The protein apotransferrin binds two Fe(ll) ons for cell transport. Consider whe

Question

The protein apotransferrin binds two Fe(ll) ons for cell transport. Consider when a 1.50 mL solution of Map apotransteimir s ndw 2 of lirl meeluo)folctmel .rhe solution tums red and the color change can be measured with a spectrophotometer First, explain why there is a difference in spectrophotometric response during titration before and after the equivalence point by placing the statements below into the appropriate categories Before the equivalence point After the equivalence point No binding sites are The color change The color change All Fe(II) added will available for Fe(lI) is dramatic is minimalbind to the protein Now, calculate the concentration of apotransferrin present in the original 1.50 mL solution. Number M apotransferrin

Explanation / Answer

Ans. #1. Before equivalence point: The color change in minimal, All Fe(III) added will bind to the protein.

Before the equivalence point, most of the protein is unsaturated. So, as soon as Fe(III) is added, all the Fe bind tot proteins. The appearance of color during titration before the endpoint is transient and minimal, i.e. the color does not persist longer than a few seconds.

After the endpoint: At the endpoint all the protein is saturated with Fe(III), so there is no free binding site available. Since Fe(III) in no longer bound by the protein, Fe3+ ions persist in the solution giving a red color to the solution. After the endpoint, when there is no free binding sites available for Fe(III) on protein, the color change is dramatic (abrupt color change) and persistent due to presence of free Fe-ions in solution.

#2. Moles of Fe(III) consumed = Molarity of Fe(III) x Volume of solution in liters

                                                = 1.71 M x (121 x 10-6 L)                             ; [1 uL = 10-6 L]

                                                = 2.0691 x 10-4 mol

Given, 1 Apotransferrin molecule binds 2 ions of Fe(III).

So,

Moles of Apotransferrin in solution = (1/2) x Moles of Fe(III) consumed

                                                            = (1/2) x 2.0691 x 10-4 mol

                                                            = 1.0346 x 10-4 mol

Molarity of Apotransferrin = Moles of Apotransferrin / Volume of solution in liters

                                                = 1.0346 x 10-4 mol / (0.0015 L)

                                                = 6.897 x 10-2 M

                                                = 0.069 M

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