Monomer-dimer equilibrium, effects

The effect of phase upon the monomer-dimer equilibrium is pronounced. The quantum yields for dimer formation in liquid-aerated water solution are low (from zero for thymine to 10"2 for other pyrimidines) but the quantum yields for dimer formation in frozen aqueous solutions or in single crystals are much higher (reaching unity in frozen water solution for thymine). The quantum yields for monomerization are uniformly high and are about the same in solution or in solid phase. The net result of this phase effect is that even at optimum wavelengths for dimer formation, the yields of dimers are low in solution and high in solid phases, for all the single bases, nucleosides, or nucleotides. 

Interestingly, one can use a Cys-Gly-Gly linker at the N- or C-terminal of the polypeptide chain in the design of disulfide-bridged coiled coils. The advantage of this approach is that the Cys-Gly-Gly linker allows complete flexibility of the polypeptide chains to adopt their most stable conformation, which includes different oligomerization states, while maintaining the polypeptide chains in a parallel manner. 49 In addition, the Cys-Gly-Gly linker eliminates the monomer-dimer equilibrium and the peptide concentration effect on stability, which is observed in two-stranded coiled-coil formation of noncovalent linked polypeptides. 49 861... 

We complete this section with a study of the effect of TMEDA on the polymerization of methyl methacrylate lithium enolate in THF262. It was concluded that TMEDA hardly affects the kinetics of the polymerization and therefore the monomer-dimer equilibrium. From these figures, TMEDA does not seem to be a better ligand for lithium ester enolates than THF, in line with previous observations by Collum on other organolithium compounds263. 

Few investigations concerning temperature effects on Grignard reagent composition in the absence of solubility effects appear in the literature. The effect of temperature on the monomer-dimer equilibrium of diethylmagnesium (R = Et) in diethyl ether was determined by Raman spectroscopy [39]. 

It seems to me that we can scarcely progress in our understanding of the structural and kinetic effects of the H-bond without knowing the AG and AH terms involved, so I intend to discuss some methods of determining them. The references will provide simple examples of the methods mentioned. The most significant AG and AH values are those evaluated from equilibrium measurements in the gas phase—either by classical vapour density measurements, the second virial coefficient [1], or from, spectroscopic, specific heat or thermal conductance [2], or ultrasonic absorptions [3]. All these methods essentially measure departures from the ideal gas laws. The second virial coefficient provides a measure of the equilibrium constant for the formation of collision dimers in the vapour as was emphasized by Dr. Rowlinson in the discussion, this factor is particularly significant as only the monomer-dimer interaction contributes to it. 

As shown in Example 24-5, deviations from Beer s law appear when the absorbing species undergoes association, dissociation, or reaction with the solvent to give products that absorb differently from the analyte. The extent of such departures can be predicted from the molar absorptivities of the absorbing species and the equilibrium constants for the equilibria involved. Unfortunately, since we are usually unaware that such processes are affecting the analyte, there is often no opportunity to correct the measurement. Typical equilibria that give rise to this effect include monomer-dimer equilibria, metal complexation equilibria when more than one complex is present, acid-base equilibria, and solvent-analyte association equilibria. 

In most cases the dimerization reactions are effectively irreversible, but N02, SO2-, and a few other species are exceptions where the dimerization equilibrium constants have been determined.11 Not all radicals dimerize. For example, NO and CIO2 are quite stable as monomers and exhibit no significant tendency to dimerize apparently, the dimerization equilibrium constants are unfavorable. In other cases, dimerization has not been demonstrated because the radicals disproportionate instead, as discussed below. However, for those radicals where dimerization has been demonstrated, the rate constants are generally close to the diffusion limit these rate constants show systematic decreases attributable to repulsion of ionic charges, as demonstrated by the series H2PO4, HPO4-, and PO42-. 

However, in sols of such small particle size, there is an appreciable concentration of monomer at equilibrium. Also, in alkaline sols at pH 9-10.5, there is an appreciable amount of ionic silica which is converted to monomer before the titration. Since monomer reacts with base at pH 9 it is therefore necessary to correct the titration for the effect of soluble silica in order to obtain a reliable value for the specific surface area of the polymer. The term soluble silica is used to include the ionic silica and dimer which react with alkali-like monomer. 

The first derivatives and in Eq. (4) vanish at the exact equilibrium geometry of the dimer. But even if we determine first the monomer equilibrium structures, in the intramolecular force fields, and next the dimer equilibrium geometry from the intermolecular potential in Eq. (4), they are very small. Moreover, they have no effect on the vibrational frequencies in first order perturbation theory. In second order they will lead to further, but small, shifts of the monomer frequencies, which we have not calculated. 

Beyond reaction kinetics and the properties of the resulting polymers, nanoconfinement also influences monomer/polymer equilibrium. Dimerization of biomolecules has been studied both theoretically and by experiment [79-81]. Using molecular simulations, the stability of protein dimers is found to depend on both confinement size and molecular concentration with maximal dimer stabilization observed at concentration levels near those observed in vivo, and this effect, which occurs only when a nanoscale volume is completely isolated has been attributed to a decrease in the entropy of mixing for example, in the limiting case of two reactant molecules in a volume, the entropy of mixing vanishes, resulting in a shift of the equilibrium completely to the right (i.e., to the dimer product) for an exothermic reaction [81]. 

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

The ability of a solute to associate with itself can be expressed by the degree of association (/). The / is obtained by dividing the stoichiometric mole fraction of the solute by the effective mole fraction of the solute. Assuming that a single multimer species in equilibrium with a monomer is a dimer,/values range from 1.0 to 2.024). 

In solution the colorless 2,4,6-triphenylphenoxyl dimer attains a rapid equilibrium with its red monomer radical (dissociation constant in benzene 4 X 10 at 20°). The radical is surprisingly stable toward oxygen and can be stored in solution for a long time when it is protected from light. The stability of the 2,4,6-triphenylphenoxyl radical is ascribed to steric and mesomeric effects. The e.s.r. spectrum and an ENDOR-spectrum of the radical are described. 

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