Magnetic non-equivalence and dynamic NMR of N-methylene protons in a Histamine-containing pseudopeptide: Alanyl-Glycyl-Histamine

Abstract The objective of the present research is to explore the potential of an additional feature of proton 1D-NMR for the study of the properties of peptides in aqueous solution, namely the magnetic non-equivalence of N-methylene protons in linear acyclic peptides induced by an asymmetric carbon atom located nearby . For this exploratory study, we have chosen an l -Alanyl terminal residue adjacent to a Glycyl residue, itself grafted to an optically inactive terminal Histamine unit, i.e., the Histamine-containing pseudopeptide l -Alanyl - Glycyl-Histamine (AGHA). The pH - and temperature-dependence of magnetic non-equivalences of Glycyl and Histamine N -methylene protons was modeled as a weighted mean over four ionization states BH 3 , BH 2 , BH, B of AGHA molecules found in the pH range explored: pH −0.78 to 12. In parallel, this research has made it possible to assess the pK 's governing the speciation curve, the base- and acid-catalyzed proton exchange rates and the pK 's of amide bonds.