12 minutes reading time (2466 words)

    NSW Branch Nyholm Youth Lecture and SA Branch D.R. Stranks Memorial Youth Lecture - Oxygen, Life and Count Dracula

    Roy Tasker and Kris Basden
    University of Western Sydney, Nepean

    What colour is liquid oxygen?
    Is Bugs Bunny a vampire under treatment?
    Why does garlic keep Count Dracula away?
    Why is eating too much bacon and orange juice unhealthy?
    How can a submerged mouse breathe in liquid without drowning?
    Why do babies born to smokers weigh about 5% lighter than average?

    In this lecture we will start with the chemistry of oxygen, and its role in living systems. We can then appreciate the challenge facing Nature of safely transporting oxygen from the air, through the body, to the tissues. We will take a close look at the "molecular handbag" haemoglobin, and see how it meets this challenge so exquisitely - encapsulating the oxygen in the lungs, and releasing it only where it is required, and with extraordinary efficiency.

    Surprisingly, this will lead us to a speculative explanation for count Dracula's unusual symptoms, Bugs Bunny's preoccupation with doctors, and the poisonous action of carbon monoxide (in cigarette smoke) and nitric oxide (from the reaction of bacon with orange juice).

    The lecture will conclude with an insight into two exciting approaches in chemical research on artificial blood substitutes.

    Oxygen - Its Physical and Chemical PropertiesThe oxygen molecules inside this balloon (Figure 1) are moving at approximately 500ms-1, colliding with themselves and the balloon's molecules. We can slow them down by dipping the balloon in boiling liquid nitrogen at -196°C (CAUTION: appropriate care required). The molecules slow down dramatically, the balloon collapses, and liquid oxygen (BP -183°C) forms inside the balloon. If the balloon is shaken in air at room temperature, the oxygen boils back to the gaseous state, and the balloon expands.

    Oxygen passed through a flask submerged in liquid nitrogen also forms a blue liquid (CAUTION - this demonstration requires care and clean glassware). A small wad of cotton wool, burning inefficiently to carbon in air, explodes dramatically when added to the liquid oxygen in an open beaker (CAUTION - appropriate safety precautions are required). 

    Due to its high charge to radius ratio the oxygen atom has a high electronegativity (attraction for bonding electrons). In the ClO3- ion (chlorate ion) the oxygen atoms enhance the oxidising power of the chlorine atom by withdrawing electron density from it. A powdered mixture of KClO3 and sugar (powdered separately from the chlorate!!) burns vigorously, with smoke and a violet flame (why?), upon addition of a drop of concentrated H2SO4 (CAUTION - depending on the scale, a fume hood may be required). The H2SO4 and KClO3 react to produce a high concentration of oxygen with heat, and the sugar acts as an excellent fuel.

    These demonstrations show that oxygen supports combustion, but only if the required activation energy is available. The large enthalpy of combustion can, in part, be explained by the formation of polar C=O bonds and O-H bonds, since these have greater electron density on the electronegative oxygen atoms than in the O=O bond.

    Oxygen - Its Role in Living SystemsMost animals use oxygen as the oxidant in respiration:

    C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

    (glucose)

    Plants replenish the oxygen by photosynthesis, the reverse reaction, using sunlight as the energy source. A patch of healthy lawn, roughly 15m by 15m, produces enough oxygen at the appropriate rate to keep a person alive. (A great excuse for not mowing the lawn!)

    Oxygen - Its Transport by Haemoglobin
    Oxygen is concentrated in red blood cells (erythrocytes) in the lungs, enabling blood to absorb over 50 times more oxygen than can water. Once inside these cells the oxygen molecules are held tightly within "molecular handbags" called haemoglobin molecules (2.8 x 1O8/cell). Each haemoglobin molecule is actually composed of four similar subunits, linked together like a jigsaw puzzle by non-covalent bonds (Figure 2). As we shall see later these subunits work cooperatively during oxygenation, making this molecule more of a "machine" than a simple "handbag".

     The key to how haemoglobin works is its structure. Each subunit consists of a "springy" protein (globin) in the form of a folded helix, stabilised mainly by hydrogen bonds (Figure 3). Within a pocket of the folded protein is the haeme group, composed of a flat porphyrin molecule with a Fe2+ ion firmly complexed in the centre. The haeme group is connected to the protein by a group bonded to the iron.

     Before the oxygen binds to the first subunit, the€ Fe2+ ion sits about 75 pm above the plane of the "strained" porphyrin, and the haeme colour is purple. After an oxygen molecule enters the hydrophobic pocket on the other side of the porphyrin, a protein group swings around like a door, hindering escape. The oxygen then bonds loosely to the iron, causing an electronic rearrangement within the metal's d orbitals. This results in a decrease in ionic radius, enabling the metal ion to slot into the porphyrin plane, relieving the strain (Figure 4). The haeme colour changes to scarlet red, as the iron is now six coordinate.

