1H NMR studies on the preferred interactions of guanidinium and C-terminal carboxylate groups in arginine-containing peptides
Interactions between positively charged side-chain residues and C-terminal carboxylate groups are of great importance in stabilizing secondary structures of many biologically active peptide sequences. The conformational properties of the hexapeptide sequence L-Arg-L-Val-Gly-L-Arg-L-Val-Gly with an Arg residue located at the third position from the carboxy terminus and a second Arg moiety at the amino terminus of the molecule, have been investigated in DMSO solution at pH 6.5, by means of 1D and 2D 1H NMR spectroscopy. The considerable downfield shift of the Arg4NÎµH resonance, when the pH of the solution is raised from 2 to 6.5, and the magnetically non-equivalent CβH and Cδ H methylene protons of the same residue at pH 6.5 provide evidence for a non-covalent hydrogen-bonding interaction between Arg4 guanidinium and C-terminal carboxylate groups. The intense backbone–backbone NOE connectivities observed for the L-Arg4-L-Val5-Gly6 C-terminal sequence, the NOE connectivities of Arg4 methylene protons and the retained ABX system of Gly6 C-terminal residue at high temperature are evidence for some mobility restrictions around this part of the molecule, and provide additional confirmation for a particular propensity of the arginine side chain, when occupying a position near the carboxy terminus, to interact with the C-terminal carboxylate group at neutral pH. Absence of ROE connectivities between protons of the Arg1 side chain, and the fact that almost all methylene protons of the same residue appear magnetically equivalent, point out that this Arg moiety does not participate in any intramolecular interaction. Thus, differences in the behaviour of guanidinium side chains of the reported peptide sequence can be used as a general probe for the study of Arg side-chain mobility and for its ability to generate hydrogen-bonding interactions in other peptides and proteins.