Use of a laboratory exercise on molar absorptivity to help students understand the authority of the primary literature.

To promote understanding of the authority of the primary literature in students taking our biochemistry laboratory courses, a biochemistry laboratory exercise on the determination of an acceptable molar absorptivity value of 2-nitrophenol (2-NP) was developed. This made the laboratory course much more relevant by linking to a thematic thread, β-galactosidase, that scaffolds concepts in one exercise with those in later exercises. The substrate for the continuous assay of β-galactosidase is the chromogenic 2-nitrophenyl-β-D-galactopyranoside that produces 2-NP. In an early laboratory exercise, students determine the wavelength of maximum absorption for the protonated and deprotonated form of 2-NP at various pH values and then determine the molar absorptivity of 2-NP. Students were encouraged to discuss apparent discrepancies not only in their own determinations of molar absorptivity values for 2-NP, but also in the published molar absorptivity values for 2-NP (2,150-21,300 M(-1) cm(-1) ) at almost the same pH and at 420 nm. Finally, the students were led to a publication that serves as an authentic source for molar absorptivity of 2-NP.