Vibrational Coupling on Stepwise Hydrogen Bond Formation of Amide I

Despite the key roles of proteins and nucleic acids in biology, understanding their labile structures and hydrogen bond interactions with guest molecules has posed a critical challenge to the scientific community. In this report, I have used dimethylformamide as a model amide to account for amide hydrogen bond interactions of protein. To quantify hydrogen bond conformation and the structural change, I have monitored the amide I infrared (IR) stretching frequencies while varying the pK_a of phenol derivatives. For all phenol derivatives, amide I has formed one hydrogen bond and two hydrogen bond conformation. It has been observed that the formation constant for one hydrogen bond is higher than that of two hydrogen bonds for all phenol derivatives. During the formation of hydrogen bond with amide I, IR absorbance of C═C transition is enhanced for all phenol derivatives. Enhancement of the IR absorbance of the C═C transition indicates hydrogen bond-assisted vibrational coupling between the amide I and phenol ring transition. The relative coupling constant is estimated to be higher for single hydrogen-bonded conformer than the double hydrogen-bonded conformer. This is an intriguing result as the frequency difference between the two coupled transitions predicts otherwise. Using IR absorption spectroscopy, a delicate interplay between hydrogen bonding conformations and intermolecular vibrational coupling between amide I and H-bond donor phenol molecules has been shown. This study can be used as a point of reference for understanding the structural information of proteins, peptides, and nucleosides having hydrogen bond interaction with any drug or ligand molecules. My results as well provide an insight into the vibrational coupling of carbonyl and C═C transition of nucleobases.