Premelting transition

The temperature dependence of the chemical shifts of the base and sugar resonances of poly(dA-dT) in 0.1 M phosphate buffer is plotted in Figure 3. There are upfield and downfield shifts associated with the noncooperative premelting transition between 5 and 55°C while only downfield shifts are observed for most of the base and sugar protons on raising the temperature above 65°C in the noncooperative postmelting transition temperature range. 

There is no information to our knowledge on premelting transitions in RNA duplexes. We have therefore investigated the NMR parameters for the related synthetic RNA duplex poly(A-U) as a function of temperature. Typical 360 MHz aromatic proton region spectra for the poly(A-U) duplex in 0.1 M phosphate (tj, = 66.5°C) at 8.2° and 44.5°C are presented in Figure 7. It is clear that the chemical shifts of the base protons vary with... 

Summary The thymidine H-3 hydrogen-bonded proton is well shielded from solvent in the premelting transition region of poly(dA-dT) in 1 M TMA+ solution based on an analysis of its line width as a function of pH and temperature. This result is consistent with parallel stacking of base pairs in the duplex state of the synthetic DNA in TMA+ solution. 

Clark et al. [30] considered premelting transitions in the context of a general theory of disorder in plastic crystals. Entropy changes were attributed to contributions from two sources increasing rotation of the molecular constituents of the crystal and the expansion of the lattice, against the van der Waals forces, necessary to accommodate this increased disorder. The observed changes were collated with molecular shapes. 

Absolute determination of AG is the most general method. From the experimentally determined enthalpies of formation of compounds (A// ) relative stabilities can easily be calculated (after consideration of AS, vide infra), but such experimental work requires large quantities (>1 g) of extremely pure material. Also, since the heat of formation of a compound is calculated from the difference of its measured heat of combustion and the heat of combustion of its constituent elements, quantities which can be in the 1000-4000 kcal mole" range, small experimental errors (>0.02%) lead to relatively large errors in AH (10). The measurement of heats of vaporization or of sublimation needed to convert A/f (l) and Af/ (s) to AH (g) are also difficult experimentally (11). Premelting transitions (5i, 12) complicate matters further. As a result, reported experimental values of AH for a given compound may differ by as much as several kilocalories per mole (e.g., adamantane and diamantane, see Table 2). 

As mentioned earlier, n-undecane exhibits a premelting transition at about 10 degrees below the melting temperature. The entropy change due to this solid-state (rotor phase) transition is reported to be 27-29 Jmol" K" [1]. The rotational isomeric (conformational) character of individual chains remains unperturbed by these low-temperature transitions. The physical meaning of this entropy should cause controversial arguments for and against... 

Figure 8 UV melting curves for 200 gM calf thymus DNA in 10 mM salt (denoted DNA), and also with 20 gM spermine and 20 gM anthracene-9-carbonyl-N -spermine (denoted anthsp). Note the premelting transition with anthracene-9-carbonyl-W -spermine.

The Ge(lll)-(2xl) cleavage structure undergoes a similar activated transition at 100 °C to a c(2 X 8) structure. At 300 C, the c(2 x 8) experiences a reversible first-order phase transition to a (1x1) structure currently regarded as a premelting transition [48]. Finally, at 785 °C, the (1 x 1) surface layer melts reversibly [49-51] before the bulk crystal melts at 937 °C. 

The tetraalkylammonium iodides have also been examined by Levkov et For this series with four identical alkyl chains on the cation they show that the total entropy of up to three premelting transitions is constant for the chains with odd numbers of carbon atoms at about 8 eu, while that for even numbers increases with chain length, starting at 11 eu for the ethyl... 

---