5. Hemoglobin can take up 4 equivalents of oxygen. a) Please explain why K4 is s

Question

5. Hemoglobin can take up 4 equivalents of oxygen. a) Please explain why K4 is significantly larger than K1? (1 point) b) Please explain what happens when an oxygen molecule binds to the Fe center of a heme including oxidation states of Fe and 02, magnetism, Fe-O2 bond angle and Fe-N distances. (4 points) 6. a) Why is the oxidation of water a difficult process? (2 points) b) Give an example of the way Nature has solved this problem and explain the critical steps in the process. (3 points) 7. Chromium(II) acetate hydrate can be prepared in a simple two-step one-pot procedure: First, CrCl3 6H20 is reacted with zinc in aqueous hydrochloric acid. In the second step, sodium acetate is added to afford the product as a red solid. a) Please give the balanced equations for each step. (2 points) b) What is the structure of the product, chromium(Il) acetate hydrate? (1 point) c) Sketch an MO scheme of the Cr d-orbitals including the valence electrons derived from Cr. (2 points)

Explanation / Answer

QUESTION 5

a) The affinity towards oxygen increases with each binding. This is beacuse the there are multiple binding sites in the same heamoglobin molecule, which allows cooperative effect, where unbound sites negihbouring a bound site switch into a state with hig affinity towards oxygen, increasing the rate of binding.

b) Heme consitis of a Fe centre with prophyrin, where iron is bound to the pyrrole rings though nitrogen. The metal centre can form two additional bonds, apart from the four existing Fe-N bonds. Iron is in its Fe(II) state (ferrous) when unbound, and doesn't fit into the prophyrin ring. Thus it lies away from the plane. When oxygen binds, ferrous is converted to ferric Fe(III) and oxygen is converted into superoxide. The smaller Fe (III) ion fits into the phophyrin ring and reduces the bond length of Fe-N and increases the bond angle to nearly 180 degrees. The oxidation state of oxygen changes from zero to - 1/2 due to the formation of superoxide. The oxygen binds at an angle close to 120 degrees, which is bent. The deoxygenated heme is magnetic whereas te oxygen boung, oxygenated form is diamagnetic.

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