UV light and peptides hit a triplet

Our bodies are full of proteins and their smaller cousins, peptides, both made up of chains of amino acids. And dangling off to one side of many of those amino acids is a so-called aromatic ring—a benzene-like ring of six carbon atoms. When a peptide containing an aromatic amino acid absorbs UV light, it has plenty of energy to break its bonds and fly apart—perhaps initiating events that can lead to skin cancer or cataracts—but it usually doesn’t. A group of physical chemists at the Swiss Federal Institute of Technology in Lausanne wanted to know whether those molecules survive on their own or whether they need help. With gas-phase peptides suitably isolated, the researchers first promoted the biomolecules to the excited singlet state with a UV laser. They then induced fragmentation with appropriate IR pulses. Through a systematic spectroscopic study involving a number of molecular resonances and pulse timings, they discovered that in just a few nanoseconds—before the molecules can dissipate their energy by returning to the ground state—a significant fraction of them are unexpectedly shunted aside into a long-lived triplet state. Left on their own, those very triplet states, persisting for tens of milliseconds, could cause lasting biological damage. Fortunately for us, peptides in our bodies are not isolated; they somehow get help from surrounding molecules in shedding their excess energy … but additional sunscreen never hurts. (A. V. Zabuga et al., J. Chem. Phys., in press, doi:10.1063/1.4882059 .)