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.