Chemistry of Everyday Things

Background Information

Proteins

What do nails, hair, tendons, antibodies and hemoglobin have in common? They all contain protein. Proteins are natural polymers that make up 15% of our bodies. These are very large macromolecules – their molar masses could be from 6,000 to no more than 1,000,000 grams.

There are two kinds of proteins. Those that serve a structural function such as keratin in nails or collagen in bones are called fibrous proteins. Globular proteins are those that do not provide structural integrity. Examples include hemoglobin which helps transport oxygen throughout the body and most enzymes which catalyze cellular reactions.

Proteins have many functions. As mentioned above, they provide structure. Proteins facilitate movement. As major components of muscles, they are responsible for the ability of muscles to contract. Most enzymes are proteins. Without enzymes, many chemical reactions in our body such as the breakdown of carbohydrates to generate energy will not readily take place.

Proteins like hemoglobin transport molecules from one part of the body to another. Hormones are proteins that regulate growth or control the activities of organs –oxytocin triggers milk secretion and contraction of the uterus. Antibodies are proteins that protect the organism from foreign substances.

Some proteins have a storage function – ferritin stores iron in the liver, spleen and bone marrow. Other proteins regulate the cell's ability to respond to its environment - rhodopsin is a protein involved in the vision process.

The building blocks of proteins are amino acids. There are over 100 amino acids but only 20 are commonly used to make proteins.

Amino acids have a common structure. Each amino acid has a carbon atom that forms a bond with an amino group (—NH ₂), a carboxylic group (—COOH), hydrogen and a side chain referred to as R group. What makes the 20 amino acids different from each other is the type of R group present.

Based on the R group, amino acids may be classified into 4 categories. Nonpolar amino acids have a side chain composed mostly of carbon and hydrogen, making their R group nonpolar and insoluble in water; hence, these amino acids are described as hydrophobic. In an aqueous medium, the nonpolar side chain turns away from water. An example of a nonpolar amino acid is alanine:

Polar amino acids have a polar side chain, making them soluble in water. They are also called hydrophilic amino acids. An example is cysteine.

Aspartic acid and glutamic acid make up the 3 ^rd category. The side chains of these two amino acids contain another carboxylic (—COOH) group. Shown below is glutamic acid.

Alkaline amino acids have a basic group that can accept a hydrogen ion. In lysine, the amino group in the side chain changes into an ammonium ion when it accepts a hydrogen ion.

Amino acids combine to form proteins. Two amino acids can react to form a dipeptide. A bond forms between carbon of the carboxylic end of one amino acid and nitrogen of the amino end of another amino acid. A side product of the reaction is water. The resulting amide group that now links the two amino acids is called a peptide linkage.

Note how the dipeptide has an open amino end and an open carboxyl end. This means that additional amino acids can react with the chain, making it longer, creating a polypeptide and eventually a protein.

The structure of a protein is described in terms of three levels. The primary structure is the order of sequence of amino acids in the polypeptide chain. This sequence is very importance because it determines the identity and function of the protein. For example, the hormones oxytocin and vasopressin have nine-unit polypeptides but two of their amino acids are different. As a result, they show two different functions. Oxytocin triggers uterus contraction and milk production while vasopressin affects blood pressure and moderates kidney functions.

Oxytocin: cys-tyr-ile-gln-asn-cys-pro-leu-gly

Vasopressin: cys-tyr-phe-gln-asn-cys-pro-arg-gly

Peptide linkages are rich sites for hydrogen bond formation. Because a polypeptide has several peptide linkages, it is possible for hydrogen bonds to form between peptide linkages within a chain. This causes the chain to fold or bend resulting in a secondary structure for the protein. The secondary structure of a protein is the configuration in space of the protein chain.

There are two common secondary structures: alpha-helix and beta-pleated sheet. In an alpha-helix, hydrogen bonds twist the polypeptide chain into a spring or spiral-like configuration in such a way that each turn is composed of 3.6 amino acid residues. Hydrogen bonds form between residues that are 4 spaces apart so that every peptide bond is involved in two hydrogen bonds: one from NH to a neighboring CO and one from a nearby CO to another NH. ⁵

The spring-like structure makes the resulting protein elastic. Keratin, the protein in hair, and collagen found in tendons have this structure.

The beta-pleated sheet involves two or more protein chains linked together by hydrogen bonds. The proteins in silk and muscles have this structure resulting in fibers that are flexible but show great strength and resistance to stretching.

