of VET. To tackle this problem, we equipped the β-hairpin fold of a tryptophan zipper with pairs of non-canonical amino acids, one serving as a VET injector and one as a VET sensor in a femtosecond pump probe experiment. This is accompanied by extensive non-equilibrium molecular dynamics simulations combined with a master equation analysis to unravel the VET pathways. The joint experimental/computational endeavor reveals the efficiency of backbone vs. contact transport, showing that even if cutting short backbone stretches of only 3 to 4 amino acids in a protein, hydrogen bonds are the dominant VET pathway.