Thanks Cleland determined that the compound dithiothreitol (DTT) could be used t

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Cleland determined that the compound dithiothreitol (DTT) could be used to help keep cysteine sulfhydryls in a reduced state. Before his lab synthesized DTT, they wanted some assurance that the reaction between it and cystine would have a favorable equilibrium constant. Considering the two half- reactions shown below, what is the value of Keq for the net reaction that will be spontaneous under biochemical standard state conditions (i.e., pH 7.00, T 25 °C)? Half-reaction 1: cystine 2H 2 e2 cysteine E°--0.220 V Half-reaction 2: DTTox + 2 H* + 2 e- DTTred Eo--0.332 V

Explanation / Answer

The net reaction will be the first one being reducing and second one being oxidzing

Net reaction:

DTTred + cystine --------------------- 2Cysteine + DTTox

Eo(reaction) = Eo(reduction) + Eo(oxidation) = -0.220 + 0.332 = 0.112V

Delta G = -nFEcell = -(2)(96500)(0.112) = -21616J

Delta G = -RT ln(K)

-21616 = -8.314 * (273+25) * ln(K)

ln(K) = 8.724

K = e^(8.724) = 6.148 * 10^3

Hence the Keq for the reaction will be 6.148 * 10^3

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