Life on our planet is based on the conversion of solar to chemical energy during photosynthesis. Pla — Analytical Chemistry Chemistry Question
Copper enzyme
Life on our planet is based on the conversion of solar to chemical energy during photosynthesis. Plastocyanin (PC), is a Cu–containing protein that mediates electron transfer in the photosynthetic electron transfer chain. The active site of PC consists of a Cu ion coordinated by the N atoms of the side chains of two histidines and the sulfur atoms of one cysteine and one methionine residue. Electron transfer by PC involves interconversion of the Cu ion between the Cu(I) and Cu(II) oxidation states. The three–dimensional structure of PC can be found at http://www.rcsb.org/, Protein Data Bank file for poplar PC: 1PNC. [VISUAL]
What is the full electron configuration of Cu, Cu(I), Cu(II)?
Model Answer
Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1,
Cu(I): 1s2 2s2 2p6 3s2 3p6 3d10,
Cu(II): 1s2 2s2 2p6 3s2 3p6 3d9
Which oxidation state of PC is EPR active ?
Model Answer
Oxidised PC
Based on the amino–acid sequence of PC (the “primary structure” of the protein), the molar mass of PC is 10500. An aqueous solution of oxidized PC has an intense blue color (PC belongs to the family of “Blue Copper Proteins”), since it absorbs strongly at 597 nm (molar absorption coefficient 4500 mol 1 dm 3 cm1 ). A sample of oxidized PC in a 1 cm UV–VIS cell has an absorption of 0,700. Calculate the amount (mg) of PC and the number of Cu(II) ions that are present in 5 cm 3 of this PC solution.
Model Answer
A = ε.c.l
c = 0.700 / (4500 × 1) = 1.56×10–4 mol dm–3
5 cm3 of the solution contain 1.56×10–4 × 5×10–3 × 10500 × 1000 = 8.2 mg PC.
N(Cu atoms) = 1.56×10–4 × 5×10–3 × 6.0221×1023 = 4.7×1017
The Cu ions of certain Blue Copper proteins can be removed to give the corresponding metal–free proteins, known as apo–proteins. These forms can bind various metal ions such as Co(II), Ni(II), Zn(II) and Cd(II), and the metal sites adopt similar structures with the “native” Cu protein. Which reconstituted Blue Copper Proteins are expected to be redox inactive?
Model Answer
Electronic configurations:
Zn(II): 1s2 2s2 2p6 3s2 3p6 3d10,
Cd(II): 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10,
Co(II): 1s2 2s2 2p6 3s2 3p6 3d7,
Ni(II): 1s2 2s2 2p6 3s2 3p6 3d8.
Redox inactive are the Zn(II) and Cd(II) reconstituted Blue Copper Proteins.