The over-all structure of a protein is its tertiary structure. So a protein could either be globular or elongated and narrow. Think of a telephone cord as the secondary structure and when some parts of the cord get tangled or form knots with its other parts, the over-all shape of the cord is no longer a nice spiral. The resulting shape with all the knots and tangles is the cord's tertiary structure. Hydrogen bonds, ionic/salt bonds and disulfide bonds between different sites of the secondary structure result in the protein taking a specific shape in space or tertiary structure. The detailed formation of these bonds is discussed in the next sub-section.

A few proteins would have one part showing an alpha-helix structure while another part would show a beta-pleated sheet configuration. An example is hemoglobin.

The presence of nonpolar and polar sections in a protein "forces" it to take a specific tertiary structure. For example, most proteins are found in an aqueous environment. Recall that water is polar. The tendency of the nonpolar side chains of the protein is to turn away from water because water is polar. At the same time, the polar and ionic side chains tend to turn towards water. This causes the protein to bend or fold in such a way that the nonpolar side chains of the protein are more in the interior while the polar chains are protruding outwards towards the water.

Hair Structure and Composition

Hair on the scalp grows out of a sac-like hole called a hair follicle. Special cells found at the bottom of the follicle reproduce to form new cells. These new cells push dead cells upward, causing hair to "grow" and come out of the scalp as hair shaft. The hair shaft is composed of three layers: the cuticle, cortex and medulla.

The cuticle is a smooth, outermost layer composed of a transparent, scale-like single layer of overlapping cells.An intercellular, lipid-rich cement is responsible for fusing together the colorless cells which are 50-70 microns long, 5-10 microns wide and at most 1 micron thick. ⁶ This layer serves as the protective barrier for the cortex and medulla.

The sheen and texture of hair depend on the conditions of the overlapping scales in the cortex. Healthy hair feels smooth to the touch because the scales are smooth, regular and lie flat. Hair feels rough because scales in the cuticle have been eroded, scratched and are now disjointed. Even the texture of a single strand of hair could vary. Closer to the root, scales are smooth since they have not been eroded, while farther away from the root, the hair strand may feel rough because the scales are more worn out.

Hair sheen depends on how the cuticle reflects light that strikes it. A smooth layer of scales in the cuticle reflects light evenly and hair appears shiny. A layer of worn out scales reflects light unevenly resulting in hair that appears dull.

Tangles are also due to worn out scales in the cuticle. Disjointed scales get caught with scratched scales in nearby strands, creating knots that tighten as the hair is combed.

The cuticle responds to pH changes. In a basic or alkaline environment, the cuticle layer swells as the scales open up which enables liquids to penetrate into the next layer. The cuticle shrinks and hardens in an acidic environment.

The cortex is a thick, middle layer composed of spindle-like cells filled with the protein keratin and melanin pigment. This is the layer that gives hair its strength, elasticity and tenacity, and color. Chemical changes take place in the cortex. Just like the cells in the cuticle, these elongated cells are held together by an intercellular cement rich in lipids.

The innermost layer is called the medulla or medullary canal. This central portion of the hair shaft has a diameter between 10-20 microns. ⁷ It is composed of cube-shaped cells that disintegrate fast, leaving behind pockets of air. The medulla may disappear along certain parts of the hair strand and may even be absent in the entire strand. Thick or coarse hair usually contains a medulla. Fine hair for the most part lacks a medulla, as does naturally blonde hair. The role of the medulla in human hair is not completely understood.

Normal hair on the average is50.65 % carbon, 20.85% oxygen, 17.14% nitrogen, 6.36% hydrogen, and 5.0% sulfur. ⁸

A type of keratin called "hard" keratin is the protein in hair that gives it structural integrity. Keratin is a fibrous, insoluble protein. This protein polymer contains 18 amino acids, the majority of which are cysteine. The amino acids present in hair include cysteine, serine, glutamic acid, aspartic acid, alanine, proline, isoleucine, tyrosine, phenyalanine, threonine, glycine, valine, histidine, phenylalanine, methionine, leucine, threonine, and arginine.

Keratin is responsible for giving hair its strength. It is organized into protofibrils, each of which is composed of 4 spiral chains of keratin twisted together. These spiral chains are cross-linked by hydrogen, ionic/salt, and disulfide binds just like the sides of a ladder held together by rungs. Where some of these bonds are found determines whether the hair is straight or curly. Changing the shape of the hair (e.g. curly to straight and vice-versa) involves manipulating these bonds.

