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Stereochemistry in Food

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Julian Coelho

on 17 December 2012

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Transcript of Stereochemistry in Food

Stereochemistry in Food The branch of chemistry that deals with spatial arrangements of atoms in molecules and the effects of these arrangements on the chemical and physical properties of substances. D,L System used for sugars and amino acids Presented by: Melanie Wightman & Julia Ma
with a little help from Julian Coelho How to distinguish between sugars and amino acids Sugars Contains an aldehyde or ketone All carbons which do not have an aldehyde or ketone have an alcohol attached Amino Acids Contains a carboxylic acid (COOH), hydrocarbon chain (R) and amino group (NH ) 2 Examples glyceraldehyde Amino Acids the molecule should be viewed with the C-H bond pointed away the CORN rule is then applied CO R N O H H 2 It is a D-isomer if the groups COOH, R and NH are arranged clockwise around the chiral carbon synthetic D-amino acids taste sweet It is an L-isomer if the groups COOH, R and NH are arranged anti-clockwise around the chiral carbon almost all naturally occurring amino acids are in the L-form and are tasteless *Often synthetic enantiomers cannot be metabolized by our bodies 2 2 the molecule should be viewed along the chain with the C=O pointed away If the -OH group is on the right it is a D-isomer If the -OH group is on the left it is an L-isomer Sugars Most naturally occurring sugars are in the D-form and taste sweet X-Ray Crystallography a method of determining the arrangement of atoms within a crystal a beam of X-rays is directed towards a crystal and diffracts into many specific directions the angles and intensities of the diffracted beams can be measured and used to produce a 3-D image of the electron density within the crystal based on this, the spatial arrangement of the atoms can be determined while the configuration of glyceraldehyde was first determined by guesswork, x-ray crystallography was used to confirm the original guesses D,L System naming is completely unrelated to the lower case d and l which represent dextrorotatory and laevarotatory structures 9 of the 19 L-amino acids are dextrorotatory D-glyceraldehyde is dextrorotatory When there is more than one possible chiral carbon, the one farthest from the aldehyde or ketone should be observed Sugars glucose fructose Amino Acids 2-aminopropanoic acid (alanine) The ability of enantiomers to rotate the plane of polarized light can be shown through the use of a polarimeter. huh? real one not real one d-form l-form RIGHT! LEFT! chemical properties of two enantiomers are IDENTICAL except when they interact with optically active substances. what the HECK does this have to do with FOOD CHEMISTRY!?!? Laevarotatory {- or (l)}: anti-clockwise rotating enantiomer Dextrarotatory {+ or (d)}: clockwise rotating enantiomer *Racemic mixture: contains a 50/50 mixture of the two enantiomers and therefore has no rotation value. well, since you asked :) smells
tastes
& toxicity Isomerism Constitutional / Structural Configurational Conformational Geometric Optical Diastereomers Enantiomers all four groups attached to the central carbon are different Stereoisomers are molecules that have the same molecular formula and the same connectivity of atoms, but differ only in the three-dimensional arrangement of those atoms in space. so just to EXPAND on enantiomers that is... A chiral molecule is non-superimposable on its mirror image
think, HANDS! :D uhh..
what's that again...? W
E
L
L... Stereoisomers in other words, a chiral carbon WHOA~ Unlike other sorts of isomers, enantiomers have identical physical and chemical properties The two non-identical mirror images are a pair of enantiomers. so going back... ohhhhh so... what's the point of having two
of them, you ask? good question it's all in the plane of polarized light, you see. metaphoric pun intended(?) so just to reiterate... the physical & All naturally-occurring amino acids exist as only one of the two possible enantiomers. amino acids are optically acitve Enantiomers may be very similar, but biological systems are much more sensitive to the shape of the molecule.
That's why they tend to form only one pair of enantiomers and have different effects such as those found in foods. Thalidomide Disaster By the time the drug was banned, more than 10,000 children had been born with major thalidomide-related problems. A common pattern of limb deformities, termed phocomelia from the Greek word for 'seal limbs', emerged. Effects included shortening or missing arms with hands extending from the shoulders, absence of the thumb and the adjoining bone in the lower arm and similar problems with the lower extremities. http://www.youtube.com/watch?v=VkyEUk1fTVk so we go on a bit of a tanget :0 remember that guys?
it's a tangent line~ Effects include: And so by extension, all proteins and enzymes are also chiral... Tastes and Smells R-form = laevorotatory = smell and flavour of spearmint S-form = dextrarotatory = smell and flavour of caraway seeds +(d) = smell of oranges
-(l) = smell of lemons Carvone Limonene but WAIT.
THERE'S MORE! like, enantiomers! but like... so what? Well, it's like this: natural = pure enantiomer
synthetic equivalent = racemic mixture Example ¿por favor? alphaionone in raspberries ;) Toxicity R,S System or CIP System - after Cahn, Ingold and Prelog who proposed it Step 1 Step 2 Step 3 The atoms bonded to the chiral carbon are ranked in order of increasing atomic number When atoms have the same atomic nmber, the second atoms are used for ranking and if those are the same, the sums on the third are used When ranking, a double bond counts for double the atomic mass Example #1 Example #2 Example #3 Example #4 Bromochlorofluoromethane (H<F<Cl<Br) H would be ranked 4th and Br would be ranked 1st The atoms should be numbered this way The molecule is then looked at with the lowest ranked substituent pointing away (this would be labelled as #4) R-Enantiomer The molecule is the ... S-Enantiomer IF the other three substitutes decrease according to ranking (from #1 - #2 - #3) in a clockwise direction the other three substitutes decrease according to ranking (from #1 - #2 - #3) in an anti-clockwise direction It is possible for a molecule to have multiple chiral carbons Diastereomers stereoisomers that are not enantiomers since they are not mirror images of each other Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the related stereocenters Menthol (2-isopropyl-5-methylcyclohexanol) 2-bromobutane 2-chloropropanal The 21 Amino Acids The End. But Not Really
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