UV Photofragmentation and IR Spectroscopy of Cold, G-Type beta-O-4 and beta-beta Dilignol-Alkali Metal Complexes: Structure and Linkage-Dependent Photofragmentation

Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing beta-O-4 and beta beta linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T similar to 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li+ and Na+ cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive FranckCondon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol center dot Li+/Na+ complexes involving the beta beta linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol center dot Li+/Na+ complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.