Vapor phases represent the simplest non polar phases, because it has no interaction with the solute.
Also, amino acid side chain affinity for water was measured using vapor phases. measured the partitioning of 14 radiolabeled amino acids using sodium dodecyl sulfate (SDS) micelles. Two scales have been developed using micellar phases. Non liquid phases can also be used with partitioning methods such as micellar phases and vapor phases.
Ethanol and dioxane are used as the organic solvents and the free energy of transfer of each amino acid was calculated. Nozaki and Tanford proposed the first major hydrophobicity scale for nine amino acids. However, organic solvents are slightly miscible with water and the characteristics of both phases change making it difficult to obtain pure hydrophobicity scale. Different organic solvents are most widely used to mimic the protein interior. The most common method of measuring amino acid hydrophobicity is partitioning between two immiscible liquid phases. Biswas et al., divided the scales based on the method used to obtain the scale into five different categories. These scales result mainly from inspection of the amino acid structures. The first and third scales are derived from the physiochemical properties of the amino acid side chains. Since cysteine forms disulfide bonds that must occur inside a globular structure, cysteine is ranked as the most hydrophobic. scales was to examine proteins with known 3-D structures and define the hydrophobic character as the tendency for a residue to be found inside of a protein rather than on its surface. The method used to obtain the Janin and Rose et al. This difference is due to the different methods used to measure hydrophobicity. Both the second and fourth scales place cysteine as the most hydrophobic residue, unlike the other two scales. There are clear differences between the four scales shown in the table. Types of amino acid hydrophobicity scales Ī table comparing four different scales for the hydrophobicity of an amino acid residue in a protein with the most hydrophobic amino acids on the topĪ number of different hydrophobicity scales have been developed. In this way, the hydrophobic effect not only can be localized but also decomposed into enthalpic and entropic contributions. A positive free energy change of the surrounding solvent indicates hydrophobicity, whereas a negative free energy change implies hydrophilicity. In terms of thermodynamics, the hydrophobic effect is the free energy change of water surrounding a solute. This leads to significant losses in translational and rotational entropy of water molecules and makes the process unfavorable in terms of free energy of the system. The mobility of water molecules in the "cage" (or solvation shell) is strongly restricted. The hydrogen bonds are partially reconstructed by building a water "cage" around the hexane molecule, similar to that in clathrate hydrates formed at lower temperatures. Introduction of hexane into water causes disruption of the hydrogen bonding network between water molecules.
However, a pure hydrocarbon molecule, for example hexane, cannot accept or donate hydrogen bonds to water. Polar chemical groups, such as OH group in methanol do not cause the hydrophobic effect. The effect originates from the disruption of highly dynamic hydrogen bonds between molecules of liquid water. The hydrophobic effect represents the tendency of water to exclude non-polar molecules. Hydrogen bonds between molecules of liquid water