Biological depressants Hypnotics

General biological depressants (hypnotics, general anaesthetics, and volatile insecticides). 611... 

Depressants may be hydrocarbons, halogenated hydrocarbons, alcohols, ethers, ketones, weak acids (like the barbiturates), weak bases, or sulphones. They are the selectively toxic agents used in medicine as hypnotics and general anaesthetics. This is the only kind of biological activity in which structure simply does not matter (there is much more about this in Chapter 15). See Section 3.3 for the general function of partition effects in securing selective distribution of drugs. 

X-ray diffraction analysis has shown that an unbranched side-chain, exactly three carbon atoms in length, conserves the psycholeptic or antidepressant effect originated by the skeleton departure from this rule introduces hypnotic properties. This chain needs to be free to take up a position where its terminal basic group can be near (for psycholeptic action) or actually overlap (for anti-depressant action) a benzene ring. These structures seem to indicate small, and yet smaller, receptor cavities for these two kinds of biological activity. 

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