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

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.