Distinguishing gas phase lactose and lactulose complexed with sodiated L-arginine by IRMPD spectroscopy and DFT calculations.

Abstract

We present the origin of the observed differentiation of lactose and lactulose achieved by complexation with sodiated L-arginine (ArgNa+). We find that the infrared multiphoton dissociation (IRMPD) bands in 3600-3650 and >3650 cm-1 regimes for gas phase lactose and lactulose, respectively, vanish when forming host-guest complexes with ArgNa+. We interpret these differences in the IRMPD spectra by scrutinizing the interactions between the functional groups (guanidium, -CO2-Na+) in ArgNa+ and -OHs in lactose/lactulose. Our calculated structures and infrared spectra of lactose/ArgNa+ and lactulose/ArgNa+ host-guest pairs indicate that the functional groups interact with the low- and high-frequency -OH stretch modes of lactose and lactulose, respectively, in the 3600-3720 cm-1 window.