Reactions

Chemistry curricula in the future

Your visit to Georgia Institute of Technology (C&EN, April 4, page 5) as well as your previous articles on meeting the chemistry challenge (C&EN, Oct. 11, 2004, page 5; Nov. 8, 2004, page 5) are also beneficial to us. They got us to think about our chemistry teaching program. But many chemistry faculty are now struggling with these questions: How much of the topics of new interdisciplinary fields should we include in our curriculum? And what should we leave out from our present curriculum?

The extremely high resolution of imaging has enabled biologists, engineers, and medical researchers to see atoms and molecules. Many of these researchers are now interested in understanding what they see quantitatively, that is, on the molecular level. This is why they need the input of chemists, because we understand molecules and molecular chemistry. Thus, we are going to be needed as long as we teach the fundamentals of molecular structure and chemical and biochemical reactions.

One would then reach the conclusion that we should emphasize fundamentals of chemistry in our teaching, but in research some of us need to reach out and conduct research with researchers in other disciplines. Of course, teaching topics in new interdisciplinary fields could be given on the graduate level as special classes. They could also be incorporated in our undergraduate courses in the form of examples or problem sets, illustrating that understanding results in new fields requires the principles of chemistry.

Mostafa A. El-Sayed
Georgia Institute of Technology, Atlanta

High-five for Mg complex

In the science concentrate "Magnesium in Flux," a pentacoordinated magnesium atom is described as a chemical curiosity (C&EN, March 21, page 40). A computational study by Stefan Kluge and Jennie Weston nicely predicts a transition from a hexacoordinated magnesium to a pentacoordinated ion upon binding of a hydroxide ligand study (Biochemistry 2005, 44, 4877). This pentacoordination had been recently proven in the crystal structure of a signal recognition particle GTPase by the group of Douglas Freymann. In the concentrate, Weston is cited as saying "pentacoordinated Mg²⁺ is a biochemical reality," while Freymann appreciates that the new work could encourage new ways of thinking about magnesium chemistry in biological systems.

I would like to point out that a pentacoordinated magnesium ion is quite common, as it is encountered in all chlorophylls and bacteriochlorophylls, which are responsible for the green color of grass, leaves, and algae. The light-harvesting complex II is the most abundant protein on Earth, and it binds within a monomer eight chlorophylls a and six chlorophylls b. All magnesium atoms within this complex, or any other chlorophyll-protein antenna complex, have the central magnesium atom within the tetrapyrrolic core bound by an extra fifth ligand. Most frequently, this ligand is a histidine residue, but other amino acids from the protein backbone or even structural water molecules are encountered.

It is unfortunate that organic chemistry and biochemistry textbooks always display (bacterio)chlorophyll formulas with a tetracoordinated magnesium atom to the four pyrrolic nitrogen atoms. This configuration is unstable and has never been encountered experimentally. A fifth ligand is always present in such tetrapyrroles.

We have published several articles describing the pentacoordinated Mg atom with an additional stereocenter within (bacterio)chlorophylls, and independently Toru Oba and Hitoshi Tamiaki have noted the strong preference for one diastereomer. The textbook formulas for such tetrapyrroles should be corrected in the future so that the pentacoordinated Mg²⁺ goes from a "biochemical reality" to an accepted banality.

Teodor S. Balaban
Karlsruhe, Germany