Amino hydrophilicity/hydrophobicity

The extrinsic pathway of coagulation is activated when circulating factor VII encounters tissue factor. Tissue factor is a transmembrane glycoprotein, which is normally expressed by subendothelial fibroblast-like cells, which surround the blood vessel. An intact endothelium normally shields the circulating blood from exposure to tissue factor. The tissue factor molecule consists of a 219 amino acid hydrophilic extracellular domain, a 23 amino acid hydrophobic region that spans the membrane, and a 21 amino acid cytoplasmic tail that anchors the molecule to the cell membrane (15,16). Other sites of tissue factor expression include activated monocytes, activated endothelial cells, and atherosclerotic plaques. 

Fife as we know it builds its proteins primarily from the same 20 amino acids, although there are many other amino acids that might have been utilized. While it is important that the collection of amino acids used in proteins includes a sufficient number of small, large, hydrophilic, hydrophobic, and charged amino acids, the exact identities of the amino acids in each of these classes may not be critical. Moreover, the amino acids utilized for protein synthesis by familiar life are all F-amino acids, and there is no reason to think that D-amino acids could not have been utilized instead. 

The two classes of amino acids that exist are based on whether the R-group is hydrophobic or hydrophilic. Hydrophobic or nonpolar amino acids tend to repel the aqueous environment and are located mostly in the interior of proteins. They do not ionize or participate in the formation of hydrogen bonds. On the other hand, the hydrophilic or polar amino acids tend to interact with the aqueous environment, are usually involved in the formation of hydrogen bonds, and are usually found on the exterior surfaces of proteins or in their reactive centers. It is for this reason that certain amino acid R-groups allow enzyme reactions to occur. 

The interactions between the amino acids and the solvent (electrostatic, hydrophilic, hydrophobic, S-S) determine the globular conformation. We can give some naive picture of the folded state in terms of a liquid-hydrocarbon model where the hydrophobic core stabilizes globular proteins. The hydrophilic (polar and charged) amino acids are exposed to the solvent and the hydrophobic (polar) amino acids are less exposed to the solvent and buried in the interior of the protein. 

The most important properties of amino acids, for the purpose of understanding protein structure and function, are electrostatic charge, hydrophilic/hydrophobic properties, and chemical reactivity, which are discussed below. 

The functional properties of proteins depend also on their structure and interactions with the environment. The functional properties of surfactants depend on their hydrophilic-hydrophobic balance, too. Protein chains modified by proteolysis, amino acid incorporation, and transpeptidation may display different functional properties. As milk proteins possess good surface activities [131], the question of the changes in the functional properties of the enzymatically modified protein products is of especial interest. 

Figure 10.12. Frequencies of hydrophilic, hydrophobic and ainphipathic amino acids from prokaryotes (Closed circles) and eukaryotes (open circles) plotted against GCj. (From D Oriofrio ct al.. 1999a).

Different sites could specialize in the hydrolysis of different categories of peptide bonds (e.g., those that join hydrophobic/hydrophilic, hydrophobic/hydrophobic, or hy-drophilic/hydrophilic amino acids, etc.) and thereby optimize the overall kinetics of degrading diverse protein sequences. 

FIGURE 13.6 Preparation of hydrophilic-hydrophobic block copolymer 34 through the coupling reaction between precursor 32 and the amino ketal 33. Block copolymer 24 is designed to associate and then dissociate upon hydrolysis of the ketal to give the more hydrophilic block copolymer 35 [99]. 

Patterning of hydrophilic/hydrophobic alkanethiols combining electron beam lithography (EBL) and self-assembly of alkanethiol molecules, (a) 150 nm thick polymethyl methacrylate (PMMA) resist spin coated onto the gold deposited on a silicon wafer (b) Patterned PMMA trenches were defined by electron beam and development (c) a hydrophilic ll-amino-l-un-decanethiol hydrochloride (MUAM) assembled in the PMMA trench area (d) PMMA resists were removed by acetone to produce patterned MUAM on gold (e) backfilled by hydrophobic octadecanethiol (ODT) yielding the final chemical pattern. (Reprinted with permission from Wiley.)... 

Amino acids show acid-base properties, which are strongly dependent on the varying R groups present in each molecule. The varying R groups of individual amino acids are responsible for specific properties polarity, hydrophilicity-hydrophobicity. Hence, the 20 a-amino acids could be categorized in the 4 distinct groups listed in Table 1. 

Interesting phase transition behavior was observed in copolymers of PVCL, which include PVCL-PEO (poly(ethylene oxide) hydrophilic) block copolymer, and copolymers of PVCL with (VA -dimethyl amino) ethyl methacrylate (DMAEMA), hydrophilic co-monomers like ethylacrylamide, polyacrylamide-PVA, or polyacrylamide-PEO copolymers. All of these combinations showed decrease in LCST of the PVCL copolymer system. The LCST behavior observed was not in accordance with conventional theory of increase/decrease in LCST with copolymerization with hydrophilic/hydrophobic... 

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