Polar acidic group; important in energy metabolism; found in DNA, sugars; additional organic side chains can replace the two hydrogens; phosphorus is usually present only in the form of phosphate
Polar; forms disulfide bonds to link molecules
Ri_S—S—R2 Formed from two sulfhydryl bonds; important in protein folding aR stands for the rest of the molecule.
is, they rotate polarized light in opposite directions due to their mirror-image asymmetry. To understand this, it is necessary first to recognize that the four bonds that a carbon atom can participate in are arranged to point to the vertices of a tetrahedron if each bond connects to identical structures. This can be seen in Figure 3.4, which shows the tetrahedral
Figure 3.4 (a) Tetrahedral structure of carbon bonding and (b) a methane molecule. (Based on Gaudy and Gaudy, 1988.)
structure and a methane molecule having that form. If the functional groups on all four bonds are identical, the angle between any two bonds will be 109°.
The difference between the two types of glyceraldehyde is not obvious in the two-dimensional representation above. It seems that it should be possible to rotate the molecule around two of the bonds to change one form to the other. That it is not possible will be clearer if the molecule is viewed in its true three-dimensional form. The central carbon atom could be viewed as being at the center of a tetrahedron, with each of its four bonds pointing to a vertex. However, each of these bonds connects to a different group: an H, an OH, a CHO, and a CH2OH. Such asymmetrical carbons are called chiral centers. Molecules with chiral centers can rotate polarized light either to the right, designated (+), or the left, designated (—). Biochemical compounds are often designated d- for dextro, meaning "right" or l- for levo, meaning "left," based on a relationship to the structure of (+)glyceraldehydes or (—)glyceraldehydes, respectively. More complex molecules may have multiple carbon atoms that can form centers for optical rotation. In such cases, the d- or l- notation indicates a relationship to the structure of glyceraldehydes, not whether the molecule actually rotates light to the right or left. Whether the molecule actually rotates polarized light to the right or left is designated by including (+) or (—) in the name, respectively.
Figure 3.5 shows three-dimensional views of d- and l-glyceraldehyde. In part (a) the two forms seem identical, but a close examination will show that bonds that point out of the plane of the page in one form point inward in the other. The difference between the two becomes more clear in part (b), which could be formed by taking hold of the hydrogen bonded to the central carbon of each compound and pointing it toward the viewer. The CH2OH group is pointed to the left (the oxygen is hidden behind the carbon). Then it can clearly be seen that the two compounds cannot be superimposed on each other
because the three groups attached to the central carbon atom are arranged in opposite directions around the carbon.
The importance of chirality is that these isomers are biochemically different and will react differently from each other. Fundamentally, optical isomers have different shapes from each other. The shape of a molecule determines how it can form complexes with other molecules. As described below, the formation of complexes, especially with enzyme molecules, is a key step in a biochemical reaction. For example, d-glucose can be used for energy by the brain, but ¿-glucose cannot. Similar differences are true for many other stereoisomeric biochemical compounds. Another name for d-glucose is dextrose, the familiar ingredient in manufactured food items.
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