LECTURE 08. GENERAL CHARACTERISTICS OF THE MACROMOLECULAR SOLUTIONS

The Macromolecules

        The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

Most macromolecules are polymers, built from monomers

        Three of the four classes of macromolecules—carbohydrates, proteins, and nucleic acids—form chainlike molecules called polymers.

         A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds.

Illustration of a polypeptide macromolecule

A macromolecule is a very large molecule commonly created by polymerization of smaller subunits. In biochemistry, the term is applied to the three conventional biopolymers (nucleic acids, proteins and carbohydrates),^[1] as well as non-polymeric molecules with large molecular mass such as lipids and macrocycles. The individual constituent molecules of macromolecules are called monomers (mono=single, meros=part).

According to the standard IUPAC definition, the term macromolecule as used in polymer science refers only to a single molecule. For example,a single polymeric molecule is appropriately described as a "macromolecule" or "polymer molecule" rather than a "polymer", which suggests a substance composed of macromolecules.

Properties

Macromolecules often have unusual physical properties. For example, individual pieces of DNA in a solution can be broken in two simply by sucking the solution through an ordinary straw. This is not true of smaller molecules. The 1964 edition of Linus Pauling's College Chemistry asserted that DNA in nature is never longer than about 5,000 base pairs. This error arose because biochemists were inadvertently and consistently breaking their samples into pieces. In fact, the DNA of chromosomes can be hundreds of millions of base pairs long.

Another common macromolecular property that does not characterize smaller molecules is their relative insolubility in water and similar solvents. Many require salts or particular ions to dissolve in water. Similarly, many proteins will denature if the solute concentration of their solution is too high or too low.

High concentrations of macromolecules in a solution can alter the rates and equilibrium constants of the reactions of other macromolecules, through an effect known as macromolecular crowding. This comes from macromolecules excluding other molecules from a large part of the volume of the solution, thereby increasing these molecules' effective concentration.

         The repeated units are small molecules called monomers.

        Monomers are connected by covalent bonds that form through the loss of a water molecule. This reaction is called a condensation reaction or dehydration reaction.

         When a bond forms between two monomers, each monomer contributes part of the water molecule that is lost. One monomer provides a hydroxyl group (—OH), while the other provides a hydrogen (—H).

         Cells invest energy to carry out dehydration reactions.

         The process is aided by enzymes.

        The covalent bonds connecting monomers in a polymer are disassembled by hydrolysis, a reaction that is effectively the reverse of dehydration.

         In hydrolysis, bonds are broken by the addition of water molecules. A hydrogen atom attaches to one monomer, and a hydroxyl group attaches to the adjacent monomer.

         Our food is taken in as organic polymers that are too large for our cells to absorb. Within the digestive tract, various enzymes direct hydrolysis of specific polymers. The resulting monomers are absorbed by the cells lining the gut and transported to the bloodstream for distribution to body cells.

         The body cells then use dehydration reaction to assemble the monomers into new polymers that carry out functions specific to the particular cell type.

An immense variety of polymers can be built from a small set of monomers.

        Each cell has thousands of different kinds of macromolecules.

         These molecules vary among cells of the same individual. They vary more among unrelated individuals of a species, and even more between species.

        This diversity comes from various combinations of the 40–50 common monomers and some others that occur rarely.

         These monomers can be connected in a great many combinations, just as the 26 letters in the alphabet can be used to create a great diversity of words.

Carbohydrates serve as fuel and building material

        Carbohydrates include sugars and their polymers.

        The simplest carbohydrates are monosaccharides, or simple sugars.

        Disaccharides, or double sugars, consist of two monosaccharides joined by a condensation reaction.

        Polysaccharides are polymers of many monosaccharides.

Sugars, the smallest carbohydrates, serve as fuel and a source of carbon.

        Monosaccharides generally have molecular formulas that are some multiple of the unit CH₂O.

         For example, glucose has the formula C₆H₁₂O₆.

        Monosaccharides have a carbonyl group (>C=O) and multiple hydroxyl groups (—OH).

         Depending on the location of the carbonyl group, the sugar is an aldose or a ketose.

         Most names for sugars end in -ose.

         Glucose, an aldose, and fructose, a ketose, are structural isomers.

        Monosaccharides are also classified by the number of carbons in the carbon skeleton.

         Glucose and other six-carbon sugars are hexoses.

