Apolar hydrogen

With the hydrophobic effect defined as the contribution to the thermodynamics that is proportional to the exposure of apolar surface area, it is then possible for a set of homologous compounds to separate the hydrophobic contribution from all other effects by plotting any thermodynamic function (for instance, AH0) versus the number of apolar hydrogens (or the apolar surface area) that become exposed to the solvent on transfer. To the extent that the other interactions make a constant contribution to the thermodynamics,... 

Fig. 1. Analysis of the apolar contribution to the dissolution thermodynamics of cyclic dipeptides into water. Each thermodynamic quantity is plotted against the number of apolar hydrogens (aH) (i.e., hydrogens bonded to carbon) (a) AC , (b) AH°, (c) AS0, and (d) AG°. Lines are the linear regression fit of the data. As described in the text, the slope gives the hydrophobic contribution. Data are from Murphy and Gill (1990).

The measured heat capacity increment associated with the transfer of an apolar hydrogen from a crystalline amino acid solid into water is 6.7 0.3 cal K-1 mol-1 or 0.45 0.02 cal (mol A2)-1 of apolar surface (Murphy and Gill, 1990, 1991). This same value is observed for the transfer of 1-alkanols into water (Hall n et al., 1986). The same value is also observed for the transfer of liquid hydrocarbons into water (Gill and Wadso, 1976), and for the dissolution of alkane... 

Contributions of Peptide and Apolar Hydrogen Groups to the Thermodynamics of Dissolution into Water ... 

Analysis of the crystallographic structure indicates that on complete unfolding of the phosphoglycerate kinase (PGK) molecule a total of 1828 apolar hydrogens (814 from the N domain and 1014 from the C domain) become exposed to the solvent. Also, an average of 73% of the amino acid residues are hydrogen bonded (Chothia, 1976) and... 

Analysis of the crystallographic structure also reveals that the two domains interact primarily through hydrophobic and hydrogen bond interactions at the interface. The number of apolar hydrogens that become exposed on the C domain on unfolding of the N domain is 49.4, and the number of apolar hydrogens exposed on the N domain on unfolding of the C domain is 44.8. These values correspond to 726 and 659 A2 of apolar surface area, respectively. In addition, nine... 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

The formation of clusters of hydrogen-bonded water molecules around different sizes of apolar molecules... 

Another important factor in the hydrogenation of a,p-unsaturated acids was the solvent by increasing the polarity of the solvent used (n-hexane < toluene < methyl acetate < THF < methanol) a progressive decrease in ee was observed. This was probably due to the fact that H2 and the a,p-unsaturated acids were more soluble in the apolar solvents and also that CD was present in the open conformation, the enantioselective conformation (8). In fact, the highest conversion and ee were obtained with n-hexane (Figure 2). 

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