Chemistry of Lipids

Chemistry of Lipids

• Lipids are derived from the Greek word "lipos", which translates to "fat"
• Lipids serve as esters composed of long-chain fatty acids and alcohols
• Fatty acids function as the primary building blocks for lipids
• Lipids consist of nonpolar organic compounds from biological origins
• Nonpolar solvents like chloroform, diethyl ether, and boiling alcohol can dissolve lipids
• Lipids do not dissolve in water
• Lipids are identified through their physical characteristics

Lipid Functions and Types

• They are efficient for energy storage
• Cells can biosynthesize most lipids
• Lipids come from both plants and animals in the diet
• Lipids include fatty acids, triacylglycerols, phospholipids, prostaglandins, waxes, and steroids, like cholesterol
• Phospholipids are the main lipid found in membranes because they contain a phosphate diester

Classification of Lipids

• Lipids are classified into simple, compound, and derived types

Simple Lipids

• These are the type of lipids that are stored within cells
• Simple lipids consist of fatty acids and glycerol
• They are esters of fatty acids and glycerol
• These lipids can undergo saponification, which involves hydrolysis under basic conditions
• They consist of a fatty acid and an alcohol
• Triglycerides and waxes are types of simple lipids

Compound Lipids

• Contain esters
• Undergo saponification, which involves hydrolysis under basic conditions
• They link with other compounds
• Most phospholipids have a diglyceride and a phosphate group
• Phosphoglycerides and sphingolipids are two types of phospholipids
• Glycolipids are lipids that feature a carbohydrate
• Proteolipids are any group of proteins with an attached lipid molecule

Derived Lipids

• Do not have ester groups
• Cannot be saponified, thus cannot be cleaved into smaller molecules through aqueous hydrolysis
• Examples include cholesterol, fat-soluble vitamins, and prostaglandin

Fatty Acids in Simple Lipids

• Fatty acids are long-chain unbranched carbons linked to a carboxyl group, denoted as -COOH

Fatty Acid Characteristics

• They are typically straight chains which range between 10 and 20 carbon atoms in length
• Usually consists of an even number of carbon atoms
• A distinction depends on the carbon chains
• Saturated chains are all single bonds
• Unsaturated chains have double bonds
• Double bonds in fatty acids are cis and not conjugated

Saturated Fatty Acids

• Have no double bonds in their long hydrocarbon chains
• Stearic acid is an example, represented as CH3(CH2)16COOH with a melting point of 71ºC
• They exist as solids at room temperature
• Stearic acid is found in palm oil and is frequently in handmade soap

Unsaturated Fatty Acids

• Have one or more double bonds that are generally cis in their long hydrocarbon chains
• Examples are oleic acid, which has one double bond, and linoleic acid, with two double bonds
• Both oleic and linoleic acid have 18 carbons in the chain, but have different melting points
• Oleic acids are derived from olive oil, sesame oil, sunflower, shea butter, and coconut oil
• Linoleic acid is found in soybean oil

Delta and Omega Notifications

• Fatty acids can be expressed in terms of Δ (delta) or ω (omega).
• 16 Δ° is a fatty acid with 16 carbon atoms and zero double bonds
• 16 Δ^6,9 is a fatty acid with 16 carbon atoms and 2 double bonds at positions 6 and 9 from COOH
• 16 ω^3 has 16 carbon atoms and a double bond at position 3 from the terminal CH3

Structure and Melting Points of Fatty Acids

• Myristic acid (C14) melts at 54°C and is found in butterfat and coconut oil
• Palmitic acid (C16) melts at 63°C and is found in lard and beef fat
• Stearic acid (C18) melts at 70°C and is also found in lard and beef fat
• Arachidic acid (C20) melts at 76°C and is in peanut oil
• Palmitoleic acid (C16, monounsaturated) melts at -1°C and comes from cod liver oil
• Oleic acid (C18, monounsaturated) melts at 13°C and comes from olive oil and peanut oil
• Linoleic acid (C18, polyunsaturated) melts at -5°C and is in soybean and corn oil
• Linolenic acid (C18, polyunsaturated) melts at -11°C and is in linseed and corn oil
• Arachidonic acid (C20, polyunsaturated) melts at -50°C and is in corn oil and animal tissues

Factors Affecting Melting Point

• The melting point of saturated fatty acids increases as the number of carbon atoms increases.
• Lauric acid (C12 saturated) has the lowest melting point while stearic acid (C18 saturated) has the highest
• Lauric acid (C12) has a lower melting point than myristic acid (C14) because of weaker Vander-Wall forces of attraction between lauric molecules than myristic acid molecules
• Saturated fatty acids have higher melting points than unsaturated fatty acids, given acids of the same length
• An example is that palmitic acid melts at 69.6°C, while oleic acid, which contains one cis double bond, melts at 13.4°C

