Betaine is a simple little molecule:
It finds use in medicine in treating elevated homocysteine levels - a molecule that deserves its own entry. It also can act as a denaturant - things like urea, formamide, and guanidinium are useful for this in biology. Betaine, at the right concentration, can help to unfold DNA a little bit, while leaving certain DNA polymerases untouched. For this reason, some people use it in DNA amplification by PCR.
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Reichardt's dye is pleasing in structure and function:
According to the Linus Pauling Institute at Oregon State University, folate (which includes folates from food and folic acid supplements) is important for amino acid metabolism and methylation reactions in the body.
Quaternized proline (stachydrine) is as interesting for its curious cyto-protective properties.