Special Topic Complex Nitrogen-Containing Biomolecules—Alkaloids

The molecule 12-crown-4 is the proper size to capture lithium ions, and 18-crown-6 has a remarkable affinity for potassium ions. These crown ethers can stabilize positive ions of different sizes, depending on the size of the cavity (Fig. 6.61). 

Three-dimensional versions of these essentially two-dimensional molecules have now also been made, using both oxygen and other heteroatoms, such as sulfur and nitrogen, as the Lewis bases. These host molecules are called cryptands and can incorporate positive ions into the roughly spherical cavity within the cage (Fig. 6.62). 

FIGURE 6.62 Three-dimensional cryptands can incorporate metal ions into a central cavity. 

11 Special Topic Complex Nitrogen-Containing Biomolecules—Alkaloids 

Alkaloids are naturally occurring amines that are powerfully active biologically. They are among the most useful medicinal agents known. The painkiller morphine and the antimalarial agent quinine are typical examples. At the same time, some of the 

---