Helix unwinding transition

The sequential and often cooperative disassembly of double-helical structure, occurring whenever the sample temperature exceeds the so-called melting temperature (Tm) for a given segment of DNA. Because of the low concentrations of intermediate states lying between helix and coil structures, the helix-coil transition can be approximated as a two-state, all-or-nothing process. See DNA Unwinding Kinetic Model for Small DNA... 

Helix-coil transition is beautifully used in PCR. Indeed, once new copies of DNA strands are produced tightly wound with their templates of the previous generation. To unwind them and to make them serve as templates once again, experimenter raises temperature, causing helices to undergo helix-coil transition and complementary strands to diffuse away from each... 

Theoretical investigations of DNA denaturation have attempted to explain the helix-coil transition through molecular models so that both a qualitative and quantitative understanding of the process can be derived. Current studies have been concerned with the unwinding mechanism of DNA, - the thermodynamic energies involved in stabilizing the helix, and base distribution information of DNA. " In recent years several review articles on the theory of denaturation have been published, and a book has now appeared. 

Basically, there are two different models that have been applied to the helix-coil transition of DNA, These models differ mainly in the treatment of a loop of broken bonds sandwiched between two helical segments. The modified Ising (MI) model considers the unwinding of the interior of a DNA molecule to take place in the same manner as the free ends. The loop entropy (LE) model accounts for a differpuce between the entropy of unbonded strands sandwiched between two helical sections and the entropy of unbonded strands at the end of the molecule. 

Another electrooptical mode is a transition of the scattering helical state, with random orientation of the helical axes into the transparent state where the helix is completely unwound [112-115]. According to (7.54), the threshold field for helix unwinding is E cx i.e., can be suflSciently small (< 0.5 Y/fiia) for high spontaneous polarization Pg. 

In CLCs with positive dielectric anisotropy, an electric field-induced cholesteric-nematic phase transition was theoretically predicted [45], [46] and experimentally observed [47], [48]. If the electric field E is applied perpendicular to the helix axis hot a CLC, the helix unwinds like in a magnetic field (Chapter 2). At sufficiently high field strengths, the homeotropic nematic structure is stabilized (Figure 6.3). The critical field strength E = Ecn depends on the pitch P, the dielectric anisotropy As, and the twist elastic constant K22 ... 

External field distortions in SmC and chiral SmC phases have been investigated [38], but the large number of elastic terms in the free-energy, and the coupling between the permanent polarization and electric fields for chiral phases considerably complicates the description. In the chiral smectic C phase a simple helix unwinding Fr6ede-ricksz transition can be detected for the c director. This is similar to the chiral nematic-nematic transition described by Eq. (83), and the result is identical for the SmC phase. Indeed it appears that at least in interactions with magnetic fields in the plane of the layers, SmC and SmC phases behave as two dimensional nematics [39]. 

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