Lecture #32
Hemoglobin...an accumulation of all the chemistry we have seen in this semester course.   CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Hemoglobin

Iron II heme

ionic radii for transition metal ions

Amino acids are oxyacids in which the -OH group is bound to a carbon. That carbon is also double-bonded to a =O and single-bonded to an "amino" (ammonia-like) -NH2 group.Amino acids are connected by peptide bonds to form polypeptides. As soon as you have a number of amino acids joined, the combinatoric possibilities become staggering.
How many different arrangements of 300 amino acids are possible?
The number of different possibilities is large...indeed! The universe isn't big enough to be packed (I mean packed) with just one of each kind.

"Heme", the complex iron ion molecule that runs the essential chemistry of oxygen transport by blood. In blood, four hemes are part of a large protein structure called "hemoglobin". Shown here is "heme" with the divalent iron ion in purple at its center and the four (blue) nitrogen donor atoms as part of the heme ring coordinating ligand.

The chemistry of oxygen in hemoglobin starts with a look at the Fe2+ at the heme site.
The chemistry of oxygen in hemoglobin starts with a look at the Fe2+ at the heme site.
In the absence of oxygen at the sixth octahedral site, the geometry of the complex ion changes and this geometric shift is mechanically transmitted to the other heme sites on the hemoglobin molecule making it easier for those to give up their oxygens as well. This leads to cooperativity among sites for both oxygen release (at cells being fed oxygen) and also oxygen takeup (by red blood cells in the lungs). Why the geometry changes has to do with the ability of a small ion to fit in the planar coordination site and a large ion to be pushed out.
The effects of adding electrons across the transition metal ion series produces some surprises, although they're understandable, showing the influence of the complexes' geometries and ligand character. This is the behavior of ionic radii within a complex ion for strong field octahedral ligands.
The counterpart to the above is the change in the trend when weak field ligands are considered.