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

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 16 (2011) pp 18-22

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

Joseph. K. Rugutt^†,*, Elliot Charobee^†, Michael Anderson^†, Nicole Christian^†, and Kipngeno J. Rugutt^‡

†Department of Chemistry, Columbus State University, 4225 University Street, Columbus, GA 31907, ‡Department of Education, Illinois State University, Normal, IL 61790-5300, rugutt_joseph@colstate.edu
Received August 6, 2010. Accepted September 23, 2010.

Published: 4 February 2011

Abstract. 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 (Ha) 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 Ha of amino acid residues, Val, Leu, Ala, and Gly, in DIPEP structures. The Ha 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 Ha protons of the N-terminal amino acids resonated at higher fields than in the micellar state. In contrast, a downfield shift of the Ha proton was observed at micellar states. The differences in Ha and NH chemical shifts and coupling constants (³J_HNC_aH) of amino acids in the dipeptide backbones are valuable conformational probes.

Key Words: In the Classroom; organic chemistry

(*) Corresponding author. (E-mail: rugutt_joseph@colstate.edu)

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