Thermal-induced reversibility

The reverse of this process, in which the same quantity of heat is added to the aqueous solution with spontaneous re-emergence (precipitation) of the salt crystal, is also fully compliant with the first law. However, such thermally induced reverse solvation has never been observed. 

Folmer-Andersen JF, Lehn J-M (2011) Thermoresponsive dynamers thermally induced, reversible chain elongation of amphiphilic poly(acylhydrazones). J Am Chem Soc 133 10966-10973... 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Fig. 9 Thermoresponsive dynamers. Top Generation of an amphiphilic poly(acyUiydrazone). Bottom Inverse thermal response to heat stimulation, with thermally induced, reversible size modification through large and reversible polymer growth in response to an increase in temperature...

The mechanism of trans-cis photoisomerization in azobenzenes proceed either via rotation about the azo bond with the mpture of the tt-character under n-n excitation, or via inversion mechanism under n-jT excitation, where the iT-bond remains intact. Both mechanisms are shown in Figure 9.3. On the other hand, thermally induced reversible cis-trans back reaction proceeds through a rotational mechanism. The trans-cis photoconversion is almost completely reversible in picoseconds without any side reactions, whereas thermal cis trans conversion usually has timescales ranging from nano- to miUiseconds, or even hours, depending upon the substituent and the local environment A variety of azobenzene systems have been designed and synthesized, including cyclodextrins [40-42], admantanes [43], polycyclics [44], bacteriorodophsin [45], and crown ethers [46]. 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

These thermal analysis studies serve to establish a direct relationship between a heat-induced AR method and the reversal of formalin-induced intra- and intermolecular protein cross-links.10 2831 Further, while formalin-treatment provides thermal stability to RNase A, this stabilization is not sufficient to prevent thermally induced protein denaturation at temperatures (>100°C) typically used in heat-induced AR methods.32 34 The implications of this finding for the mechanism of AR will be discussed further in Section 15.6. 

In summary, formalin-treated does not significantly perturb the native structure of RNase A at room temperature. It also serves to stabilize the protein against the denaturing effects of heating as revealed by the increase in the denaturation temperature of the protein. However, formalin-treatment does not stabilize RNase A sufficiently to prevent the thermal denaturation of the protein at temperatures used in heat-induced AR methods as shown by both DSC and CD spectropolarimetry. This denaturation likely arrises from the heat-induced reversal of formaldehyde cross-links and adducts, as shown in Figure 15.4 of Section 15.4. Further, cooling formalin-treated RNase A that had been heated to 95°C for 10 min does not result in the restoration of the native structure of the protein, particularly in regard to protein tertiary structure. 

Some exchange reactions of complexes 112 and 115 have been studied. From the X-ray crystal structure analyses, it appears that trimethylgallium is rather loosely bound to the (r 2-aryne)- and (r 2-cydohexyne) metallocene building blocks in the dimetalla-bicyclic complexes 112 and 115, respectively (Scheme 7.34). Therefore, it was tempting to investigate whether it was possible to reverse the reactions depicted in Schemes 7.32 and 7.33 using these specific examples to carry out thermally induced exchange reactions. 

In most experimental studies, the environmental conditions of a given polypeptide sample are varied by changing either the temperature of the system or the composition of a solvent mixture consisting of a helix-supporting solvent and a helix-breaking solvent. A curve of helical fraction fN versus temperature at fixed solvent composition is called the thermally induced or thermal transition curve, while a curve of fN versus solvent composition at fixed temperature is called the solvent-induced transition curve. The former is classified into two types normal and inverse (or reverse), depending on whether fN decreases or increases with the rise in temperature. 

Photochromism can be defined as a reversible change between species having different absorption spectra, the initial change being induced by the action of electromagnetic radiation, usually in the ultraviolet, visible and infrared regions. The product is generally thermodynamically less stable, and consequently the reverse reaction is spontaneous and thermally induced. 

Recently, other thermally induced image-reversal processes have been... 

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