Food Chemistry

1. Food Chemistry Chapter 17 in Green / Damjii

2. Homework F.9: Texture • Read F9– Texture pp. 488-490 • Do Qs 43-50 • on p 494

3. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.

4. Types of Isomerism – II cont. • Stereoisomers – Enantiomers (aka Optical Isomers) • Requires a molecule with a chiral center • An atom (usu C) with four different groups attached to it • aka an asymmetric carbon atom • Two forms are mirror images of one another that cannot be superimposed on each other (like gloves)

5. achiral chiral 24.2

7. Types of Isomerism – II cont. • Stereoisomers – Enantiomers • Common example are amino acids • only glycine does not have an enantiomeric form

8. Types of Isomerism – II cont. • Stereoisomers – Enantiomers • Common example are amino acids • only glycine does not have an enantiomeric form - why?

10. Types of Isomerism – II cont. Thalidomide • a drug once prescribed to counteract pregnancy-related morning sickness, is an effective sedative as the R enantiomer (left side), but the S enantiomer (right side) is a potent teratogen (it causes birth defects). Source - http://www.answers.com/topic/asymmetric-synthesis-1

11. Types of Isomerism – II cont. • Stereoisomers – Enantiomers • have different effects on polarised light • said to be optically active in a polarimeter • if a mixture has both enantiomers it is said to be a racemic mixture and is not optically active

13. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. CONVENTION 1: • dextrorotary [ + or (d) ] – clockwise rotating enantiomer; positive specific rotation value • laevorotatory [ - or (l) ] – counter clockwise rotating enantiomer; negative specific rotation value Does differentiate between the 2 enantiomers… but does NOT indicate absolute configuration (spatial arrangement).

14. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. CONVENTION 2: D, L (no relation to d and l ) • older convention… used mostly for sugars and amino acids • Most naturally occurring sugars are in the D-form ! (and taste sweet) • Most amino acids are in the L-form ! (and are tasteless) • Enantiomers that don’t occur naturally are typically NOT metabolized by our bodies.

15. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. CONVENTION 2: D, L (no relation to d and l ) • For sugars… absolute configuration related to glyceraldehyde • Locate chiral center • Orient aldehyde group away from you • If OH is on right, the sugar is D- • If OH is on left, the sugar is L- • Most naturally occurring sugars are in the D-form ! (and taste sweet)

16. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. CONVENTION 2: D, L (no relation to d and l ) • For amino acids… use “CORN” rule • Locate chiral center and orient C-H bond away from you • If the groups COOH, R, NH2 are arranged clockwise around the chiral carbon the amino acid is the D-form • If the groups COOH, R, NH2 are arranged counterclockwise around the chiral carbon the amino acid is the L-form • Most amino acids are in the L-form ! (and are tasteless)

17. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. CONVENTION 3: R, S • Newer convention… used for most other compounds • aka CIP system – named after originators: Cahn, Ingold, and Prelog

18. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. SYSTEM 3: R,S • Locate chiral center • Rank atoms bonded to the chiral carbon in order of increasing atomic number. (One through four, one being the group of highest priority… highest atomic number). • H < C < N < O < F < Cl < Br • If there are two or more atoms with same atomic number the second atoms are used to rank the substituents… then the third… etc. • Orient the molecule so that the lowest ranking (4) substituent points away from you. • If the other three substituents decrease in a clockwise direction, it is the R-enantiomer • If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

19. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. • Orient the molecule so that the lowest ranking substituent points away from you. • If the other three substituents decrease in a clockwise direction, it is the R-enantiomer • If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

20. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. 2 2 4 4 3 3 1 1 • Orient the molecule so that the lowest ranking substituent points away from you. • If the other three substituents decrease in a clockwise direction, it is the R-enantiomer • If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

21. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. (S)-(+)-lactic acid (left) and (R)-(–)-lactic acid (right)

22. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. Enantiomers of citalopram(anti-depressant). The top is (S)-citalopram The bottom is (R)-citalopram 2 4 1 1 4

23. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms. 3 This molecule (on left side) needs to be re-oriented so the C-H bond is facing away from us 2 1 1 4 4 2 3 • Enantiomers of mecoprop, 2-(4-chloro-2-methylphenoxy)propanoicacid – a herbicide

24. F.9.2: Distinguish between the properties of the different enantiomeric forms of stereoisomersfound in food. • Natural flavors tend to be pure enatiomers – biosynthesis is stereospecific • Synthetic flavors tend to be racemic mixtures Alpha-ionone (found in raspberries) Natural = R-alpha-ionone

25. Stereoisomers – Enantiomers • Usually have similar physical properties • except when they interact with other optically active substances – which happens often in the human body • Usually have similar chemical properties • except when they interact with other optically active substances – which happens often in the human body • general, only one enantiomer of a drug, agrochemical (herbicide, pesticide), flavoring agent, or other molecule (when asymmetric) has the desired biological effect, while the other enantiomer has very different effects or, at least, places a metabolic burden on the body.

26. Types of Isomerism – II cont. amino acid asparagine - taste • one form tastes bitter, the other tastes sweet • each of the two enantiomers binds differently to chemoreceptors in the tongue. Source - http://www.answers.com/topic/asymmetric-synthesis-1

27. Carvone forms two mirror image forms or enantiomers: • R-(–)-carvone smells like spearmint (happens to be laevorotary) • S-(+)-carvone, smells like caraway seeds. • each of the two enantiomers binds differently to chemoreceptors in the nose

28. Limone forms two mirror image forms or enantiomers: • R + (d) enantiomer - smells like orange • S – (l) enantiomer - smells like lemon

29. (S)-(-)-(E)-alpha-ionone – • woody • cedar wood like • fresh juicy greenish flavor(aroma) • ( Yamamoto et al., 2009) • (R)-(+)-(E)-alpha-ionone • violet-like • fruity • raspberry-like • flowery • ( Yamamoto et al., 2009)