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Chapter 18: Q26P (page 458)

18-26. A 2.00mL solution of apotransferrin was titrated as illustrated in Figure 18-11. It required 163L of 1.43 mM ferric nitrilotriacetate to reach the end point.

(a) Why does the slope of the absorbance versus volume graph change abruptly at the equivalence point?

(b) How many moles of Fe(III) (= ferric nitrilotriacetate) were required to reach the end point?

(c) Each apotransferrin molecule binds two ferric ions. Find the molar concentration of apotransferrin in the 2.00mL solution.

Short Answer

Expert verified

a)Seetheexplanation.b)n(Fe)=2.33.10-7molc)[apotransferrin]=5.85.10-5M

Step by step solution

01

Reason for volume graph change abruptly at the equivalence point

a) The slope of the absorbance versus volume graph changes abruptly at the equivalence point because before the equivalence point Fe binds to a protein and forms a red complex, so the absorbance increases. After the equivalence point, there are no binding sites.

02

Find moles

b) The moles of Fe (III) that were required to reach the end point is:

n(Fe)=c.V=1.43.10-3M.163.10-6Ln(Fe)=2.33.10-7mol

03

Find apotransferrin

c) The n of apotransferrin is:

napotransferrinn(Fe)=n(Fe)2=2.33.10-7mol2napotransferrin=1.17.10-7mol

The concentration of apo transferrin is:

apotransferrin=napotransferrinVapotransferrin=1.17.10-7mol2.10-3Lapotransferrin=5.85.10-5M

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Most popular questions from this chapter

18-29. Biotin-streptavidin fluorescence titration. Biotin is a cofactor in enzymatic carboxylation reactions. Biotin activatesCO2for biosynthetic reactions.


Streptavidin is a protein isolated from the bacterium Streptomyces avidinii that binds biotin with a formation constant of ~1014M-1. The biotin-streptavidin complex is widely used in biotechnology because the noncovalent complex is stable in the presence of detergents, protein denaturants, and organic solvents, and at extremes of pH and temperature.

The stoichiometry of the biotin-streptavidin complex was measured by a fluorescence titration. Fluorescein (page 453 ) covalently attached to biotin via the biotin carboxyl group fluoresces at 520 nm when irradiated at 493 nm. When biotin-fluorescein (BF) binds to streptavidin (SA), fluorescence decreases. The table gives emission intensity for addition of BF to SA and also for addition of SA to BF. Data are already corrected for dilution.

(a) Make a graph of fluorescence versus mole ratio for each titration and state the stoichiometry of binding of biotin to streptavidin.

(b) Explain the shape of each titration curve.

In formaldehyde, the transition n→π*(T1)occurs at 397 nm,and the n→π*(S1)transition comes at 355 nm. What is the difference in energy (kJ/mol) between the S1andT1states? This difference is due to the different electron spins in the two states.

Calculate the frequency (Hz), wavelength(cm-1), and energy (J/photon and J/mols of photon]) of visible light with a wavelength of 562 nm.

Standard addition. Selenium from 0.108 g of Brazil nuts was converted into the fluorescent product in Reaction 18-15, and extracted into 10.0 mL of cyclohexane. Then 2.00 mL of the cyclohexane solution were placed in a cuvet for fluorescence measurement. Standard additions of fluorescent product containingSe/mL are given in the table. Construct a standard addition graph to find the concentration of Se in the 2.00-mL unknown solution. Find the wt%of Se in the nuts and its uncertainty and 95% confidence interval

In a different titration, the absorbance after adding 75 mL of ferric nitrilo-triacetate to 1.500 mL of Apo transferrin was 0.222. Find the corrected absorbance.
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