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Chapter 19: Q18P (page 488)

A study was conducted with derivatives of the DNA nucleotide bases adenine and thymine bound inside micelles () in aqueous solution.

Sodium dodecyl sulfate forms micelles with the hydrocarbon tails pointed inward and ionic headgroups exposed to water. It was hypothesized that the bases would form ahydrogen-bonded complex inside the micelle as they do in DNA:

To test the hypothesis, aliquots of 5.0 mMadenine derivative were mixed with aliquots of 5.0 mMthymine derivative in proportions shown in the table. Each solution also contained 20mMsodium dodecyl sulfate. The concentration of product measured by nuclear magnetic resonance also is shown in the table. Are the results consistent with formation of a 1:1complex? Explain your answer.

Short Answer

Expert verified

The mole fraction of the complex formation is 0.5

Step by step solution

01

Define mole fraction.

It is the ratio of number of mole of one component to the total number of moles in given mixture.

02

Determine spreadsheet.

The column E is calculates as:

n(thymine)n(thymine)+n(adenine)

03

Draw graph.

If we see, the peak is at mole fraction 0.5, which will fit in 1:1 complex formation as:

11+1=0.5

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Most popular questions from this chapter

Fluorescence quenching in micelles. Consider an aqueous solution with a high concentration of micelles and relatively low concentrations of the fluorescent molecule pyrene and a quencher (cetylpyridinium chloride, designated Q), both of which dissolve in the micelles.


Quenching occurs if pyrene and Q are in the same micelle. Let the total concentration of quencher be [Q] and the concentration of micelles be [M]. The average number of quenchers per micelle isQ=[Q]/[M]. If Q is randomly distributed among the micelles, then the probability that a particular micelle has n molecules of Q is given by the Poisson distribution:

Probability of n molecules of Q in micelle =Pn=Qnn!e-Q

whereis n factorial(=n[n-1][n-2]....[1]). The probability that there are no molecules of Q in a micelle is

Probability ofmolecules of Q in micelle = Pn=Q00!e-Q=e-Q

because 0!=1

Let l0be the fluorescence intensity of pyrene in the absence of Q and let IQbe the intensity in the presence of Q (both measured at the same concentration of micelles). The quotient lQ/l0must be e-Qwhich is the probability that a micelle does not possess a quencher molecule. Substituting Q=[Q]/[M]gives

lQ/l0=e-Q=e-[Q]/[M]

Micelles are made of the surfactant molecule, sodium dodecyl sulfate. When surfactant is added to a solution, no micelles form until a minimum concentration called the critical micelle concentration (CMC) is attained. When the total concentration of surfactant, [S], exceeds the critical concentration, then the surfactant found in micelles is[S]-[CMC]. The molar concentration of micelles is

[M]=[S]-[CMS]Nav

where Nav is the average number of molecules of surfactant in each micelle.

Combining Equationsandgives an expression for fluorescence as a function of total quencher concentration, [Q]:

ln=l0lQ=[Q]Nav[S]-[CMS]

By measuring fluorescence intensity as a function of [Q] at fixed [S], we can find the average number of molecules of S per micelle if we know the critical micelle concentration (which is independently measured in solutions of S). The table gives data for 3.8渭惭

pyrene in a micellar solution with a total concentration of sodium dodecyl sulfate [S]=20.8mM

(a) If micelles were not present, quenching would be expected to follow the Stern-Volmer equation. Show that the graph of l0/lQversus [Q] is not linear.

(b) The critical micelle concentration is 8.1mM.Prepare a graph ofln(l0/lQ)versus [Q]. Use Equation 5 to find Nav, the average number of sodium dodecyl sulfate molecules per micelle.

(c) Find the concentration of micelles, [M], and the average number of molecules of Q per micelle,Q, when[Q]=0.200mM

(d) Compute the fractions of micelles containing,, andmolecules of Q when[Q]=0.200mM

What is the advantage of a time-resolved emission measurement with Eu3+versus measurement of fluorescence from organic chromophores?

Find the concentration of [X] if the absorbance are 0.700 at 272 nm and

0.550 at 327 nm.

The figure shows spectra of1.00脳10-4MMnO4-,1.00脳10-4and an unknown mixture of both, all in1.000鈥�cm鈥�path length cells. Absorbances are given in the table. Use the least squares procedure in Figure 19-3 to find the concentration of each species in the mixture.

Visible spectrum ofMnO4-,Cr2O72-, and an unknown mixture containing both ions.

Simulating a Job鈥檚 plot. Consider the reaction A+2B鈬�AB2,forwhichK=[AB2]/[A][B]2. Suppose that the following mixtures of A and B at a fixed total concentration of

10-4 M are prepared:

(b)Prepare a graph by the method of continuous variation in which you plot[AB2]versus mole fraction offor each equilibrium constant. Explain the shapes of the curves.
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