        Monosaccharides may also exist as enantiomers. Monosaccharides, particularly glucose, are a major fuel for cellular work.         They also function as the raw material for the synthesis of other monomers, such as amino acids and fatty acids.

        Two monosaccharides can join with a glycosidic linkage to form a disaccharide via dehydration. Maltose, malt sugar, is formed by joining two glucose molecules. Sucrose, table sugar, is formed by joining glucose and fructose. Sucrose is the major transport form of sugars in plants. Lactose, milk sugar, is formed by joining glucose and galactose. Polysaccharides, the polymers of sugars, have storage and structural roles. Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages.

        Starch is a storage polysaccharide composed entirely of glucose monomers.

Lipids are a diverse group of hydrophobic molecules. Unlike other macromolecules, lipids do not form polymers.

Proteins have many structures, resulting in a wide range of functions. Proteins account for more than 50% of the dry mass of most cells. They are instrumental in almost everything that an organism does. All protein polymers are constructed from the same set of 20 amino acid monomers. Polymers of proteins are called polypeptides. A protein consists of one or more polypeptides folded and coiled into a specific conformation. Amino acids are the monomers from which proteins are constructed. Amino acids are organic molecules with both carboxyl and amino groups.

Amino acids are joined together when a dehydration reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen from the amino group of another. The resulting covalent bond is called a peptide bond. Repeating the process over and over creates a polypeptide chain. At one end is an amino acid with a free amino group (the N-terminus) and at the other is an amino acid with a free carboxyl group (the C-terminus). Polypeptides range in size from a few monomers to thousands.

        Three levels of structure—primary, secondary, and tertiary structures—organize the folding within a single polypeptide. Quaternary structure arises when two or more polypeptides join to form a protein. The primary structure of a protein is its unique sequence of amino acids. Coils and folds are referred to as secondary structure and result from hydrogen bonds between the repeating constituents of the polypeptide backbone. The weakly positive hydrogen atom attached to the nitrogen atom has an affinity for the oxygen atom of a nearby peptide bond. Each hydrogen bond is weak, but the sum of many hydrogen bonds stabilizes the structure of part of the protein. Typical secondary structures are coils (an alpha helix) or folds (beta pleated sheets). The structural properties of silk are due to beta pleated sheets.

Tertiary structure is determined by interactions among various R groups. These interactions include hydrogen bonds between polar and/or charged areas, ionic bonds between charged R groups, and hydrophobic interactions and van der Waals interactions among hydrophobic R groups. While these three interactions are relatively weak, strong covalent bonds called disulfide bridges that form between the sulfhydryl groups (SH) of two cysteine monomers act to rivet parts of the protein together.

Quaternary structure results from the aggregation of two or more polypeptide subunits.

Superabsorbent polymer

Superabsorbent polymers (SAP) (also called slush powder) are polymers that can absorb and retain extremely large amounts of a liquid relative to their own mass.

Water absorbing polymers, which are classified as hydrogels when cross-linked, absorb aqueous solutions through hydrogen bonding with water molecules. A SAP's ability to absorb water is a factor of the ionic concentration of the aqueous solution. In deionized and distilled water, a SAP may absorb 500 times its weight (from 30–60 times its own volume) and can become up to 99.9% liquid, but when put into a 0.9% saline solution, the absorbency drops to maybe 50 times its weight. The presence of valence cations in the solution will impede the polymer's ability to bond with the water molecule.

The total absorbency and swelling capacity are controlled by the type and degree of cross-linkers used to make the gel. Low density cross-linked SAP generally have a higher absorbent capacity and swell to a larger degree. These types of SAPs also have a softer and more sticky gel formation. High cross-link density polymers exhibit lower absorbent capacity and swell, but the gel strength is firmer and can maintain particle shape even under modest pressure.

The largest use of SAP is found in personal disposable hygiene products, such as baby diapers, adult protective underwear and sanitary napkins. SAP was discontinued from use in tampons due to 1980s concern over a link with toxic shock syndrome. SAP is also used for blocking water penetration in underground power or communications cable, horticultural water retention agents, control of spill and waste aqueous fluid, artificial snow for motion picture and stage production. The first commercial use was in 1978 for use in feminine napkins in Japan and disposable bed liners for nursing home patients in the USA.

