FOOD CHEMISTRY STEREOCHEMISTRY IN FOOD By: AZHAR AIZAT
INTRODUCTION• Chiral carbon atoms - carbon atom that is bonded to four different groups – which bring about two distinct spatial arrangement of molecule, called enantiomers.Similarity• Enantiomers are mirror image of each other, have identical physical and chemical properties
Differences One of them will rotate the plane of polarization of plane polarized light in a clockwise direction This enantiomer is said to be dextrorotatory (+ or d) and has a positive specific rotation value. Another enantiomer will rotate the plane of polarization of the plane polarized light in an anticlockwise direction. It is said to be laevorotatory (- or l) where it has negative specific rotation value.
F9.1: Explain the three differentconventions used for naming the differentenantiometric forms1) D, L notation• An older convention used for sugar and amino acids.• The absolute configuration of other sugars about each chiral centre is then named by analogy: o If the C=O pointed away, the –OH group on the right then it is a D-isomer. o And if the C=O pointed towards you , the –OH group on the left then it is a L-isomer
• The D,L system is also applied to amino acids.• Using the “CORN” rule, the “COOH”, “R-side chain group”, “NH2” are around the Carbon atom.• CORN rule: o Molecule is viewed with the C-H bond pointed away from the observer (same as in the R,S system) o Made up of COOH, R and NH2 arrange around the carbon atom. If it is arranged clockwise – D-form If anticlockwise – L-form
• Almost all naturally occurring amino acids are the L-form and are usually tasteless.• Synthetic D-amino acids taste sweet and most naturally occurring sugars are D-form.• Enantiomers that does do not occur naturally cannot be metabolized by our body.• SUGARS contain an aldehyde or ketone, and all carbons which do not have it is attached to an alcohol.• Amino acids contain carboxylic acid (COOH), Hydrocarbon chain (R) and amino group (NH2)
1) R,S system (CIP system) Cahn, Ingold & Prelog• Used for other groups of compounds.• Involve 3 steps: i. Atom bonded to the chiral carbon are ranked in order of increasing atomic number – most common would be (H<C<N<O<F<Cl<Br) ii. When 2 @ more atoms have the same atomic no. (eg: -CH3 & -COOH) , the 2nd atoms are used to rank the substituents (so –COOH >CH3, since atomic no of 3xH < O) . And if the 2nd atom is also the same, then move on the next atom and so on. (remember that double bond count as double - COOH = 3O, since one of O have double bonds. iii. View the double molecule with the lowest ranking is pointing away from you, other three substituent must either decrease in order in a clockwise direction (R-enantiomer) @ in an anti clockwise direction (S-enantiomer)
F9.2: DISTINGUISH BETWEEN THEPROPERTIES OF ENANTIOMERIC FORMSOF STEREOISOMERS FOUND IN FOOD
• Enantiomers may be similar but different enantiomeric forms of molecule have different taste smells and toxicity.• This is because biological systems are much more sensitive to the shape of molecule• E.g.: Carvone: R-form= Laevorotatory= has smell and flavour of spearmint. S-form= Dextrarotatory= has the smell and flavour of caraway seeds. Limonene: +(d) = Smells of oranges -(l)= Smells of lemons
• Sometimes, a natural flavour is a pure enantiomer, because it is biosynthesis tends to stereo specific.• Synthetic equivalence is often a racemix mixture mixture of equal amounts of the enantiomers . (easier to synthesis)• E.g.: alphaionone in raspberries.• As for toxicity it can varies tremendously between the different enantiomer, as become tragically apparent in the Thalidomide disaster.
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