Carbohydrate-Aromatic Interactions: Vibrational Spectroscopy and Structural Assignment of Isolated Monosaccharide Complexes with p-Hydroxy Toluene and N-Acetyl L-Tyrosine Methylamide

The nature of carbohydrate binding first to p-hydroxy toluene and then the capped amino acid, N-acetyl L-tyrosine methyl amide (AcTyrNHMe), has been investigated in a solvent-free environment under molecular beam conditions. A combination of double resonance IR-UV spectroscopy and quantum chemical calculations has established the structures of complexes with the alpha and beta anomers of methyl D-gluco- and D-galacto- and L-fucopyranosides (alpha/beta MeGlc, MeFuc MeFuc). The new results, when combined with dispersion-corrected DFT calculations, reveal gas phase structures which are dominated by hydrogen bonding but also with evidence of CH-pi bonded interactions in complexes with alpha/beta MeGal. These adopt stacked intermolecular structures in marked contrast to those with alpha/beta MeGlc; p-OH -> O bonds linking AcTyrNHMe, or p-hydroxy toluene, to the carbohydrate provide an anchor that facilitates further binding, both rough OH -> O and NH -> O hydrogen bonds to the peptide backbone and through CH-pi dispersion interactions with the aromatic side group Stacked structures. associated with dispersion interactions with the aromatic ring are not detected in the corresponding complexes of capped phenylalanine, despite their common occurrence in bound carbohydrate-protein structures.