A Guided Inquiry Approach Toward Understanding Structures of Chiral Dipeptides Using Amide and Alpha Protons as Conformational Sensors

The overarching goal of our on-going project funded by the National Science Foundation is to develop guided inquiry chiral science experiments for organic chemistry curriculum at Columbus State University. To better understand the structural basis of the chiral discrimination of aqueousN-undecylenyl dipeptide surfactants (DIPEP), one-dimensional proton nuclear magnetic resonance (1H NMR) was used to investigate the conformation adopted by the dipeptide backbone. A salient feature of the 1H NMR spectra of DIPEP surfactants is the extensive overlap of a large number of proton resonances in the region 0.5-3.5 ppm. The alpha-proton (Hα) resonances of amino acid residues, which have chemical shifts between 3.50 and 4.75 ppm, were observed, once the water (HOD) resonance had been eliminated by selective presaturation. In the monomeric and micellar states, significant differences were found between the chemical shifts of the amide (NH) and Hα of amino acid residues, Val, Leu, Ala, and Gly, in DIPEP structures. The Hα protons of N-terminal amino acids resonated at higher fields than the corresponding protons on the C-terminal. Because of fast rapid exchange with H2O, the NH resonances of Gly were not observed in spectra of some DIPEP surfactants. At monomeric states, the Hα protons of the N-terminal amino acids resonated at higher fields than in the micellar state. In contrast, a downfield shift of the Hα proton was observed at micellar states. The differences in Hα and NH chemical shifts and coupling constants (3JHNCaH ) of amino acids in the dipeptide backbones are valuable conformational probes.