CH3F8 Advanced Coordination & Bio-Inorganic
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
CH3F8
Advanced Coordination & Bio-Inorganic
ADVANCED ORGANIC CHEMISTRY (CH3F8)
1. ANSWER ALL PARTS:
(a) Consider the ions of Eu and Tb.
(i) Referring to their electronic structures, suggest why Eu2+ is more stable with respect to oxidation than Tb2+ .
(ii) Using the equations below, calculate the pH range over which Eu2+ is stable. E = E0 – 0.059pH
Eu3+ + 1e- → Eu2+ E0= –0.35 V
(iii) Eu3+ and Tb3+ have the same number of unpaired electrons in their valence
shell. Suggest why Tb3+ has a much larger magnetic moment.
(iv) Briefly explain why the magnetic moment of Eu3+ increases with
temperature.
(v) In a crystalline Eu3+ compound, the emission band associated with the electronic transition 5 D4 → 7 F2 is split into four closely-spaced lines. Briefly discuss whether the compound has a rock salt crystal structure or a D4h coordination environment.
[50%]
(b) Figure 1 shows the primary and tertiary structure of a mammalian
metallothionein. The vertical lines indicate the boundary between the N-
terminal and the C-terminal domains.
Figure 1. Amino-acid sequence and 3-dimensional structure of rat metallothionein-2. Bound Cd2+ ions are shown as purple spheres, and sulfur atoms are shown as orange sticks. N- and C-termini are labelled with N and C, respectively.
(i) How many cysteines participate in binding the three Cd2+ ions in Domain 1, and how many bind the four ions in Domain 2?
Figure 2 shows a titration of this protein with EDTA, monitored by 111Cd NMR spectroscopy, with each Cd2+ site labelled as seen in Figure 1.
Figure 2. Titration of 1 mM rat metallothionein-2 with EDTA (from bottom to top), monitored by 111Cd NMR spectroscopy. The ratios shown on the right are molar ratios.
(ii) Explain what can be concluded from the spectrum where 7 molar
equivalents of EDTA have been added to the protein.
(iii) Explain what can be concluded from the spectra collected in the presence
of lower EDTA molar ratios.
[25%]
(c) Figure 3 shows pH titrations of the Zn2+ and Cd2+ complexes of another, very small metallothionein with the sequence SPCTCSTCNCAGACNSCSCTSCSH. For this, the H1 proton resonance of
the C-terminal histidine residue was monitored using 1 H NMR spectroscopy.
Figure 3. 1 H NMR titration of a small metallothionein as a function of pH.
(i) Explain what can be concluded from these data about the metal-binding status of the C-terminal histidine in the two preparations.
(ii) Which coordination chemistry principle can account for the different
behaviour of the two forms during the pH titrations seen in Figure 3? Are your conclusions from part (i) in agreement with this principle? Justify your answer.
[25%]
2. ANSWER ALL PARTS:
(a) A cytochrome P450-dependent oxygenase enzyme has been identified that
catalyses the sequential oxidation of 1, via alcohol 2, to tetrahydrofuran 3 (Figure 4).
(i) Draw out the mechanism for the enzyme-catalysed oxidation of 1 to 2.
(ii) By consideration of the normal cytochrome P450-catalysed reaction
mechanism, draw a mechanism for the unusual oxidation of 2 to 3.
Figure 4.
Replacement of Phe-89 in the active site of this enzyme by tryptophan gave a F89W mutant enzyme which catalysed a different oxidation reaction: converting 1 to alcohol 4 and then carboxylic acid 5.
(iii) Rationalise why the F89W mutant catalyses a different oxidation, compared
to the wild-type enzyme, and explain the further oxidation to 5.
[50%]
(b) A 1:2 (Cu:ligand) Cu(II) complex of the quinolone derivative shown in Figure 5
(EXN) has been proposed as a metallo-antibiotic. The complex was
characterised by EPR (Figure 6) and EXAFS (Figure 7) analysis.
Figure 5. EXN antibiotic compound
Figure 6. Experimental EPR spectrum (solid line) for the Cu(II)-EXN complex. The
dotted line is a simulation (and can be ignored for this question).
Figure 7. EXAFS data (radial distribution function) for the Cu(II)-EXN complex (solid line). The dotted line is the fit according to the structural parameters shown in the table.
(i) What is the d-electron configuration of Cu(II)?
(ii) What can be deduced about the Cu(II) coordination geometry from the EPR
spectrum? How does this correlate with the d-electron configuration?
(iii) The molecular formula of the complex was determined as C38 H46 F2N6O8Cu.
Using this information and the EPR and EXAFS data, propose a structure for the complex (depicting first and second coordination shell around Cu(II) is sufficient). Explain how the experimental data support your answer.
(iv) How would you expect the EPR and EXAFS data to change if the copper
ion was reduced?
[50%]
3. ANSWER ALL PARTS:
(a) Consider the bifunctional chelator 3 (shown below) that binds amyloid beta peptides (a disease marker for Alzheimer’s disease).
Figure 8. Bifunctional chelator 3
(i) 3 can be coordinated with copper-64 or gallium-68 to produce a PET imaging agent. Identify the best coordination site for a metal cation and give the coordination number of the resulting complex.
(ii) Using Table 1, compare the 64Cu-labelled PET agent with its 68Ga-labelled
analogue for imaging purposes. Discuss the advantages and disadvantages of using a bifunctional metal chelator rather than a 18 F- tagged marker for diagnosis.
Table 1. Parameters for some radioisotopes
Isotope |
t1/2 |
Main emissions |
|
|
|
Decay mode |
Decay energy |
18 F |
1.8 h |
Positron (97 %) |
0.63 MeV |
64Cu |
12.7 h |
Positron (18%) Beta decay (39%) Gamma radiation (0.5 %) |
0.58 MeV 0.65 MeV 1.35 MeV |
68Ga |
1.1 h |
Positron (89%) |
0.89 MeV |
(iii) How might you modify 3 to become a diagnostic agent that uses a different
imaging modality for detection?
(iv) Why is it difficult to design a responsive PET imaging agent?
[50%]
(b) Figure 9 shows the active site of manganese superoxide dismutase (Mn-SOD)
in its Mn(II) resting state.
Figure 9. Active site of Mn-SOD. The central manganese ion is highlighted in green, carbon atoms are grey, nitrogens in blue, and oxygens in red
(i) Describe the coordination sphere of manganese in this enzyme, including geometry and bound amino acids.
(ii) How does the coordination geometry differ from what is most frequently
observed for Mn(II)?
(iii) The redox potential for the Mn(II)/Mn(III) couple in aqueous solution at pH
7 is +1096 mV. In the enzyme Mn-SOD, it is 393 mV. Suggest, qualitatively, how the protein may influence the redox potential in this way.
(iv) Write down the dismutation reaction for superoxide.
(v) The catalytic cycle of all SODs follows a “ping-pong” mechanism. Taking into account your answers to (ii) and (iii), and starting with the resting state shown in Figure 9, suggest a catalytic cycle with four different states.
[50%]
2022-08-11