Swelling  - an increase in volume of solid bodies caused by their absorption of liquids or vapors from the environment. The capacity to swell is a characteristic property of bodies consisting of macromolecular compounds (polymers). Swelling is caused by diffusion processes, which are usually accompanied by solvation (the binding of a low-molecular-weight substance by a polymer).

A distinction is made between limited and unlimited swelling. In the first case the macromolecules are bonded fairly strongly, and swelling stops after having reached a certain limit. The swelled body retains its shape and a distinct boundary with the liquid phase. In the second case, mutual diffusion of the solvent and the polymer gradually leads to the disappearance of the interphase boundary between the swelling body and the liquid. Such swelling culminates in complete dissolution of the polymer. For example, limited swelling is exhibited by gel-like ion-exchange resins in water and by vulcanized rubber in benzene; unlimited swelling is exhibited by all polymers that are soluble in a particular solvent. In some cases, such as the gelatin-water system, limited swelling gives way to unlimited swelling with increasing temperature. Swelling is also a property of some minerals with a lamellar crystal lattice—for example, the montmorillonites. Upon swelling in water, such materials may undergo spontaneous dispersion, leading to the formation of highly disperse colloidal systems.

Swelling has wide use in industry and in everyday life. It frequently accompanies bonding of polymer materials, processing of polymers to produce various articles, production of rubber adhesives, and other processes, such as preparation of many foods and many natural processes (germination of seeds and spores).

A gel (from the lat. gelu—freezing, cold, ice or gelatus—frozen, immobile) is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state. By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. It is the crosslinking within the fluid that give a gel its structure (hardness) and contribute to the adhesive stick (tack). In this way gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase.

Composition

Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects. This internal network structure may result from physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water (hydrogels), oil, and air (aerogel). Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly is a common example of a hydrogel and has approximately the density of water.

Cationic polymers

Cationic polymers are positively charged polymers. Their positive charges prevent the formation of coiled polymers. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space. Gel is a colloid solution of dispersion phase as liquid and dispersion medium as solid.

Types of gels

Hydrogels

Hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are highly absorbent (they can contain over 99.9% water) natural or synthetic polymers. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. Common uses for hydrogels include

• currently used as scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells to repair tissue.
• hydrogel-coated wells have been used for cell culture
• environmentally sensitive hydrogels which are also known as 'Smart Gels' or 'Intelligent Gels'. These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.
• as sustained-release drug delivery systems.
• provide absorption, desloughing and debriding of necrotic and fibrotic tissue.
• hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors, as well as in DDS.
• used in disposable diapers where they absorb urine, or in sanitary napkins
• contact lenses (silicone hydrogels, polyacrylamides)
• EEG and ECG medical electrodes using hydrogels composed of cross-linked polymers (polyethylene oxide, polyAMPS and polyvinylpyrrolidone)
• water gel explosives
• rectal drug delivery and diagnosis

Other, less common uses include

• breast implants
• now used in glue.

• granules for holding soil moisture in arid areas
• dressings for healing of burn or other hard-to-heal wounds. Wound gels are excellent for helping to create or maintain a moist environment.
• reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis (see ion exchange resin)

Common ingredients are e.g. polyvinyl alcohol, sodium polyacrylate, acrylate polymers and copolymers with an abundance of hydrophilic groups.

Natural hydrogel materials are being investigated for tissue engineering; these materials include agarose, methylcellulose, hyaluronan, and other naturally derived polymers.

Organogels

An organogel is a non-crystalline, non-glassy thermoreversible (thermoplastic) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an organic solvent, mineral oil, or vegetable oil. The solubility and particle dimensions of the structurant are important characteristics for the elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules.

Organogels have potential for use in a number of applications, such as in pharmaceuticals, cosmetics, art conservation, and food. An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.

Xerogels

A xerogel is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15-50%) and enormous surface area (150–900 m²/g), along with very small pore size (1-10 nm). When solvent removal occurs under hypercritical (supercritical) conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively transforms the porous gel into a dense glass.

Properties

Many gels display thixotropy - they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. By replacing the liquid with gas it is possible to prepare aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.

Naturally occurring gels in the animal kingdom

Some species secrete gels that are effective in parasite control. For example, the long-finned pilot whale secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent other organisms from establishing colonies on the surface of these whales' bodies.

Applications

Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in manufacture of wide range of products, from foods to paints and adhesives.

In fiber optics communications, a soft gel resembling "hair gel" in viscosity is used to fill the plastic tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material is extruded around it.

