Long-chain a-amino acids

The self condensation of esters of long-chain a-amino acids (methyl 2-aminoocta-decanoate (39), docosanyl 2-aminooctadecanoate (40), methyl 2-aminohexacosanoate (41), docosanyl 2-aminohexacosanoate (42) is used to prepare oriented polypeptide films, which are models for biological membranes.52 ... 

The long-chain a-amino acid esters (40), (41), and (42) form bilayers on sonication in water under acidic conditions. Liposomes prepared from (40) and (42) precipitate if the aqueous medium is neutralized by titration with NaOH. Only liposomes made from (41) are stable even in basic solutions, as shown by electron microscopy52). Polypeptide formation in oriented spherical vesicles was confirmed by FT-IR spectroscopy. The liposomal solution of (41) was freeze-dried and the spectrum obtained from the residue was comparable with one of the polycondensed monolayers. The formation of polypeptide vesicles is illustrated in Scheme 4. 

Besides pol3nnerization another type of polyreaction, the polycondensation, can be used to stabilize membrane systems. Recently, Fukuda et al. (5) described polyamide formation in monolayers. Long chain esters of glycine and alanine were polycondensed to yield non-oriented polyamide films of polyalanine and polyglycine. In analogy to this reaction it is possible to prepare stabilized, oriented membrane systems via polycondensation using long chain a-amino acid esters (6). 

In analogy to the polycondensation of long chain a -amino acid esters in monolayers the formation of polypeptide vesicles from these compounds is also possible (6). 

Figure 2.2 Diagram of a voltage-activated sodium channel protein. The channel is composed of a long chain of amino acids intercormected by peptide bonds. The amino acids perform specific functions within the ion channel. The cylinders represent amino acid assemblies located within the membrane of the nerve cell and responsible for the foundation of the ion pore.

A long chain of amino acids attached end-to-end has many possible ways to fold. The final shape, or conformation, of a folded protein molecule is determined by its unique sequence of amino acids and by the effects of environmental conditions on amino acid side chains. The conformation selected is the one that is most stable because it has the lowest free energy (Bloomfield 1979). This conformation is designated the native state of the protein. 

The important molecule inside these red blood cells is hemoglobin. Hemoglobin is a protein molecule consisting of four long chains of amino acids. In the center of each one is a unit like the one shown here. This porphyrinlike group is a complex of iron the iron atom is the point of attachment of the incoming oxygen molecule. 

The names and the structure of the a-amino acids present in proteins are shown in Table 12.1.1. Besides the amino acids that are protein constituents, many other amino acids occur naturally in living systems. Amino acids are linked in proteins through an amide bond (or peptide bond) formed between the amino group of one amino acid and the carboxyl group of the other amino acid. The peptide bond contains the planar group of atoms CaNCC. When a small number of amino acids are linked through F>eptide bonds they form simple peptides (dipeptides, tripeptides, etc.). Long chains of amino acids are commonly named polypeptides. The sequence of amino acids in a polypeptide is known as primary structure. A polypeptide chain can be described schematically as follows ... 

Our protein-tube hypothesis shows that long chains of amino acids have a tendency to form amyloids rather than maintain their protein-like shape. Indeed, nature has on suitable occasions thwarted this tendency by dividing the protein into substantially independent domains that fold autonomously and are then assembled together. This suggests that the variety of protein folds increases with length up to a certain point at which they are supplanted by the formation of domains or amyloids. 

When the long chains of amino acids link to form protein structures, not only do they arrange themselves into secondary structures, but the whole chain also arranges itself into a very specific overall shape called the tertiary structure of the protein. The protein chain is held in its tertiary structure by attractions between the side-chains of its amino acids. For example, covalent bonds that form between sulfur atoms in different parts of the chain help create and hold the... 

In all forms of digestion (whether of proteins, carbohydrates, or fats), larger molecules are broken down into smaller molecules by a reaction with water in which a water molecule is split in two, each part joining a different product molecule. This type of reaction is called hydrolysis. Remember that proteins are long chains of amino acids linked together by amide functional groups called peptide bonds. When protein molecules are digested, a series of hydrolysis reactions convert them into separate amino acids. 

PROTEIN A large, complex compound made of long chains of amino acids. Proteins have a number of essential functions in living organisms. 

Peptides and proteins are macromolecules made up from long chains of amino acids joined head-to-tail via peptide bonds. The three-dimensional structure of a protein is very well defined and is essential for it to function. Proteins are found... 

Polypeptide A long chain of amino acids joined to one another via peptide bonds. 

In a protein or a peptide, a long chain of amino acids is held together in a head-to-tail arrangement the a-carboxyUc group of one amino acid is joined by the a-amino group of another amino acid via an amide linkage, also called the peptide... 

In 1902, Emil Fischer proposed that proteins were long chains of amino acids joined together by amide bonds formed between the a-carboxyl group of one amino acid and the a-amino group of another. For these amide bonds, Fischer proposed the special name peptide bond. Figure 18.5 shows the peptide bond formed between serine and alanine in the dipeptide serylalanine. 

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