27 Oct 2009

Siderophore Building: In class Exercise

In-Class Activity

Submitted by Sheila Smith, University of Michigan- Dearborn
Categories
Description: 
This is an in-class exercise (brainstorming) designed to lead the student through the design of a siderophore (or "iron lover") by applying his knowledge of Lewis acid- base chemistry (specifically HSAB theory), geometry and entropy. This is a good start for a discussion of iron transport and storage in the bioinorganic section of Inorganic II (transition metals). I also use it in my junior level descriptive course when we discuss coordination chemistry.
AttachmentSize
PDF icon SiderophorebuildingICE.pdf31.92 KB
Learning Goals: 
The student should be able to apply his knowledge of Lewis acid base chemistry to predict good ligands for a transition metal ion in a given oxidation state. The student should be able to use his knowledge of preferred geometry, VSEPR to predict coordination number for a metal in a given oxidation state. The student should be able to apply his knowledge of entropic considerations to the "chelate effect"
Equipment needs: 
none.
Implementation Notes: 
I use this an an in-class discussion exercise at the introduction of Metal ion transport and storage. It provides a good review of Lewis acid-base chemistry (including HSAB) and VSEPR, as well as a discussion of the thermodynamic considerations. At the end of the exercise, I present the students with several real examples of siderophores, such as ferrioxamine B and enterobactin, along with their (extremely high) stability constants. This provides an excellent segue into the idea of how to release the iron that you've gone to so much trouble to accumulate.
Time Required: 
10 minutes
Evaluation
Evaluation Methods: 
This is an in-class exercise and therefore most assessment is based on the quality of the discussion that ensues. A student may also be assessed by asking them to apply the same principles to a metal other than iron at a later date.
Creative Commons License: 
Creative Commons Licence

Comments

I used this exercise in my junor/senior level bioinorganic course.  I had meant to do it as an in class exercise, but ran out of time, so instead gave it to the students as a homework assignment.  All of my 5 students were able to correctly answer the questions.  This wasn't too surprising given that we had just reviewed coordination compounds and HSAB theory.  I think it was a nice exercise for them to think about the concepts we had just discussed in class.

I used this activity today in my advanced inorganic course after about a week of lecture on the topic (I focused mainly on oxygen activation and electron transfer strategies).  In addition to the material on the handout, gave each group an inorganic model kit and had them build their ligands before drawing them.  This helped stress the need for 5-6 membered chelate rings because they could feel the strain in the bonds when they tried to make 4-membered rings.

I also stipulated that whatever compounds they built needed to be water-soluble (i.e., not neutral overall), which was a bit like pulling teeth at first, because nobody was thinking about the charge of the ligands they were making, other than the fact that they had oxygen in them.

In the end, my kids managed to make some ligands that I've never seen before, and which weren't completely insane, so I'd call the activity a success.

Gerard-  I love the modelling addition.  That never even occured to me... maybe you could share with us more about what they came up with .

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