On the photostability of the disulfide bond: An electronic or a structural property?

Photostability is an essential property of molecular building blocks of nature. Disulfides are central in the structure determination of proteins, which is in striking contradiction to the result that the S-S bond is a photochemically labile structural entity that cleaves to form free radicals upon light exposure. In an earlier contribution we hypothesized that the key to the photostability of some disulfides may be found in a cyclic structural arrangement. Here we provide further evidence to support this hypothesis by showing that straight chain disulfides undergo ultrafast S-S dissociation on a sub 50 fs timescale without further ado. In a cyclic motif resembling the cysteine-disulfide bond in proteins, light can perturb the S-S bond to generate short-lived diradicaloid species, but the sulfur atoms are conformationally restricted by the ring that prevents the sulfur atoms from flying apart. Conversely, in a straight chain conformation, light perturbation results in two separated R S · radicals because there is no restoring force to counteract the repulsive motion of the sulfur atoms. For the cyclic conformation this restoring force is provided by the cyclic framework, and thus the photostability of disulfide-bonds must be ascribed a cyclic structural arrangement.