Simulated refolding of stretched titin immunoglobulin domains
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Steered molecular dynamics (SMD) is used to investigate forced unfolding and spontaneous refolding of immunoglobulin I27, a domain of the muscle protein titin. Previous SMD simulations revealed the events leading to stretch-induced unfolding of I27, the rupture of hydrogen bonds bridging b-strands A and B, and those bridging b-strands A9 and G, the latter rupture occurring at an extension of ;15 Å and preceding the complete unfolding. Simulations are now used to study the refolding of partially unfolded I27 domains. The results reveal that stretched domains with ruptured interstrand hydrogen bonds shrink along the extension direction. Two types of refolding patterns are recognized: for separated b-strands A9 and G, in most simulations five of the six hydrogen bonds between A9 and G stably reformed in 2 ns, whereas for separated b-strands A and B hydrogen bonds seldom reformed in eight 2-ns simulations. The mechanical stability of the partially refolded intermediates has been tested by re-stretching.