Can re-entrance be observed in force induced transitions?

A large conformational change in the reaction co-ordinate and the role of the solvent in the formation of base-pairing are combined to settle a long standing issue related to the force induced transitions of DNA. Prediction of re-entrance i.e a strand goes to the closed state from the open state and again to the open state with temperature has not been observed so far experimentally. An experimental protocol in the constant force ensemble is being proposed for the first time that will enable the observation of the re-entrance behavior in the force-temperature plane. Our exact results for small oligonucleotide that forms a hairpin structure provide the evidence that re-entrance can be observed at the ambient condition. It took almost four decades to realize that unzipping of double stranded DNA (dsDNA) may be achieved with a force applied solely at one end instead of varying the temperature or pH of the solvent [1–5]. This became possible only because of recent advancement in experimental techniques (optical tweezers, atomic force microscope, magnetic tweezers etc.) which provided important information about elastic, structural and functional properties of DNA [6–12]. Furthermore, elucidated the mechanism involved in crucial processes like transcription and replication [1, 2, 13–15] . Consequently by using force as a thermodynamic variable, many theoretical and numerical studies were done establishing a wealth of definite falsifiable predictions [3, 5, 17–20]. Among these, a very important role was played by simple models which are amenable to analytical solution or very accurate numerical treatments [3,17,19]. Such models despite their simplicity, have been proved to be quite predictive and provided many important information about the cellular processes.) One of the most important predictions is the existence of a reentrance behavior [17–19] in the force-temperature plane at low temperature region. Notably now the prediction of re-entrance is not only confined to DNA but also for other bio-polymers [21–24]. However, such re-entrance has so far remained elusive in the experimental studies. One of the possible reasons for not observing re-entrance may be attributed to the fact that at low temperature most of the solvents would freeze, precluding any unzipF (b) (a)