Slow folding of cross-linked alpha-helical peptides due to steric hindrance.

The folding process of a 16-residue alpha-helical peptide with an azobenzene cross-linker (covalently bound to residues Cys3 and Cys14) is investigated by 50 molecular dynamics simulations of 4 micros each. The folding kinetics at 281 K show a stretched exponential behavior but become simpler and much faster when a distance restraint is used to emulate a nonbulky cross-linker. The free-energy basin of the helical state is divided into two subbasins by a barrier that separates helical conformations with opposite orientations of the Arg10 side chain with respect to the azobenzene cross-linker. In contrast, such barrier is not present in the helical basin of the peptide with the nonbulky cross-linker, which folds with speed similar to the unrestrained peptide. These results indicate that the cross-linker slows down folding because of steric hindrance rather than its restraining effect on the two ends of the helical segment.