Stretching single-domain proteins: Phase diagram and kinetics of force-induced unfolding

Single-molecule force spectroscopy reveals unfolding of domains in titin on stretching. We provide a theoretical framework for these experiments by co mputing the phase diagrams for force-induced unfolding of single-domain pro teins using lattice models. The results show that two-state folders (at zer o force) unravel cooperatively, whereas stretching of non-two-state folders occurs through intermediates. The stretching rates of individual molecules show great variations reflecting the heterogeneity of force-induced unfold ing pathways. The approach to the stretched state occurs in a stepwise "qua ntized" manner. Unfolding dynamics and forces required to stretch proteins depend sensitively on topology. The unfolding rates increase exponentially with force f till an optimum value, which is determined by the barrier to u nfolding when f = 0. A mapping of these results to proteins shows qualitati ve agreement with force-induced unfolding of Ig-like domains in titin. We s how that single-molecule force spectroscopy can be used to map the folding free energy landscape of proteins in the absence of denaturants.