Crown ethers

Crown ethers are present in nature, primarily in certain micro-organisms.
The antibiotics Nonactin and valinomycin are a couple of such examples.
The cells maintain a specific concentration of sodium and potassium ion inside and outside the cell. This concentration gradient is an absolute necessity for a variety of functions including maintaining osmotic pressure balance. The crown ether antibiotics solvate potassium ions selectively and are capable of passing through the cell membrane easily thus upsetting the balance of sodium and potassium ions, which is how they function as antibiotics.
Bicyclic and cyclics of higher order(tri tetra and so on) or termed Cryptrands and the corresponding metal complexes are cryptates.

Cryptates are three dimensional hence they have a more pronounced ability to selectively encapture the cations.
Crown ethers with nitrogen as the hetero atom are known to solvate anions.
Chiral crown ethers have been used in resolution
They are useful in phase transfer catalysis and S_N2 reactions.
In comparison with crown ethers cryptands are suited better for molecular recognition.
Crown ethers though commonly interact with cations they are also known to complex with small molecules like amines and phenols