W12

Question 1

Types of isomerisation in complex ions

Answer 1

Question 2

Coordination isomersation in complex ions

Answer 2

Same formula but formula of compex ion is different

Question 3

Linkage isomers

Answer 3

Complex ion has same formula but ligand is ambidentate and can bind through different donor atoms

cyanide

M:CN-

M:NC-

cyanido-κC

cyanido-κN

thiocynate

M:SCN-

M:NCS-

thiocynato-κS

thiocynato-κN

nitrite

M:NO₃-

M:ONO₂-

nitrito-κN
nitrito-κO

Question 4

Colours

Answer 4

Isomers have different colours - even linkage isomers

Question 5

Geometric isomers

Answer 5

Atoms of groups of atoms assume different ligand binding positions in the complex

• Cis/trans isomerisation: two constitutent on the same/different side
• Fac/mer isomerisation: (octahedral complexes) three constituents making up a face/meridian

Question 6

Cis/trans isomerisation

Question 7

Fac/mer isomerisation

Answer 7

1 axial, 2 equatorial atom

2 axial, 1 equatorial atom

Takes up 1 octahedral face

Cuts through meridian

Question 8

Optical isomerisation

What is it

naming

example

Answer 8

Nonsuperimposable mirror images called delta and lambda

For 2x bidentate ligands in octahedral structure:

• Delta: 1x (equatorial to axial) 1x (equatorial to equatorial)
• Lambda: 2x (equatorial to equatorial)

Triscatecholates metal complexes have optical isomerisaion

Question 9

Sidephores

Answer 9

Chelates that can bind to iron irons very strongly

Are secreted by micro-organisms

Examples

• catecholates (Fe³⁺)
• enterobactin (Fe³⁺)
• hydrogen phosphate (Fe³⁺)
• haem group (Fe²⁺)

Question 10

Catecholate

Answer 10

Bidentate, dianionic/divalent chelating ligand whne bound to metal

Each donor atom (oxygen) is uninegative

Formed by deprotonating catechol

Question 11

Triscatecholatoferrate(III)

Answer 11

• 3 catecholates bind to one Fe(III)
• Overall 3- charge
• Very high equilibrium constant

Question 12

Enterobactin reaction

Answer 12

• Compound produced by aerobic bacteria
• Forms [FeEb]^₃- = lipophilic - able to transfer Fe into cells
• Cyclic polyester forms amide links to 3 catechol-type ligands
• It becomes a hexadenate, hexavalent ligand, binding to Fe(III) in the middle
• Overall 3- charge

Question 13

Enterobactin reaction in aerobic bacteria

Answer 13

Triscatecholato is LESS stable than enterobactin

Because of chelate effect:

• Enterobactin is a hexidentate ligand while triscatecholatoferrato(III) is three bidentate ligands
• If one of the tricatecholato bonds to metal is broken, it is lost from the complex (because of entropic driving force) but in case of enterobactin a dissociated catecholate will reattach itself (no entropic driving force)

Question 14

Hydorgen phosphate

Answer 14

• Hydrogen phosphate is a bidentate ligand that binds to Fe3+
• Both O- act as donor atoms
• forms [Fe^III(H₂O)₄(HP^VO₄^2-)]⁺
• iron complex stable at pH 2 of stomach
• used to supply blood to transferrin

Question 15

Naming coordination complexes

Answer 15

Order

1. Ligands are named in alphabetical order before metal ion
2. Cation is named before anion

Ligands

1. Anion endings change
   
   1. Ide -> ido
   2. Ate -> ato
2. Di, tri, tetra, penta, hexa indicate number of “simple” ligands
3. Bis, tris, tetrakis are used to indicate number of “complex” ligands

Metal

1. Oxidation state of metal ion is indicated by roman numerals
2. For metals in anionic complexes: sometimes latin name is used
   
   1. Ferrate Fe
   2. Cuprate Cu
   3. Platinate Pt
   4. Plumbate Pb
   5. Argenate Ag
   6. Aurate Au
   7. Stannate Sn

Question 16

Secondary structure of proteins

Answer 16

Alpha helix

Beta pleated sheet

Structure: Helical

• Intra-chain hydrogen bonding
• C=O…H-N
• All side chains point away from helix

Function: Gives elasticity to proteins

• Wool, hair: alpha keratin
• Tendons: collagen

Structure:

• Inter-chain hydrogen bonding
• C=O…H-N
• Side chains point alternatively above and below plane of sheet

Function: Gives rigidity to proteins

• Silk

Question 17

Transferin structure

Answer 17

• 2 identical subunits of 40k Da
  
  • Contains both beta and alpha sheets
• Held together by:
  
  • Ionic bonds
  • H-bonds
  • Etc.
• Fe binding constant: 10²⁶

Question 18

Transferrin structure

Answer 18

• Transport iron INTO cells (<1% of body’s total Fe content)
  
  • Travels in blood
  • Docks with transferrin receptors on cell surfacers
  • Taken into cell, drop in pH causes release of Fe3+
  • Demetalled protein is returned to outside of cell, process continues
• Bactericidal
  
  • Transferrin is in mucus, egg whites, milk
  • Scavenges for iron - deprives bacteria of it
• Passes Fe to ferritin (iron storage protein)
• Can remove Fe3+ from phosphate and citrate

Question 19

How does transferrin carry iron?

