Isomerism in Biomolecules.pdf

2013

Assignment No. 2 Isomerism in Biomolecules Fakhereddin Babiker Ali Musa

When two or more different compounds have the same molecular formula but, different structural

formula; they are called isomers and the phenomenon is isomerism. Isomers are classified into structural or constitutional or positional isomers, which occur when atoms and groups are linked together in different ways; and stereo- or configurational isomers, which occur when atoms and groups are connected in the same way, yet having different spational arrangements. Stereoisomers are classified according to the ability to interconvert by rotation about single bonds, to two types; configurational and conformational isomers. Configurational isomers are then split up in accordance with the nonsuperposability, into two types; optical isomers (enantiomers, epimers, anomers) and geometric isomers (diastereomers). Also the conformational isomers are subdivided to optical isomers (enantiomers only) and geometric isomers, and if these optical isomers (enantiomers) can not readily interconvert at ambient temperature, they will be called atropisomers (Figure-1). Isomers

Structural Stereo

Configurational

A B C D Figure-2: A; showing pyran ring B; furan ring C; keto- and D; aldo- form

Figure-1: Classes of Isomers

isomerism in biomolecules so far. And almost all classes of biomolecules show stereoisomerism.

Conformational

Diastereomers Enantiomers

Stereo isomerism is the most important class of Configurational

isomerism a type of stereoisomerism, is found in carbohydrates in various forms; D and L isomers, α and β anomers (Figure-3), and epimers (Figure-4). In lipids, it is found in the most important cis and trans forms, α and β orientation especially in steroids (Figure5). And in peptides and proteins only L isomer (Lα-amino acids (Figure-6)) is found; this is true for humans but, in micro-organisms both D- and L-αamino acids are found. (Berg, 321)

Diastereomers Enantiomers

Atropisomers

The diversity of biomolecules and the many

functional groups they contain have increased their propability of occuring in an amzing number of isomers.

Structural isomerism occurs in biomolecules

in a variety of forms; in carbohydrates the pyran and furan ring structures and aldo- and keto- forms (Figure-2), and in some carboxylic acids.

A

B

C

D

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

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2013

Assignment No. 2

Conformational

because biomolecules for the most part chiral, and in living systems they are almost enantiomerically pure. Chiral compounds have the ability to rotate plane polarized light (PPL), and this property has been used historically to differentiate between enantiomers; when an enantiomer turns the PPL to the right it is called dextrorotatory and, when it turns it to the left it is called levorotatory and designated as (+) and (-) respectively. Many stereochemical descriptors are found which has nothing to do with the optical activity, as they only descripe how different groups are distributed arround the chiral center (e.g.. D and L, R and S, E and Z, Erythro and Threo). Now adays their importance has lessened somewhat with the development of powerful NMR and chiral chromatographic methods. When a solution contains equivalent amounts of the two enantiomers, it is refered to as a racemic mixture.

isomerism, another type of stereoisomerism, which occurs widely in biomolecules especially the biologically active macromolecules. Different types of conformational isomers are called conformers, and even small molecules can perform conformation. In macromolecules conformation is stabilized by inter- and intramolecular interactions. The most important types of conformers are; staggered and eclipsed; the former is the most stable so far.

Figure-4: α-D-Glucose and its epimers

Chirality In Biological World

has great significance, as it affects enzyme-substrate, antigen-antibody, hormone-receptor interactions and hence; the biological activities atained through them, this because these interactions are highly enantio-selective or stereo-selective in the biological world.

Figure-5: Steroid nucleus showing; A: all-trans configuration between adjacent rings and B: a cis configuration between rings A and B

Figure-6: L-α-Alanine

Chirality of a compound is its existence in two

References

nonsuperposable forms (i.e mirror images), and to be chiral a compound must possess at least one asymmetric or stereogenic center (i.e a carbon with four different groups attached to) (Figure-7).

1. William H. Brown, Christopher S. Foote, Brent L. Iverson, Eric V. Anslyn. Organic Chemistry. Belmont : Cengage Learning, 2012; 116:133:134:136:137:138. 2. Robert K. Murray, Daryl K. Granner, Peter A. Mayes, Victor W. 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. 2nd Edition. Stuttgart · New York : Thieme, 2005; 8. 4. Jeremy M. Berg, John L. Tymoczko, Lubert Stryer. Biochemistry. 7th Edition. New York : W. H. Freeman and Company, 2012; 321. 5. David L. Nelson, Michael M. Cox. Lehninger Principles of Biochemistry. 5th Edition. New York : W. H. Freeman and Company, 2008; 15:16:18.

Figure-7: Chiral Centers in Different Biomolecules

Chirality

O

A

and ptical ctivity is very important to be discussed throughout here; 2