Organic Chemistry

Module 8.Fatty acids and lipids

Lesson 27A
STRUCTURE AND PROPERTIES OF NEUTRAL LIPIDS, PHOSPHOLIPIDS AND CHOLESTEROL

27.A.1 Introduction

• Include –fats, oils and waxes
• Uncharged- therefore termed as non-polar or neutral lipids
• On hydrolysis yield only alcohol (glycerol) and fatty acids
• Chemically -fat and oil - glycerides

27.A.2 Glycerides

• Esters of fatty acids with glycerol
• Classified - mono- , di- and triglycerides - depending upon number of –OH of glycerol esterified. (Fig. 27.1 Simple Lipids)
• Also classified as - simple and mixed - depending upon fatty acids composition
• Simple glyceride- all fatty acids are alike- all –OH groups are esterified with same (one) type of fatty acid.
• Mixed glyceride- different fatty acids get esterified in same glycerol molecule at different positions.

27.A.3 Phospholipids

• Heterogeneous group of compounds belonging to class lipid and contain ester phosphate- therefore complex lipid
• White waxy solids- turns dark on exposure to air or light - because polymerization- because contained excessive USFAs.
• Not readily soluble in acetone, but soluble in other fat solvents-used for precipitation from extract
• Have net negative charge at phosphate group at around pH 7.0- therefore known as polar lipids
• Hygroscopic and mix well with water and form cloudy colloidal solution.

27.A.4 Importance In Dairy And Food Industry

1. Used as emulsifier in product formulation

• Purified preparation (Lecithin)- moderately effective in lowering surface tension of aqueous solutions, but when combined or adsorbed to carbohydrates or proteins – become remarkably active and constitute valuable agent for emulsification of fats and oils
• Large quantities of soyabean lecithin- used as emulsifying and smoothing agents in food industry
• Used as a wetting agent in manufacturing of instant milk powder

2. Component of milk fat globule membrane

• Play vital role in stability of fat globule emulsion form- provides net negative charge on the globule – therefore causes electrostatic repulsion between the globules

3. Used as synergistic antioxidant – specifically cephalin – prevents or delay oxidative deterioration of lipids.

27.A.5 Classification of Phospholipids

Classified into a variety of groups and sub-groups - based on structural components present in the molecule
I. Phosphoglycerides

1. Phosphatidyl choline (Lecithin)
2. Phosphatidyl ethanolamine (Cephalin)
3. Phosphatidyl serine
4. Plasmalogens
5. Diphosphatidyl glycerol (cardiolipin)

II. Phosphoinositides

Inositol or Lipositol

III. Phosphosphingosides

Sphingomyelins

27.A.6 Phosphoglycerides

• The most common phospholipids
• Derived from a phosphatidic acid

Fig. 27.2 Formation of phosphatidic acid

• One of the primary –OH group esterified with phosphoric acid and remaining two –OH groups by fatty acids (or in some causes forms ether linkage with a long unsaturated aliphatic chain)

Fig. 27.3 Formation of phosphoglycerides

• Second –OH group of phosphate gets esterified with amino alcohol

Fig. 27.4 Structure of serine

27.A.7 Lecithin

• On hydrolysis – yields – fatty acids, phosphoric acid and choline

Fig. 27.5 Structure of lecithin

• Variation in fatty acids composition – give rise to different lecithin
• Fatty acid at C₂ is generally unsaturated – susceptible to oxidation
• Phospholipase D- hydrolyses choline (base)
• Choline- heated – decomposes into trimethylamine and ethylene glycol

Fig. 27.6 Thermal decomposition of choline

• Trimethylamine has fishy flavour therefore lecithin of fat globule membrane creates fishy flavor defect in butter

27.A.8 Cephalin

• On hydrolysis yields- fatty acids , phosphoric acid and ethanolamine or serine (Fig. 27.7)
  
• Physical and chemical properties are similar to lecithin

27.A.9 Plasmalogens

• Structure- similar to lecithin or cephalin- the only difference – one of the fatty acid is replaced by enol form of long chain aldehyde, linked by ether linkage (Fig. 27.8)
• Vinyl ether linkage- generally on position 1
• Aldehyde – aliphatic and long chain (C₁₂ to C₁₈)

27.A.10 Cardiolipin

• Two primary hydroxyl group of glycerol molecule- linked to two molecules of diglyceride through phosphodiester bridges (Fig. 27.9)

27.A.11 Phosphoinositides

• Contains carbohydrate like inositol (Fig. 27.10)

27.A.12 Sphingomyelins

• In place of glycerol contain sphingosine – a complex unsaturated amino alcohol (Fig. 27.11)