In this Letter, we investigate the link between thermal denaturation and mechanical unzipping for two models of DNA, namely the Dauxois-Peyrard-Bishop model and a variant thereof we proposed recently. We show that the critical line that separates zipped from unzipped DNA sequences in mechanical unzipping experiments is a power-law in the temperature-force plane. We also prove that for the investigated models the corresponding critical exponent is proportional to the critical exponent α, which characterizes the behaviour of the specific heat in the neighbourhood of the critical temperature for thermal denaturation.(#) email : Marc.Joyeux@ujf-grenoble.fr
-INTRODUCTIONSeveral fundamental biological processes, like transcription and replication, involve the separation and recombination of the two strands that compose double-stranded (zipped) DNA molecules. Since transcription and replication are quite complex mechanisms, many early studies focused on thermal denaturation, that is the separation of the two strands upon heating [1][2][3][4][5][6][7]. In cells, unzipping is however not mediated by thermal activation but rather by proteins, which apply forces to separate and stretch complementary DNA strands. Compared to thermal denaturation studies, the recent mechanical unzipping experiments, where forces are applied to adjacent 5' and 3' strands of individual DNA molecules [8][9][10][11][12][13][14][15][16], therefore represent a great step towards the understanding of real biological processes. These experiments triggered in turn much theoretical effort (see [17][18][19][20][21][22][23][24][25][26][27] and references therein).The purpose of the present paper is to point out that there actually exists a tight link between thermal denaturation and mechanical unzipping. Indeed, we will show that for the two 1-dimensional mesoscopic DNA models we investigated it is possible to establish a mapping between the order of the thermal denaturation phase transition and the shape of the critical line of mechanical unzipping in the temperature-force plane. More precisely, we will show that the critical line that separates zipped from unzipped sequences in mechanical unzipping experiments is a power-law, whose exponent is proportional to the critical exponent α that characterizes the behaviour of the specific heat in the neighbourhood of the critical temperature for thermal denaturation. This work therefore complements that of Singh and Singh, who showed that the critical force varies as the square root of the temperature gap to denaturation for the Dauxois-Peyrard-Bishop model with a large anharmonic stacking parameter ρ [26]. The coupling constant of this interaction drops from ) 1 ( ρ + K to K as the paired bases separate, which decreases the rigidity of DNA sequences close to dissociation and results in a sharp first-order transition. Numerical values of the parameters used in this work are those of Ref. [26], that is D=0.063 eV, a=4.2 Å -1 , K=0.025 eV Å -2 , α=0.35 Å -1 , except that we