Molecular Arrangement and Unsaturation

• Regular arrangement of saturated fatty acids enable molecules to pack closely, forming a crystalline structure with strong van der Waals attractions
• Cis double bonds in unsaturated fatty acids create a bend in the molecule, causing chains to pack poorly and resulting in weaker Van der Waals attractions
• Melting point decreases as the degree of unsaturation increases.
• Stearic acid, with no double bonds, has a higher melting point than oleic acid, which contains one double bond
• Oleic acid has a higher melting point than linolenic acid, which has three double bonds
• Shorter chain length and unsaturation enhance the fluidity of fatty acids and their derivatives

Properties and Food Sources

• Unsaturated fatty acids contain one or more double bonds and have nonlinear chains that prevent molecules from packing closely
• They exhibit weak attractions and low melting points, appearing as liquids at room temperature
• Saturated fatty acids have molecules fitting together in a regular pattern with strong attractions and high melting points and exist as solids at room temperature
• Saturated fats can be found in animal fats, coconut oil, and palm oil
• Monounsaturated fats are in olive oil, nuts, and avocados
• Polyunsaturated fats are in most plant oils, oily fish, and seeds

Triglycerides

• Animal fats and vegetable oils are esters composed of three fatty acid molecules linked to a glycerol, forming a triacylglycerol
• Triglycerides break down into water, an acid catalyst through hydrolysis or by lipases

Types of Triacylglycerols

• Simple triacylglycerols consist of three identical fatty acid side chains
• Mixed triacylglycerols contain two or three different fatty acids
• Differences in fatty acid residues structure affect the melting point of fats and oils

Triglycerides Molecules

• Fatty acids in a triglyceride are not always the same
• Natural triglycerides are mixtures of different triglyceride molecules
• Triacylglycerols with more saturated fatty acids have higher melting points
• Animal fats are mostly saturated fatty acids and are solids at room temperature
• Oils contain more unsaturated and polyunsaturated fatty acids, making them liquids at room temperature

Fats and Oils

• Fats are triglycerides that are solids at room temperature, usually from animal sources and contain saturated fatty acids
• Oils are triglycerides that are liquids at room temperature, from plants or fish and contain unsaturated fatty acids
• Butter is a saturated fat because it contains more saturated fatty acids than unsaturated ones
• Vegetable oils are unsaturated fats because they have more unsaturated fatty acids

Health Fats

• Healthy fats and oils consist of high amounts of both monounsaturated and polyunsaturated fats with low amounts of saturated fat

Chemical Properties of Fats and Oils

• The two main reactions fats and oils undergo are hydrolysis and saponification

Hydrolysis of Triglycerides

• Hydrolysis occurs through adding water with an acid catalyst or through digestive enzymes known as lipases, it can break apart triglycerides

Saponification of Triglycerides (Basic Hydrolysis)

• Triglycerides react with strong bases like NaOH or KOH, creating carboxylate salts called soaps

Soap Properties and Action

• NaOH produces a "hard" soap in bar forms
• KOH produces a "soft" soap in shaving creams and liquid soaps
• Soaps are amphiphilic, combining a long, nonpolar, water-insoluble hydrocarbon “tail” with a charged, water-soluble "head"
• In water, the "tails" tangle, with the charged heads sticking out and forming a structure called a micelle, through hydrogen bonds with water molecules
• Soaps clean by merging with greasy hydrophobic dirt molecules into the hydrophobic alkyl portion of the micelles
• Polar carboxylate groups of the soap micelles suspend the micelle in water

Synthetic Detergents

• Regular soaps precipitate in the presence of hard water which has Ca2+, Fe2+, Fe3+ and Mg2+
• Soaps exchange their cations and become water-insoluble, non-biodegradable, and create foam
• Synthetic detergents avoids problems with regular soaps
• Polar groups of most synthetic detergents are sodium sulfonates or sulfates
• The calcium, magnesium, and iron salts of these compounds are water-insoluble

Hydrogenation

• Alkenes transform into alkanes using hydrogen gas, H2, and a metal catalyst, Pt or Ni.
• Used to change liquid unsaturated vegetable oils at room temperature into solid saturated fats.

Hardening of Oils

• Natural unsaturated fatty acids have cis double bonds, upon vegetable oil partial hydrogenation become more saturated oils and the cis fatty acids become isomerized to trans configuration.
• Trans acids are more linear than cis, raising their melting points.
• Trans fats may mimics saturated fat properties and contribute to cardiovascular diseases and cancers

Hydrogenolysis

• Splitting occurs through hydrogen
• Carried out at high temperature and pressure
• A copper chromite catalyst is used
• Glycerol and three long-chain alcohols form as the products
• The long-chain alcohol is used in the manufacturing of detergents

Coconut Oil Reduction

• Coconut oil is reduced to yield a large amount of lauryl alcohol
• Lauryl alcohol can be esterified with sulfuric acid, yielding lauryl hydrogen sulfate.

Biological Functions of Triacylglycerols

• Triacylglycerols are primarily used as an energy reser