Aspects of Transition Metal Chemistry Applied to Biological Systems (II) Flashcards
What is the Chelate Effect?
When two or more donor atoms from the same ligand bind to the metal centre then the ligand is chelating
Chelated complexes of polydentate ligands are always thermodynamically stable than those of the same metal with an equivalent number of comparable monodentate ligands
Why are Chelated complexes of polydentate ligands more thermodynamically stable than those of the same metal with an equivalent number of comparable monodentate ligands
1)** Statstical effect** - when one end of a chelating ligand binds, the other end is close by
2) ΔS entropy - more particles are released when using polydentate ligands, meaning ΔS is more positive and ΔG (stability constant) goes up
3) ΔH contributions - as multiple ligands approach the metal there is repulsion from the LPs on the ligands, however with a polydentate ligand the repulsion has already been overcome when forming the ligand
4) Hydration energy - when the ligand binds to the metal, we have to disrupt the hydrogen bonding network that the ligand has with the solvent environment (8 sites for H-bonding on 2xNH₃ while 6 site for H-bonding on en - less hydration energy to disrupt and more favourable ΔH)
Metals in biology usually bind to ligands according to the Hard Soft Acids Bases principle (HSAB)
What does this mean?
- Hard acids bind to hard bases
- Soft acids bind to soft bases
What is a hard acid?
A metal with high charge and/or small ionic radius
What is a Hard Base?
A ligand with high electronegativity
What is a Soft Acid?
Metal with low charge and relatively large ionic radius
What is a soft base?
A ligand with low electronegativity and relatively polarisable
Which of the following acids and bases would bond together based upon HSAB
- Cu(II) is the hard acid due to having a higher charge and hence smaller radius
- (O Asp)₂ (S Cys) is the hard ligand due to having 2 x of the more electronegative (O Asp)₂
The redox potentials for cytochrome a3 (top) and cytochrome c (bottom)
Explain the difference in the two values
- Both values for mV are positive, indicating Fe(II) is more stable than Fe(III)
- This is due to the more positive Eθ, the more negative ΔG - more stable (ΔG = -nFEθ)
- With Eθ being larger for Cytochrome C, it would indicate this is the more stable compound
Why is Cytochrome C the more stable compound
- Fe(II) is the softer acid
- (NHis)(SMet) is the softer ligand
- Hence are more likely to bond together + the soft environment will stabilise Fe(II)
- And Eθ is more positive
How would you work out the CFSE for the following orbital splitting?
3e x -0.4Δ0 = -1.2ΔO
What is the difference in CFSE from Fe(II) to Fe(III)
- Fe(II) = (4x-0.4)+(2x0.6) = -0.4ΔO
- Fe(III) = (3x-0.4)+(2x0.6) = 0ΔO
- CFSE increase of -0.4ΔO for Fe(II)
Why is there a difference in E1/2 vs. SHE/mV for the two compounds
It is higher in [Fe(bipy)₃] due to bipy giving a large ΔO - pi acceptor ligand
How do these metal centres link to biology?
The coordination sphere of the metal centres, will affect the electron structure and hence its function in biology
What is X-ray diffraction?
- A powerful structural probe of proteins which enables the development of a model of the 3D structure of a molecule
- It uses x-rays to provide data which enables the formation of a model of the structure of a particular molecule
- X-ray over light, as the waves of light are 1000x to big a wavelength to see molecules with
How does X-ray diffraction work?
- shine x-rays through a crystal and measure the ways the x-rays are diffracted which results in a picture taken around a whole crystal
- Results in a range of spots with a multitude of intensities associated with it - due to the electron densities within the crystal
- We then work backwards to work out what the electron density might look like to give that pattern which we can then fit atoms to
- Ultimately resulting in a 3D structure of the protein/molecule etc
