Anomeric Study Reinterpreted

A computational study disputes the interpretation of previous work describing a peptide sensor that responds to differences between anomers caused by the anomeric effect. Anomers are sugar isomers that differ in the configuration at C1 of the sugar ring, and the anomeric effect is the energetic stability of one anomer relative to the other resulting from an electronegative substituent at C1. Earlier this year, Benjamin G. Davis and coworkers at the University of Oxford reported that the infrared spectrum of the peptide sensor with methyl-d-galactopyranoside reveals information about the shape and bonding of complexes with the two anomers (C&EN, Jan. 10, page 5). Yirong Mo of Western Michigan University and coworkers have now used computational methods to calculate the bond distances and vibrational frequencies of the previously reported peptide-sugar complex and those of modified complexes in which the electronegative substituent at C1 has been removed or the sugar’s endocyclic oxygen has been replaced with a methylene group (J. Am. Chem. Soc., DOI: 10.1021/ja205613x). Calculations with the modified complexes show shifts in the vibrational frequencies between the two anomeric forms, albeit of different magnitude than that observed with the original complex. Mo and coworkers propose that the observed spectral differences are the result of conformational structure differences rather than the anomeric effect.