Aldopentose

An analogous pattern of configurations can be seen m the aldopentoses when they are arranged m the order ribose arabinose xylose lyxose (RAXL is an easily remem bered nonsense word that gives the correct sequence) This pattern is discernible even m the aldotetroses erythrose and threose... 

During the discussion of hemiacetal formation in d ribose in the preceding section you may have noticed that aldopentoses have the potential of forming a six membered cyclic hemiacetal via addition of the C 5 hydroxyl to the carbonyl group This mode of ring closure leads to a and p pyranose forms... 

Like aldopentoses aldohexoses such as d glucose are capable of forming two fura... 

Of the eight stereoisomeric aldopentoses Figure 25 2 shows the Fischer projections of the d enantiomers (d ribose d arabinose d xylose and D lyxose) Likewise Figure 25 2 gives the Fischer projections of the eight D aldohexoses... 

Chain extension of the aldopentose (—) arabinose by way of the denved cyanohydnn gave a mixture of (+) glucose and (+) mannose... 

Note The dipping solution, which can also be used as a spray solution, can be employed with silica gel, kieselguhr, cellulose, RP, NH2 and CN phases. Sugars (exceptions include, for example, fructose, melezitose and raffinose) yield brilliantly colored zones on an almost colorless background when the spray solution is employed. Aldohexoses appear brown, aldopentoses bright red and hexuronic acids orange in color [3]. The detection limit differs for different substances it ranges from 10 ng (pyridoxal) over 100 ng (cinnamaldehyde) up to 2 pg (citral). 

Among the aldopentoses, D-ribose is a component of many biologically important substances, most notably the ribonucleic acids, and D-xylose is very abundant and is isolated by hydrolysis of the polysaccharides present in corncobs and the wood of trees. 

Aldopentoses have three chirality centers and a total of 23 = 8 possible stereoisomers, or four d,l pairs of enantiomers. These four pairs are called ribose, arabinose, xylose, and lyxose. All except lyxose occur widely. r>Ribose is an important constituent of RNA (ribonucleic acid), t-arabinose is found in many plants, and D-xylose is found in wood. 

Fischer projections of the tour-, five-, and six-carbon d alcloses are shown in Figure 25.3. Starting with D-glyceraldehyde, we can imagine constructing the two d aldotetroses by inserting a new chirality center just below the aldehyde carbon. Each of the two d aldotetroses then leads to two d aldopentoses (four total), and... 

The structures of the four n aldopentoses can be generated in a similar way and named by the mnemonic suggested by a Cornell University undergraduate "ribs are extra lean."... 

Problem 25.10 > The following model is that of an aldopentose. Draw a Fischer projection of the sugar, name it, and identify it as a n sugar or an ). sugar. 

Problem 25.22 What aldopentose would give a mixture of L-gulose and L-idose on Kiliani-Fischer chain extension ... 

Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

Problem 25.23 Two of the four d aldopentoses yield D-threose on WohJ degradation. What are their structures ... 

Which of the other three n aldopentoses gives the same aldaric acid as D-lyxose ... 

Compound A is a D aldopentose that can be oxidized to an optically inactive aldaric acid B. On Kiliani-Fischei chain extension, A is converted into C and D C can be oxidized to an optically active aldaric acid E, but D is oxidized to an optically inactive aldaric acid F. What are the structures of A-F ... 

Just as proteins are biopolymers made of amino acids, nucleic acids are biopolv-mers made of nucleotides joined together to form a long chain. Each nucleotide is composed of a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar linked through its anomeric carbon to the nitrogen atom of a heterocyclic purine or pyrimidine base. 

The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biological polymers that act as chemical carriers of an organism s genetic information. Enzyme-catalyzed hydrolysis of nucleic acids yields nucleotides, the monomer units from which RNA and DNA are constructed. Further enzyme-catalyzed hydrolysis of the nucleotides yields nucleosides plus phosphate. Nucleosides, in turn, consist of a purine or pyrimidine base linked to Cl of an aldopentose sugar—ribose in RNA and 2-deoxyribose in DNA. The nucleotides are joined by phosphate links between the 5 phosphate of one nucleotide and the 3 hydroxyl on the sugar of another nucleotide. 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

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