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Using the stopped-flow circular dichroism (SFCD) technique,we investigate the kinetics of pH-induced folding and unfolding processes of DNA i-motif.It is.found that the molecule can fold or unfold at a timescale of 100 ms when the solution pH is changed and that the folding and unfolding rates strongly depend on the solution pH.Based on quantitative data,we propose a model to decipher the folding and unfolding kinetics.Our model suggests that cooperativity of protons is crucial for either folding or unfolding process.In the unfolding pathway,cooperative neutralization of two protons is the only rate-limiting step.In the folding process,there are three protons cooperatively binding to the DNA strand in the rate-limiting step.These results offer in-depth understanding of the folding and unfolding kinetics of the DNA i-motif and may give precise guidance to construct novel nanostructures.