9 Dec 2014

Literature Discussion of Hexamminecobalt(III) – Probing Metal Ion Binding Sites in Nucleic Acids by NMR Spectroscopy

Literature Discussion

Submitted by Erica Gunn, Simmons College

I use this literature discussion in my second year inorganic class as a follow-up to a lab experiment where students synthesize Werner complexes and then (with much guidance) analyze their IR spectra using symmetry and group theory arguments. This paper provides an excellent example of how cobalt complexes are used in modern applications, and serves as a bridge to bioinorganic chemistry, which is a central feature later in the course.

Rowinska-Zyrek, M.; Skilandat, M.; Sigel, R. Hexamminecobalt(III) – Probing Metal Ion Binding Sites in Nucleic Acids by NMR Spectroscopy Z. Anorg. Allg. Chem. 2013, 639 (8-9), 1313-1320. DOI: 10.1002/zaac.201300123

Learning Goals: 

In answering these questions, a student will:

- Gain a greater appreciation for the use of transition metal complexes in solving problems of structure and binding in biological systems.

-  Discuss specific features of cobalt hexammine complexes that allow them to substitute for magnesium hexaaqua complexes  when binding to biological molecules

-  Appreciate the effects of metal size, charge, lability, and spectral activity on the ease with which a system can be characterized

-  Identify the advantages and limitations of using a structural mimic to study molecular structure.

Implementation Notes: 

I passed this paper out in the week after students synthesized a series of Werner complexes in the laboratory. Students had been exposed to descriptive inorganic chemistry and periodic trends, symmetry and (rudimentary) group theory prior to this exercise.  In the second half of the laboratory session, they had compared the rates of hydration of hexammine, pentammine, and cis-tetrammine chloride cobalt complexes using UV/VIS, so some students were quite familiar with the stability of the hexammine complex. My students had not yet been introduced to ligand field theory, HSAB theory, redox mechanisms, or the 18-electron rule; incorporation of those ideas would be an interesting extension to the activity if your class has already been prepared to discuss those theories. The paper also explicitly mentions the large number of structures in the RCSB protein data bank that make use of cobalt complexation. This could be another option for extending the exercise for an upper-level course.

Most of the students in my class are juniors or seniors, despite the fact that this is a 200-level course. Many of them have had organic and biochemistry, and several are biochemistry majors. Biological applications are of great interest to them, and bioinorganic chemistry was one of the favorite topics of the course. Students were excited to see class topics applied to current research problems. This paper was a nice example to foreshadow more advanced topics, and later served as a familiar reference point for discussions of inner and outer sphere binding mechanisms, redox chemistry, catalysis, and bioinorganic chemistry.


Evaluation Methods: 

I assigned this paper as a 10-point literature activity, intended mostly to expose students to the chemical literature and enhance their interest in class topics. Questions focus largely on reading comprehension, but could easily be expanded for use in a more advanced course (see implementation notes for some suggestions). Students answer the questions listed in the activity sheet to verify that they have read the paper, and I grade them largely based on participation. We took 10-15 minutes in class to discuss the interesting points of the paper (could easily be extended to a longer discussion if you have the time), but this was used as an independent activity to expose them to the ideas and enhance their interest in coordination chemistry. The paper is extremely readable, and students did not appear to have difficulty understanding it on their own. 


Evaluation Results: 

Though brief, the in class student discussion indicated a high level of interest in the paper, and excitement about a "real" application of course material. The reading question answers were largely correct and complete, indicating that students had achieved at least basic understanding of the paper, though some students had difficulty explaining how the authors used substitution experiments to infer Mg binding types.

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