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, -CO 2 - 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.