DNA microarrays or \"chips\" first appeared on the market in 1996. These chips a

Question

DNA microarrays or "chips" first appeared on the market in 1996. These chips are divided into square patches, with each patch having strands of DNA of the same sequence attached to a substrate. The patches are differentiated by differences in the DNA sequence. One can introduce DNA or mRNA of unknown sequence to the chip and monitor to which patches the introduced strands bind. This technique has a wide variety of applications in genome mapping and other areas. Modeling the chip as a surface with binding sites, and modeling the attachment of DNA to a patch using the Langmuir model, what is the required difference in the Gibbs energy of binding needed to modify the fractional coverage on a given patch from 0.90 to 0.10 for two different DNA strands at the same concentration at 298 K? In performing this calculation replace pressure (P) with concentration (c) in the fraction alcoverage expression. Also, recall that K= exp(-Delta G/RT).

Explanation / Answer

Fractional coverage, is the ratio of the two values. They are the number of surface sites to which adsorbant attaches and the number of total adsorbant sites available.

The fractional coverage is represented as, = KP/ (1+KP)

The change in Gibbs free energy, G = -RT Ln (concentrations of products/reactants)

A)

0.9 = C/1+C

0.9+0.9C = C

0.9=0.1C

C=0.9/0.1=9

B)

0.1=C/1+C

0.1+0.1C = C

0.1=0.9C

C=0.1/0.9= 0.11

Difference in the free energy, G = -8.314*298*ln(9) - (-8.314*298*ln(0.11))

=> -10912 J/mol

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