Phosphate ester linkage

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

A phosphate-phosphonate rearrangement process has also been explored in which a strong base is used to abstract a proton from the position adjacent to an aryl phosphate ester linkage. The product, an or f/io-phosphonopheno I, is generated in excellent yield (Figure 6.16).70 Further exploration of the variability of structure for this type of reaction seems desirable. 

The phosphate group of nucleotides is attached via a phosphate ester linkage, and may be attached to... 

Derivatives of phosphoric acid, pyrophosphoric acid, and related compounds are very important in biological systems. Pyrophosphoric acid is an anhydride of phosphoric acid. Adenosine triphosphate, an energy carrier that is universally found in living organisms, has a phosphorus dianhydride connected to an adenosine group by a phosphate ester linkage. Phosphorus ester bonds are used to form the polymeric backbone of DNA (see Chapter 27). 

Phosphate esters play a central role in biochemistry. Figure 11-3 shows how phosphate ester linkages compose the backbone of the nucleic acids RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). These nucleic acids, which carry the genetic information in the cell, are discussed in Chapter 23. 

Figure 23-24 shows how the individual ribonucleotide units are bonded into the RNA polymer. Each nucleotide has a phosphate group on its 5 carbon (the end carbon of ribose) and a hydroxyl group on the 3 carbon. Two nucleotides are joined by a phosphate ester linkage between the 5 -phosphate group of one nucleotide and the 3 -phosphate group of another. 

Many of the phospholipids in membranes are glycerol derivatives that have two esterified fatty acids plus a charged side chain joined by a phosphate ester linkage. A typical example is phosphatidylcholine (lecithin), a major component of most membranes ... 

The reaction between two nucleotides to form a phosphate ester linkage can be approximated as follows ... 

In both DNA and RNA, the heterocyclic amine base is bonded to Cl of the sugar, and the phosphoric acid is bonded by a phosphate ester linkage to the C5 sugar position. The names and structures of all four deoxyribonucleotides and all four ribonucleotides are shown in Figure 28.4 (p. 1162). 

A rapid FTIR method for the direct determination of the casein/whey ratio in milk has also been developed [26]. This method is unique because it does not require any physical separation of the casein and whey fractions, but rather makes use of the information contained in the whole spectrum to differentiate between these proteins. Proteins exhibit three characteristic absorption bands in the mid-infrared spectrum, designated as the amide I (1695-1600 cm-i), amide II (1560-1520 cm-i) and amide III (1300-1230 cm >) bands, and the positions of these bands are sensitive to protein secondary structure. From a structural viewpoint, caseins and whey proteins differ substantially, as the whey proteins are globular proteins whereas the caseins have little secondary structure. These structural differences make it possible to differentiate these proteins by FTIR spectroscopy. In addition to their different conformations, other differences between caseins and whey proteins, such as their differences in amino acid compositions and the presence of phosphate ester linkages in caseins but not whey proteins, are also reflected in their FTIR spectra. These spectroscopic differences are illustrated in Figure 15, which shows the so-called fingerprint region in the FTIR spectra of sodium caseinate and whey protein concentrate. Thus, FTIR spectroscopy can provide a means for quantitative determination of casein and whey proteins in the presence of each other. 

Phosphatidic acids are the simplest phosphoacylglycerols and are present only in small amounts in membranes. The most common phosphoacylglycerols in membranes have a second phosphate ester linkage. The alcohols most commonly used to form this second ester group are ethanolamine, choline, and serine. Phosphatidylethanolamines are also called cephalins, and phosphatidylcholines are called lecithins. Used as emulsifying agents, lecithins are added to foods such as mayonnaise to prevent the aqueous and fat components from separating. 

In addition to inorganic orthophosphates, phosphorus is bound in organic phosphates (ester linkages) within humus, from which plant available phosphate is set free by microorganisms (mineralization). Examples are nucleic acids, inisitol hexaphosphates as the largest group, and phospholipids. In agricultural soils, the C P ratio is in the order of 50 1. 

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