Draw

Answer 19

• Fe3+ is buried deep into transferrin
• Coordination environment contains anionic and a neutral ligands
  
  • Monodentate: 2 tyrosines(-), 1 histadine, 1 aspartic acid(-)
  • Bidentate: 1 carbonate

Question 20

Ferritin structure and function

Answer 20

Structure

• Rust coated with soluble protein

1. Protein coat or shell:
   
   1. Subunits are roughly cylindrical, 163 amino acids
   2. 24 subunits form hollow sphere with large diameter (120 angstroms)
2. Rust core
   
   1. 75 angstroms across
   2. All iron is Fe3+
   3. Mainly FeO(OH) with phosphate ligands
3. Core-protein interface
   
   1. Fe ions enter through channels at corners
   2. Ion channels are surrounded by oxygens
   3. Helps polar iron complexes pass through

Function

• Store iron (takes it from transferrin)

Question 21

O2 binding proteins

Answer 21

Uses Fe2+ in heme functional unit

• Haemoglobin (blood): transports oxygen
• Myoglobin (muscles): stores oxygen

Question 22

Haem unit

Answer 22

• Based on a deprotonated porphyrin ligand with Fe²⁺ at centre
• Square planar, tetradentate, dianionic/divalent ligand
• Aromatic so flat
• Has two propanato groups attached to porphyrin ring
  
  • Bulky
  • Negatively charged
  • Hydrophilic
• Also has methyl and vinyl subunits (not shown)
• Oxygen binds to open face opposite the two propanato side groups
• When deoxygenated, water binds to open face opposite the propanato side groups

Question 23

Structure of a protonated porphyrin ligand

Answer 23

• Aromatic (26 pi electrons) with many resonance forms
• Flat, square planar coordination geometry

Question 24

Myoglobin structure, structure of haem group binding site, function

Answer 24

Structure

• Monomeric (only one protein subunit)
• 8 alpha helices fold around 1 heme unit
• Helices are held together by:
  
  • Hydrogen bonding
  • Covalent (disulfide bridges)

Structure of Haem group

• Haeme group is held to protein side chain through histidine side chain
  
  • through nitrogen on imidazole side group binding to propanato)
• Oxygen binds to site opposite (trans) to histidine group
• Crowding of coordination site allows only very small molecules to have access to Fe(II)
  
  • Oxygen is small enough to pass through and bind

Function

• Stores oxygen in tissues (muscles)
• Myoglobin converted to oxymyoglobin even at low O2

Question 25

Haemoglobin structure and function

Answer 25

Structure * α₂β₂ tetramer (2 alpha and 2 beta subunits) * 4 haem units - minor changes to haem structure Function * Oxygen transport in blood (from lungs to muscles)

Question 26

Cooperative binding in Hb

Answer 26

* Protein subunits in Hb don't act independently * **One oxygen being taken up -\> conformational changes that allow successive oxygens to bind more easily** * Also means that when O₂ starts to be lost, successive O₂ molecules are lost more easily * This is a function of the quaternary structure -\> not found in myoglobin (separate subunits) * Without cooperativity, we would suffer asphyxiation in pure O2

Question 27

Mechanism for cooperativity in Hb

Answer 27

**Deoxygenated Hb** * Fe2+ is too big to sit in plane of porphyrin so it sits slightly below **Oxygenated Hb** * Coordination with oxygen redistributes d electrons of Fe2+ -\> makes Fe2+ centre smaller * **Fe2+ now sits in plane of porphyrin ring** * **Causes imidazole on histadine group to move up 0.08 A** * -\> leads to conformational change in other heam groups

Question 28

Carbon monoxide posioning Explain O2 vs CO coordination to haem unit

Answer 28

CO binds much more strongly than O2 to Fe(II) centre in Hb * Linear bond forms: Fe-C≡O * Lies within plane of haem -\> more stable Haem has some protein groups around coordination site * Oxygen forms a bent structure so no steric clash * CO is linear -\> leads to a steric clash btwn CO and surrounding proteins -\> BUT still bound more strongly that O2

Question 29

Carbonic anhydrase Biological function Role of catalyst Molar mass

Answer 29

Biological function: transport carbon dioxide from metabolising tissues to lungs Role of catalyst: speed up rate of forward and backward reactions of CO₂ + H₂O = HCO₃^- + H⁺ 30kDA

Question 30

**Carbonic anhydrate** Mechanism for catalyst

Answer 30

Zinc attached to three histadines (through N on imidazole) and water =\> water becomes a hydroxyl at biological pH (weakening of H-O bond) Hydroxide is what catalyses reaction

Question 31

Carbonic anhydrase Environment of metal

Answer 31