Simulated refolding of stretched titin immunoglobulin domains

Steered molecular dynamics (SMD) is used to investigate forced unfolding an d spontaneous refolding of immunoglobulin 127, a domain of the muscle prote in titin. Previous SMD simulations revealed the events leading to stretch-i nduced unfolding of 127, the rupture of hydrogen bonds bridging beta -stran ds A and B, and those bridging beta -strands A' and G, the latter rupture o ccurring at an extension of similar to 15 Angstrom and preceding the comple te unfolding. Simulations are now used to study the refolding of partially unfolded 127 domains. The results reveal that stretched domains with ruptur ed interstrand hydrogen bonds shrink along the extension direction. Two typ es of refolding patterns are recognized: for separated beta -strands A' and G, in most simulations five of the six hydrogen bonds between A' and G sta bly reformed in 2 ns, whereas for separated beta -strands A and B hydrogen bonds seldom reformed in eight 2-ns simulations. The mechanical stability o f the partially refolded intermediates has been tested by re-stretching.