Hydrogels existing naturally in the body include mucus, the vitreous humor of the eye, cartilage, tendons and blood clots. Their viscoelastic nature results in the soft tissue component of the body, disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively developing synthetically derived tissue replacement technologies derived from hydrogels, for both temporary implants (degradable) and permanent implants (non-degradable). A review article on the subject discusses the use of hydrogels for nucleus pulposus replacement, cartilage replacement, and synthetic tissue models.

Thixotropy is shear thinning property. Certain gels or fluids that are thick (viscous) under normal conditions flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. They then take a fixed time to return to a more viscous state. In more technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

Some fluids are anti-thixotropic: constant shear stress for a time causes an increase in viscosity or even solidification. Constant shear stress can be applied by shaking or mixing. Fluids which exhibit this property are usually called rheopectic. They are much less common.

Natural examples

Some clays are thixotropic, with their behavior of great importance in structural and geotechnical engineering. Landslides, such as those common in the cliffs around Lyme Regis, Dorset and in the Aberfan spoil tip disaster in Wales are evidence of this phenomenon. Similarly, a lahar is a mass of earth liquefied by a volcanic event, which rapidly solidifies once coming to rest.

Drilling muds used in geotechnical applications can be thixotropic. Honey from honey bees may also exhibit this property under certain conditions.(heather honey).

Another example of a thixotropic fluid is the synovial fluid found in joints between some bones. The ground substance in the human body is thixotropic, as is semen.^[1]

Some clay deposits found in the process of exploring caves exhibit thixotropism: an initially solid-seeming mudbank will turn soupy and yield up moisture when dug into or otherwise disturbed. These clays were deposited in the past by low-velocity streams which tend to deposit fine-grained sediment.

A thixotropic fluid is best visualised by an oar blade embedded in mud. Pressure on the oar often results in a highly viscous (more solid) thixotropic mud on the high pressure side of the blade, and low viscosity (very fluid) thixotropic mud on the low pressure side of the oar blade. Flow from the high pressure side to the low pressure side of the oar blade is non-Newtonian. (i.e.: fluid velocity is not proportional to the square root of the pressure differential over the oar blade).

Applications

Thread-locking fluid is a thixotropic adhesive that cures anaerobically.

Thixotropy has been proposed as a scientific explanation of blood liquefaction miracles such as that of Saint Januarius in Naples.

Semi-solid casting processes such as thixomoulding use the thixotropic property of some alloys (mostly light metals) (bismuth). Within certain temperature ranges, with appropriate preparation, an alloy can be put into a semi-solid state, which can be injected with less shrinkage and better overall properties than by normal injection molding.

Solder pastes used in electronics manufacturing printing processes are thixotropic.

Many kinds of inks—used in silkscreen textile printing—made from plastisol, exhibit thixotropic qualities. Some, such as those used in CMYK-type process printing, are designed to quickly regain viscosity once they are applied to protect the structure of the dots for accurate color reproduction.

The swelling capacity of a polymer is determined by the amount of liquid material that can be absorbed. This test can done by two methods:

1. Beaker test method

2. Tea bag test method

Beaker test method

In this method

• A small amount of superabsorbent polymer material is taken (0.1g) and it is placed in the beaker.
• 100 ml of doionised water is poured into the beaker.
• After 20 min the swollen polymer was separated by using filter paper
• By weighing the polymer, one can find the swollen capacity of the SAP material.

Tea bag test method

• In this method, Take 0.1g of SAP material and put the bag suspended over the excess watered beaker.
• Note the time 20 min. And weigh the bag and we calculate the percentage of swelling through the following formula:

(w2-w1)/(w1)  %

w1= weight of the polymer (Before swelling)

w2= weight of the polymer (After swelling)

• Note: Filter paper only for removing water.

References:

1.The abstract of the lecture.

2. intranet.tdmu.edu.ua/auth.php

3. Atkins P.W. Physical chemistry. – New York. – 1994. – P.299-307.

4. en.wikipedia.org/wiki

5.Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA, 1999

6.John B.Russell. General chemistry. New York.1992. – P. 550-599

7. Lawrence D. Didona. Analytical chemistry. – 1992: New York. – P. 218 – 224.

8. http://www.pharmainfo.net/free-books/pharmaceutical-suspensionsa-review

Prepared by PhD Falfushynska H.