Vacuum ultraviolet light provides clues for ultrafast amide dynamics

Amide motifs abound in biomolecules, being found in systems such as amino acids, DNA bases and plant phenylpropanoids. One lesser-understood feature of amides is their resistance to ultraviolet radiation damage. New work using vacuum ultraviolet (VUV) light looks to offer clues to the dynamics of excess energy redistribution in this class of molecules.

Larsen et al. demonstrated a technique for studying the ultrafast excited state dynamics of formamide and two of its methyl-substituted analogues. Using time-resolved photoelectron spectroscopy in conjunction with quantum chemistry calculations, the authors provide a view of the energy redistribution processes following 160 nm VUV excitation.

Of particular interest to the study was the role of mixed Rydberg/valence states, which have been proposed as important dynamical doorways for non-adiabatic radiationless transitions back to ground state. In their experiment, the authors used an initial pulse as a “pump” to excite the molecules followed by a second “probe” pulse to remove an electron, effectively make a movie showing the energy redistribution in real time.

All three amides exhibited ultrafast processes occurring in less than 100 femtoseconds, which the authors argue is then followed by dissociation. Calculations indicated subtle differences in how the excited state dynamics were mediated in methyl-substituted species when compared directly to formamide.

The authors suggest that longer VUV pump laser wavelengths will help reveal more about these differences and the role of amide functionality in resistance to UV damage. They hope their work will also help drive innovation toward improved bench-top VUV light sources.

Source: “Vacuum ultraviolet excited state dynamics of small amides,” by Martin A. B. Larsen, Theis I. Sølling, Ruaridh Forbes, Andrey E. Boguslavskiy, Varun Makhija, Kévin Veyrinas, Rune Lausten, Albert Stolow, Magdalena M. Zawadzki, Lisa Saalbach, Nikoleta Kotsina, Martin J. Paterson, and Dave Townsend, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5079721 .