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David E. Hansen, Dengda Tang, Jon A. Sanborn, and Mark D. Marshall. 2006. (Strept)Avidin-Biotin: Two Interrelated Experiments for the Introductory Chemistry Laboratory. Journal of Chemical Education 83: 777–779. Full Article

A two-experiment sequence focusing on the noncovalent complex between the egg white protein avidin (or the similar protein streptavidin, which is expressed by the bacterium Streptomyces avidinii) and the essential cofactor biotin has been developed. The association equilibrium constant for the formation of the (strept)avidin–biotin complex is 1015, one of the largest ever measured for a noncovalent interaction between a protein and small molecule. In the first of the two experiments, students employ molecular modeling techniques to examine a series of noncovalent interactions, beginning with the argon dimer and culminating with (strept)avidin–biotin complex. In the second experiment, students titrate a solution of avidin with the dye 2-[(4-hydroxyphenyl)azo]benzoic acid (HABA). HABA has an absorption maximum at 348 nm, but upon binding to avidin the maximum redshifts to 500 nm, thus allowing the quantity of complex present to be determined. From these data, the equilibrium constant for the binding of HABA to avidin is calculated.

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