     This small movement of the iron signals the attachment of an oxygen molecule to the other subunits in the haemoglobin. As the Fe2+ moves into the plane it pulls the attached protein group with it, triggering a sequence of intramolecular and intermolecular bond distortions which are transmitted throughout the subunits. The effect of these movements on the subunits is to open their "pockets", exposing their haeme groups to oxygenation. In this way the four subunits act cooperatively, such that the successive O2-binding constants of the four haemes are 1:4:24:9. This currently accepted mechanism for the machine-like uptake of oxygen (and in reverse, release of oxygen) was painstakingly elucidated by Perutz(2) and others over many y€ears.

    The narrow channel leading to the haeme pocket is just wide enough for a small molecule to enter. The pocket is also hydrophobic, excluding the H+ ions (on water molecules) required for the reduction of the O2 molecule:

    O2 + 4H+ + 4e- → 2H2O

    More than one electron is needed to reduce each oxygen molecule so one of the protective functions of the globin folds is to prevent the haeme units (one electron providers) coming together to provide the extra electrons. In addition, the protein group and the porphyrin bonded to the Fe2+ also prevent irreversible oxidation to Fe3+. Haeme molecules are easily oxidised by oxygen if the globin protein is not present, and in the oxidised form they are unable to exchange oxygen.

    Oxygen binding is an equilibrium process (Figure 5), and is thus reversible. The equilibrium constant is 5.5 x 104 showing that the oxyhaemoglobin product is favoured. A haemoglobin solution containing this equilibrium reaction can be prepared simply by lysing red blood cells, obtained from the Blood Bank, in ammonia (CAUTION - appropriate safety precautions are required when handling human blood).

     Reversibility of oxygen uptake can be demonstrated by firstly adding just enough Stokes' Reagent (iron (II) tartrate complex - a weak reducing agent) to decrease the concentration of free oxygen in this solution, producing purple deoxyhaemoglobin. Then shaking the solution in air replaces the oxygen, reforming the red oxyhaemoglobin. This serves as an excellent illustration of Le Chatelier's Principle.

    The poisoning effect of molecules with a similar size to oxygen, like CO and NO, can also be demonstrated. When cigarette smoke, containing CO, is bubbled through an oxyhaemoglobin solution, the CO molecules compete with the oxygen for the haeme groups (Figure 6). But the CO molecule binds about 150 times more strongly to the haeme group. The result is that the body's haemoglobin becomes clogged with CO. In heavy smokers up to 20% of the oxygen combining sites in their blood can be blocked by CO (2,3). This means less oxygen for the growing foetus if mum smokes!!

    NO molecules bind to haemoglobin up to 50,000 times more strongly than O2 molecules. NO can be generated in situ in a haemoglobin solution by adding solid NaNO2 (an additive in preserved meats like bacon) to solid vitamin C (an ingredient in orange juice). The next tim€e you are in the supermarket look at the food additives in preserved meats like bacon. You will see nitrates (Nos. 249,250) and ascorbic acid (No. 300) mentioned on the label. Why? Because HbNO is stable to air oxidation to metHb, Hb with Fe(III), which is brown and aesthetically unappetising. The latter is formed from HbO2 if ordinary meat is exposed to the air, or heated in a pan.

    Vampires and Blood Disorders
    The natural biosynthesis of haeme is a complex process. Porphyria is the hereditary disorder that results in overproduction of the porphyrin component, and deposition of the excss porphyrin in the skin. When sunlight hits the skin these porphyrin molecules become electronically excited, and their excitation energy is passed onto oxygen in the skin cells. The now excited oxygen, singlet oxygen, is very reactive towards molecules with double bonds, and hence causes considerable disruption to cellular metabolism. Skin cells can form ugly lesions - a kind of light-induced leprosy (5).

    Iron deficiency is a common medical problem, due to poor absorption (only about 10%) of dietary iron. This problem results from the insolubility of Fe(II) and Fe(III) hydroxides in the alkaline conditions within the intestine:

    Fe(OH)2 ⇌ Fe2+ + 2OH- , Ksp = 6.0x10-15

    Fe(OH)3 ⇌ Fe3+ + 3OH- , Ksp = 8.0x10-40

    The combination of the above medical conditions - iron-deficiency porphyria - may explain the persistence of the Dracula myth(5). Since haeme is absorbed 5 - 10 times more efficiently than uncomplexed iron, an excellent source of iron for the iron-deficiency porphyria sufferer would be the haeme in fresh blood! He would also be expected to be pale, with receding gums exposing protruding teeth (both symptoms of iron deficiency), and he would also have to avoid daylight due to the skin's light sensitivity (Figure 7).

