Probing conformational propensities of histidine in different protonation states of the unblocked glycyl-histidyl-glycine peptide by vibrational and NMR spectroscopy
David DiGuiseppi, Reinhard Schweitzer-Stenner
Journal of Raman Spectroscopy
Histidine has been shown to play a major role in a number of biological systems. Being able to understand unconstrained conformational distributions of histidine and their dependence on the environment can shed light on the structure–function relation with regard to its multiple roles in biochemical processes such as proton transfer, ligand binding, protein folding and maintaining protein intrinsic disorder. We utilized polarized Raman, FTIR, vibrational circular dichroism and 1H NMR spectroscopy to probe the conformational distribution of the central histidine in the unblocked tripeptide H-Gly-His-Gly-OH in D2O. Our results show that in the double protonated state (GHG++), 94% of the histidine residue resides in the upper left quadrant of the Ramachandran plot with an equal partition between polyproline II (pPII) and β-strand conformations. In this protonation state, enthalpic and entropic differences between the two conformations are practically eliminated, which indicates reduced backbone and/or side chain hydration. On the contrary, the single protonated state (GHG+), while only marginally different with regard to the pPII/β partition at room temperature (β-strand is now slightly favored), shows an enthalpic stabilization of pPII by 43.02 kJ/mol, which is being compensated by an entropic stabilization of β-strand (0.145 kJ/(mol K)). This indicates a much stronger coupling between peptide and water. At neutral pH, where the imidazole side chain of the histidine residue is deprotonated, GHG in D2O forms a hydrogel above a peptide concentration of approximately 25 mm. Some experimental evidence suggests that the preceding peptide aggregation involves hydrogen bonding between the C-terminal groups and the imidazole NH group.
Circular dichroism, Secondary structure, Biochemistry