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Assignment No. 2 2013

Isomerism in BiomoleculesFakhereddin Babiker Ali Musa

When two or more different compounds have the same molecular formula but, different structuralformula; they are called isomers and the phenomenon is isomerism. Isomers are classified into structuralor constitutional or positional isomers, which occur when atoms and groups are linked together indifferent ways; and stereo- or configurational isomers, which occur when atoms and groups areconnected in the same way, yet having different spational arrangements. Stereoisomers are classifiedaccording to the ability to interconvert by rotation about single bonds, to two types; configurational andconformational isomers. Configurational isomers are then split up in accordance with thenonsuperposability, into two types; optical isomers (enantiomers, epimers, anomers) and geometricisomers (diastereomers). Also the conformational isomers are subdivided to optical isomers(enantiomers only) and geometric isomers, and if these optical isomers (enantiomers) can not readilyinterconvert at ambient temperature, they will be called atropisomers (Figure-1).

Figure-1: Classes of IsomersThe diversity of biomolecules and the manyfunctional groups they contain have increasedtheir propability of occuring in an amzingnumber of isomers.Structural isomerism occurs in biomoleculesin a variety of forms; in carbohydrates thepyran and furan ring structures and aldo- andketo- forms (Figure-2), and in some carboxylicacids.

A B C DFigure-2: A; showing pyran ring B; furan ring C; keto- and D; aldo- formStereo isomerism is the most important class ofisomerism in biomolecules so far. And almost allclasses of biomolecules show stereoisomerism.Configurational isomerism a type ofstereoisomerism, is found in carbohydrates invarious forms; D and L isomers, α and β anomers(Figure-3), and epimers (Figure-4). In lipids, it isfound in the most important cis and trans forms,α and β orientation especially in steroids (Figure-5). And in peptides and proteins only L isomer (L-α-amino acids (Figure-6)) is found; this is true forhumans but, in micro-organisms both D- and L-α-amino acids are found. (Berg, 321)

A B C D

Figure-3: A & B; showing L and D isomers, C & D; showing α and βanomers

Structural

Isomers

Stereo

ConformationalConfigurational

Enantiomers

Diastereomers

Atropisomers

Diastereomers

Enantiomers

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Assignment No. 2 2013

Conformational isomerism, another type ofstereoisomerism, which occurs widely inbiomolecules especially the biologically activemacromolecules.Different types of conformational isomers arecalled conformers, and even small moleculescan perform conformation. In macromoleculesconformation is stabilized by inter- and intra-molecular interactions. The most importanttypes of conformers are; staggered andeclipsed; the former is the most stable so far.Figure-4: α-D-Glucose and its epimers

Figure-5: Steroid nucleus showing; A: all-trans configuration betweenadjacent rings and B: a cis configuration between rings A and BFigure-6: L-α-AlanineChirality of a compound is its existence in twononsuperposable forms (i.e mirror images),and to be chiral a compound must possess atleast one asymmetric or stereogenic center (i.ea carbon with four different groups attachedto) (Figure-7).Figure-7: Chiral Centers in Different BiomoleculesChirality and Optical Activity is veryimportant to be discussed throughout here;

because biomolecules for the most part chiral,and in living systems they are almostenantiomerically pure. Chiral compounds havethe ability to rotate plane polarized light (PPL),and this property has been used historically todifferentiate between enantiomers; when anenantiomer turns the PPL to the right it is calleddextrorotatory and, when it turns it to the left it iscalled levorotatory and designated as (+) and (-)respectively. Many stereochemical descriptorsare found which has nothing to do with theoptical activity, as they only descripe howdifferent groups are distributed arround thechiral center (e.g.. D and L, R and S, E and Z,Erythro and Threo). Now adays their importancehas lessened somewhat with the development ofpowerful NMR and chiral chromatographicmethods.When a solution contains equivalent amounts ofthe two enantiomers, it is refered to as a racemicmixture.Chirality In Biological World has greatsignificance, as it affects enzyme-substrate,antigen-antibody, hormone-receptor interactionsand hence; the biological activities atainedthrough them, this because these interactions arehighly enantio-selective or stereo-selective in thebiological world.References1. William H. Brown, Christopher S. Foote, Brent L. Iverson,Eric V. Anslyn. Organic Chemistry. Belmont : CengageLearning, 2012; 116:133:134:136:137:138.2. Robert K. Murray, Daryl K. Granner, Peter A. Mayes, VictorW. Rodwell. Harper's Illustrated Biochemistry. 26th Edition.New York : Lange Medical Books/McGraw-Hill, 2003; 103:104.3. K. Koolman, K. H. Roehm. Color Atlas of Biochemistry. 2ndEdition. Stuttgart · New York : Thieme, 2005; 8.4. Jeremy M. Berg, John L. Tymoczko, Lubert Stryer.Biochemistry. 7th Edition. New York : W. H. Freeman andCompany, 2012; 321.5. David L. Nelson, Michael M. Cox. Lehninger Principles ofBiochemistry. 5th Edition. New York : W. H. Freeman andCompany, 2008; 15:16:18.