     What about Count Dracula's aversion to garlic? Well, garlic breaks down old red blood cells. The enzyme cytochrome P-450 in garlic removes iron from the haeme group, spoiling Count Dracula's reason for drinking blood in the first place!

    One treatment for porphyria is carotene (7) - a precursor of vitamin A found in carrots. It works by absorbing the excitation energy in porphyrin molecules bombarded by sunlight, preventing the formation of singlet oxygen. Well, could it be that Bugs Bunny is a vampire, who can only venture into daylight if he munches continuously on carrots? He is obviously under medical treatment for some condition since he often absent-mindedly says "What's up doc?" after eating a carrot! Very suspicious!!

    The Search for an Artificial Blood
    Oxygen is remarkably soluble (up to 50% v/v at room temperature) in perfluorocarbon emulsions containing, for example,

     Such a high dissolved oxygen concentration enables a mouse to obtain sufficient oxygen by breathing such liquids (Figure 8), with no adverse after effects. In contrast, breathing water drowns because the solubility of oxygen in water is only about 3%v/v at room temperature.

     By understanding the critical design features of haemoglobin, chemists, as molecular engineers, can build molecules that mimic its ability to reversibly exchange oxygen, as yet without the cooperative efficiency of the subunits. The most successful complexes to date use an iron porphyrin with a hydrophobic pocket formed by organic groups attached to the porphyrin (Figure 9). In one respect the "capped" porphyrin structure is superior to haemoglobin. CO and O2 bind to the haeme iron differently.

     So, by designing the height of the cap low enough to accommodate only the 02 molecule, the complex cannot be "poisoned" by CO. This approach has been substantiated by experimental results.

     Once chemists fully understand how biological molecules work there remains the challenge of mimicking, and even improving upon, Nature at the molecular level. The future of molecular engineering looks very exciting.

    References

    1. Tasker, R.F,, Basden K. and Roberts, P., "Presenting a Chemistry Youth L€ecture". Chemistry in Australia, 1992, 108 - 110.
    2. Perutz, M.F., Scientific American, 1978, Vol 239, p68.
    3. Huheey, J.E., Inorganic Chemistry, second edition, 1985.
    4. Rix, C.J., J Chem Ed, 1982, 389.
    5. Milgrom, L., New Scientist, 1984, April, 9.
    6. Wells, C.H.J., Education in Chemistry, 1990, May, 77.

    About the Authors
    Roy Tasker discovered the joy of chemistry as an undergraduate at the University of Queensland, and as a postgraduate at the University of Otago where he completed his PhD in 1982. He developed a new method of synthesising peptides using Co(III) complexes.

    After teaching positions at the University of Tasmania (1982), Brisbane Grammar School (1983), and the University of Adelaide (1984), he was appointed as a foundation lecturer at the then Nepean CAE in 1985. As part of a team that developed the first undergraduate and postgraduate applied chemistry courses in Western Sydney he was later promoted to Senior Lecturer (1989) in the newly established University of Western Sydney, Nepean.

    Apart from his interests in experimental research in bioinorganic chemistry he is actively involved in communicating chemistry to the general public, both young and old. He has scripted three chemistry videos, served on the HSC Chemistry Examination committee (1988-1990), and co-presented the RACI NSW Nyholm Youth Lecture and the RACI SA Stranks Lecture in 1991 with Kris Basden (UWS, Nepean). As Public Relations representative for the NSW RACI Branch he enjoys the challenge of communicating chemistry through the media. He is also involved in the Understanding Science and your Environment (USE) Program as a presenter of its pilot module. "Everyday Chemicals - Balancing Benefits and Risks" that he developed with Diedre Tronson (UWS, Hawkesbury). In May 1992 he was presented with an inaugural Award for Teaching Excellence at the University.

    He sees improving the scientific literacy of the public, and the image of chemists and chemicals in particular, as an urgent and vital priority for the 1990s.

    Kris Basden graduated from the University of Technology, Sydney in 1983 with a BAppSc in Biomedical Science. After graduation, she joined the then Centre for Applied Science at Nepean College of Advanced Education as a Technical Officer in Chemistry/Biology and a Foundation Staff Member. She is currently employed at the same institution, now known as the University of Western Sydney, Nepean, in the Faculty of Science and Technology as Assistant Technical Manager.

    She is currently enrolled in a MSc at Macquarie University, her research project involving the study of the development of immune competence in marsupials. Her other interests include the exciting and rapidly developing area of DNA technology.

    Kris is keen to raise the level of interest in science of young students. This has led to co-presenting the RACI NSW Nyholm Youth Lecture and the RACI SA Stranks Lecture in 1991 with Roy Tasker (UWS, Nepean), and many other guest-speaking opportunities at high schools in the area of Western Sydney, where she encourages all students to further their knowledge of science. She also hopes to promote science within the community by showing that science can be entertaining and fun.

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