Oil-like side chains

The vinegar-like side chains can exist in either of two different states, for example, the car-boxyl/carboxylate, COOH/COO, chemical couple and the amino/ammonium, -NH2/NH3, chemical couple. As demonstrated below, the uncharged state of the couple favors association of oil-like domains, whereas the charged state of the couple disrupts association of oillike domains by having destroyed the special hydration of oil-like groups in the process of achieving its own hydration. 

Oil and Vinegar Don t Mix, But, When Oil-like and Vinegar-like Side Chains of Proteins Are Forced to Coexist by Virtue of Structure (Primary,... 

Another type of gel expands and contracts as its structure changes in response to electrical signals and is being investigated for use in artificial limbs that would respond and feel like real ones. One material being studied for use in artificial muscle contains a mixture of polymers, silicone oil (a polymer with a (O—Si—O—Si—) — backbone and hydrocarbon side chains), and salts. When exposed to an electric field, the molecules of the soft gel rearrange themselves so that the material contracts and stiffens. If struck, the stiffened material can break but, on softening, the gel is reformed. The transition between gel and solid state is therefore reversible. 

The first category is composed of the nine amino acids having nonpolar side chains, identified as those with side chains that are largely hydrocarbon in nature. The single exception is methionine, which contains a sulfur atom in its side chain. This is not a problem, since sulfur atoms are quite hydrophobic. Basically, hydrocarbons do not like water hydrophobic means water-hating. Think about fats, oils, waxes. 

In globular proteins, the folding of the polypeptide chain is such that the amino acids with nonpolar side chains are assembled in the interior to form a hydro-phobic core, whereas the amino acids with polar and charged side chains tend to be at the surface to interact with the (aqueous) solvent. This oil-drop-like distribution of hydrophilic and hydrophobic amino acids is of importance for the functionality and stability of a protein because pK values of acidic and basic side chains can be shifted in nonpolar environment by several units, and internal hydrogen bonds are strengthened because the donors and acceptors do not have to compete with water molecules [133, 134J. 

Very recently it was demonstrated that tocopherol moieties in kerogen are likely precursors of prist-l-ene (Figure 7) (30). This idea was supported by the fact that tocopherols are widely distributed in photosynthetic tissues and that they also occur as such in several recent sediments (31). It is tempting to conclude that during "natural pyrolysis" the generated pristene will be transformed to the well known component, pristane, in ancient sediments and oils. This example nicely illustrates that we have to be very careful when we conclude that acyclic isoprenoid hydrocarbons such as pristane originate from the chlorophyll side chain, phytol, based solely on structural similarities. 

Extended tricyclic alkanes with isoprenoidal side chains (C19-C30 Fig. 4.25), which can be named after the C25 member 13p,140C-cheilanthane (a sesterterpane), are common in oils and ancient sediments and they may be microbial in origin (Aquino Neto et al. 1983). The hexaprenol in Fig. 5.26, a common eubacterial and archaebacterial component, seems a likely precursor (Ourisson et al. 1982). Occasionally, the cheilanthane series has been found to extend to C54 (de Grande et al. 1993), suggesting that hexaprenol either undergoes further polymerization reactions or is not the sole precursor. 

Oxidation of coniine with chromic acid produces butyric acid (115, 124), while dehydrogenation of the alkaloid with silver acetate converts it to abase, conyrine (125), which is also obtained from coniine hydrochloride by distillation with zinc dust (126). Conyrine, CgHnN, is a colorless, fluorescent oil, b.p. 166-168°, which forms a chloroplatinate and an aurichloride it can be converted back to coniine by reduction with hydriodic acid, it behaves on methylation like a pyridine base, and further, it gives rise on oxidation to a-pyridinecarboxylic acid. Therefore, conyrine is a 2-propylpyridine (XCIV) while coniine is a 2-propylpiperidine (XCV) (126), in which the side chain is normal since conyrine is not identical with 2-isopropylpyridine (127). 

Among the phenolic compounds, phenol vjhich is absent in oil 5 is foiond as a major component in residua. Cracking reactions involving side chains like propenyl and methoxy groups on phenolic compounds have therefore occurred. 

Proteins, on the other end of the scale of molecular complexity, act as emulsifiers but behave differently from the small molecules, because of their individual molecular structures, and, indeed, it is the particular proteins present which give many food emulsions their characteristic properties. Most, if not all, proteins in their native states possess specific three-dimensional structures which are maintained in solution, unless they are subjected to dismptive influence such as heating (6). When they adsorb to an oil-water interface, it is unlikely that the peptide chains of proteins dissolve significantly in the oil phase, as they are quite hydro-philic as a result of the presence of carboxyl or amido groups it is more likely that the major entities penetrating the interface are the side chains of the amino acids (Table 1). It is possible, for example, for an a-helical portion of a protein to have a hydrophobic side, created by the hydrophobic side chains which lie outside the peptide core of the helix. However, even proteins lacking such regular structures possess amino acids with hydrophobic side chains which will adsorb to the oil-water interface. When a protein is adsorbed, the structure of the protein itself will... 

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