How does each of these bonds form? One end of a keratin chain contains the amino group or –NH ₂ while the other end has the carboxyl group, —COOH. In hair, these groups have been ionized. The carboxyl group becomes a negative ion because it loses its H ⁺ while the amino group is now a positive ion because it gains a H ⁺. The attraction between the two opposite charged ions from neighboring chains creates an ionic or salt bond that helps hold the protein strands together.

Hydrogen bonds can also form between keratin chains. Hydrogen bonds are intermolecular forces that form between the positively charged H of one molecule covalently bonded to a small but quite electronegative atom such as F, O, or N and a negatively charged atom such as F, O or B of another molecule covalently bonded to H. Even though hydrogen bonds are weak, keratin has numerous sites in which hydrogen bonds form not only within but also between molecules. This makes hydrogen bonds a significant cross-link force.

But hydrogen bonds can be broken by water. Water itself forms hydrogen bonds with sites in the protein chains that used to be connected by hydrogen bonds. This enables water molecules to insert themselves between the protein chains. Hair, which is already 30% water by weight, absorbs more water, causing the hair shaft to swell and open up the cuticle.

The strongest cross links are called disulfide bonds. These are covalent bonds that form between two cysteines that are found in different parts of the twisted keratin chains.

Cysteine is an amino acid which has a –SH part protruding from the protein backbone. When two cysteines from neighboring protein chains react, the hydrogen from the SH of each cysteine is lost. Now without their hydrogen, the two sulfur atoms form a covalent bond with each other. This bond is called disulfide bond or disulfide linkage. The most abundant amino acid in keratin is cysteine; thus, disulfide bonds play a major role in giving hair its strength.

Among the three bonds, hydrogen bonds are the easiest to break. Water, for instance, can break hydrogen bonds. This explains why curly hair becomes temporarily straight when it is wet. But once it dries, new hydrogen bonds form between adjacent keratin strands and the hair turns curly again.

An acidic or basic solution can break ionic bonds; hence, hair products that are too acidic or basic can make hair weaker and brittle.

Because disulfide bonds are a lot stronger, a redox reaction is needed to break them. It is these bonds that must be broken when hair is permed or relaxed.

In straight hair, atoms found at more or less the same sites on adjacent keratin chains form the bonds that hold the chains together. This results in straight hair.

In wavy or curly hair, most of the bonds are formed between atoms located along different sites of nearby chains. This causes keratin to fold and bend a lot, resulting in curly hair.

Two types of melanin pigment in the cortex account for natural hair color. Brown to black color is due to the pigment eumelanin. Red and yellowish colors are due to phaeomelanin. How light or dark hair color is depends on the density of distribution of the melanin granules. When skin cells grow old and stop producing eumelanin or pheomelanin, a person's hair turns gray or white.

Hair produces its own natural conditioner in the form of sebum, an oil-like material. Sebum is produced by sebaceous glands found next to the hair follicles. It coats the hair shaft, serving as a protective barrier for the cuticle. It also traps moisture in which makes hair soft and smooth. The smooth and even surface it creates on the cuticle makes the hair shiny. Finally, the waxy coating prevents bacteria from growing on the hair shaft.

But sebum can trap dirt. Sebum and dirt build up makes hair not only unhygienic but also dull.

Water alone is not effective in removing dirt from hair. Water and sebum do not mix because water is polar while sebum is nonpolar. Hence, shampoo is used to wash hair.

Shampoos contain detergent. A detergent molecule has both polar and nonpolar components. Sebum and dirt stick to the nonpolar end of the detergent. Water is used for rinsing because water attracts the polar end of the detergent. As water pulls out the polar end of the detergent, sebum and dirt dissolved in the nonpolar side of the detergent are pulled away, leaving the hair clean.

Shampoo is better than soap in washing hair. Soap is basic which will cause the scales in the cuticle to swell and open up. Shampoo has a lower pH which makes the cuticle cells lie flat.

Most shampoos have a built-in conditioner. Conditioners, whether they are part of the shampoo formulation or separate, coat the hair shaft with a waxy material. Just like sebum, a conditioner serves as a protective layer, smoothens the rough edges of cuticle scales and traps moisture in, preventing hair from tangling and getting dry and rough.

But there is also danger to using too much conditioner. Residue from over conditioning builds up and weighs the hair down, making it limp.