Monomer dimer equilibrium

The dithionite ion has a remarkable eclipsed stmcture of approximate C2v symmetry (Fig. 15.32b). The extraordinarily long S-S distance (239 pm) and the almost parallel SO2 planes (dihedral angle 30°) are other unusual features. Electron-spin-resonance studies have shown the presence of the S02 radical ion in solution ( 300ppm), suggesting the establishment of a monomer-dimer equilibrium 8204 ... 

Baryshnikova OK, Sykes BD (2006) Backbone dynamics of SDF-lalpha determined by NMR interpretation in the presence of monomer-dimer equilibrium. Protein Sci 15 2568-2578 Becker EB, Bonni A (2004) Cell cycle regulation of neuronal apoptosis in development and disease. Prog Neurobiol 72 1-25... 

Veldkamp CT, Peterson FC, Pelzek AJ, Volkman BF. The monomer-dimer equilibrium of stromal cell-derived factor-1 (CXCL 12) is altered by pH, phosphate, sulfate, and heparin. Protein Sci 2005 14 1071-81. 

Until very recently, the only example of a quasi-3-coordinate aluminum phosphide was (Me3SiCH2)2AlPPh2,76 which was reported to exist in solution as a monomer-dimer equilibrium. However, a crystal structure of... 

Pt/Al2C>3-cinchona alkaloid catalyst system is widely used for enantioselective hydrogenation of different prochiral substrates, such as a-ketoesters [1-2], a,p-diketones, etc. [3-5], It has been shown that in the enantioselective hydrogenation of ethyl pyruvate (Etpy) under certain reaction conditions (low cinchonidine concentration, using toluene as a solvent) achiral tertiary amines (ATAs triethylamine, quinuclidine (Q) and DABCO) as additives increase not only the reaction rate, but the enantioselectivity [6], This observation has been explained by a virtual increase of chiral modifier concentration as a result of the shift in cinchonidine monomer - dimer equilibrium by ATAs [7],... 

Variable temperature C NMR data of the Y derivative were consistent with a monomer/dimer equilibrium for compounds 85 in solution as depicted in Scheme... 

Scheme 26. Proposed monomer/dimer equilibrium for compounds 85.

A series of titanasilsesquioxane alkoxides, including [R7Si70i2Ti0zPr] (107, R = C-C5H9 108, c-CfiHn = 1,2) and [Cy7Si70i2Ti(p-0Me)(Me0H)]2 (109) were published by Crocker and co-workers. " Treatment of Cy7Si70i2Ti(0Pr) (108) with methanol affords the six-coordinate titanasilsesquioxane dimer [Cy7Si70i2 Ti(p-OMe)(MeOH)]2 (109), which was structurally characterized. The monomer/ dimer equilibrium of 108 and the formation of 109 are depicted in Scheme 34. " ... 

The structure of [CofCNljNO] " has been studied by spectral means. I.r. and electronic spectra indicate it should be reformulated as [(CN)5Co(N202)-CoICNjs] - however, the appearance of v(NO) in range 1770—1860 cm" in dilute solution is taken as evidence for a monomer-dimer equilibrium. ... 

Fluorescence quantum yield and emission maximum determinations as a function of peptide concentration may also permit the detection of peptide self-aggregation at concentrations below 10-4 M, because the peptide fluorophore is likely to be located in a different environment in the peptide aggregate. For example, the concentration-dependent changes in the tryptophan fluorescence emission maximum of mellitin were monitored to determine the equilibrium dissociation constant and thermodynamic parameters of the monomer-tetramer self-association reaction of this peptide. 25 Similarly, measurement of the changes in the tryptophan fluorescence intensity of gramicidin A as a function of its concentration permitted the determination of an average monomer-dimer equilibrium con-stant. 26 ... 

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. 

In 1976, Lappert and co-workers " reported the first stable dialkylstan-nylene (154, Scheme 14.71) in solution. They found that 154 exists as a monomer in the gas phase and as a dimer 153 in the solid state, whereas it exists as a monomer-dimer equilibrium mixture in solution. Extensive studies on the reactions of 154 were carried out, especially on oxidative addition and insertion reactions leading to a variety of new organotin compounds such as 165 and 166 (Scheme... 

Soon after the isolation of 136, Tokitoh et described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 °C in solution, and it showed a deep purple color (A,max =561 nm) in hexane. The Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between —30 and 60 °C, indicating the absence of a monomer-dimer equilibrium. 

For the aluminum systems so far described, only the monomer-dimer equilibrium has been observed. For very bulkyl R groups, the monomer predominates for small alkyls, cyclopropyl, and phenyl-bridged species, the symmetrical dimer is the stable form. When the phenylethynyl group is present, this equilibrium is complicated further by the additional feature arising from the different possible types of bridge bonds, which may be described in the following manner ... 

Chauvin, E Brand, L. Roseman, S. Sugar transport by the bacterial phosphotransferase system. Characterization of the Escherichia coli enzyme I monomer/dimer equilibrium by fluorescence anisotropy. J. Biol. Chem., 269, 20263-20269 (1994)... 

Abstract-—The hydrogen bonding chemical shift of the —OH proton of methyl alcohol has been separated into two parts, one due to the formation of dimer and the other to all other polymeric species. A molecule which is hydrogen bonded as dimer contributes only about one-third as much as a molecule in a higher polymer. The separation is based on correlation with the —OH stretching infrared data, and is confirmed by the examination of sterically hindered alcohols—both aromatic and aliphatic—which because of bulky substituents, can form dimers but not higher polymers. In these substances, the small H-bond shifts are also correlated with monomer-dimer equilibrium studies of the infrared spectra. 

Fig. 5. The monomer-dimer equilibrium in methyl ethyl 2-butyl carbine ...

Propargyl alcohol HC = 0—CHgOH ako gives an infrared spectrum indicative of only a monomer dimer equilibrium, although since here there can be no steric hindrance there is no obvious reason why this should be the case. Its nuclear resonance shift with dilution is more akin to the kind shown by polymer-forming alcohols, and the anomaly... 

Ftg. 7. The Lr, spectrum of propargyl alcohol showing (two right-hand peaks) that only monomer-dimer equilibrium appears to be involved... 

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