Photoinitiated intramolecular diradical cross-linking of polyproline peptides in the gas phase

Polyproline is a fascinating polymer with interesting structural properties that have been studied in both solution and the gas phase. Herein, a method capable of measuring structural dynamics over long timescales is developed and applied to examination of polyproline in the gas phase. This method is based on measuring the probability of two radicals recombining to form a new covalent bond within a single molecule, which provides distance constraint information. To examine polyproline peptides of various lengths, radical precursors were selectively placed at the termini. Photoactivation with 266 nm light can then be used to create a diradical species, and recombination of the two radicals can be used to evaluate end-to-end distances and structural flexibility. The results reveal that interaction of the polyproline termini is more favorable for shorter chain lengths and lower charge states. As charge states increase, Coulombic repulsion favors formation of more extended structures where the termini no longer come in close contact. With increasing chain length, the greater conformational space also appears to decrease the likelihood of the termini being able to recombine. The amount of radical recombination observed for short polyproline peptides in low charge states is not consistent with what would be expected for helical conformations. Rather, molecular mechanics calculations reveal that lower charge state polyproline peptides tends to adopt globular conformations in the gas phase.