Photoswitchable distance constraints in the
form of photoisomerizable chemical cross-links offer a general
approach to the design of reversibly photocontrolled proteins.
To apply these effectively, however, one must have guidelines
for the choice of cross-linker structure and cross-linker
attachment sites. Here we investigate the effects of varying
cross-linker structure on the photocontrol of folding of the Fyn SH3 domain, a well-studied model protein. We develop a theoretical framework based on an explicit-chain model of protein folding, modified to include detailed model linkers, that allows prediction of the effect of a given linker on the free energy of folding of a protein. Using this framework, we were able to quantitatively explain the experimental result that a longer, but somewhat flexible, cross-linker is less destabilizing to the folded state than a shorter more rigid cross-linker. The models also suggest how misfolded states may be generated by cross-linking, providing a rationale for altered dynamics seen in nuclear magnetic resonance analyses of these proteins. The theoretical framework is readily portable to any protein of known folded state structure and thus can be used to guide the design of
photoswitchable proteins generally.