Observation of Conformational Simplification upon N-Methylation on Amino Acid Iodide Clusters

This Letter reports a counterintuitive observation that methylation of the glycine-iodide cluster leads to fewer conformations and spectroscopic simplicity. Cryogenic "iodide-tagging" negative ion photoelectron spectroscopy (NIPES) is used to probe specific binding sites of three N-methylated glycine derivatives, i.e., N-methylglycine (sarcosine), N,N-dimethylglycine, and N,N,N-trimethylglycine (glycine betaine). NIPES reveals a progressive spectral simplification of the iodide clusters with increasing methylation due to fewer contributing structures. Low energy conformers and tautomers of each cluster are computationally identified, and those observed in the experiments are assigned based on excellent agreement between the NIPE spectra and theoretical simulations. Zwitterionic cluster structures are found to be less stable than their canonical forms and do not contribute to the observed spectra. This work demonstrates the power of iodide-tagging NIPES in probing conformations of amino acid-iodide clusters and provides a molecular level understanding on the effect of methyl substitution on amino acid binding sites.