What is a Methyl Group?
A methyl group is an alkyl derived from methane, having a molecular structure equivalent to methane (methane is one carbon bonded to four hydrogen atoms: CH4) minus one hydrogen atom: CH3.
The prefix “meth”- in methyl indicates the presence of a single carbon atom.
When one methyl group is transfered to another compound, this is called methylation. The difference of methylation and demethylation is a matter of point of view: the molecule that looses a methyl group is demethylated, the molecule that gains a methyl group is methylated.
How Methyl Groups Changes Biological Activity
The methyl group allows for a substance to more easily cross the blood brain barrier, thus affecting the potency of the substance.
For example, methamphetamine and amphetamine are very similar compounds, but there is an extra methyl group (-CH3) on methamphetamine.
The extra methyl group in methamphetamine makes it more lipid soluble (fat soluble). That allows methamphetamine to cross the blood-brain barrier a lot more quickly than it’s amphetamine counter part. This difference in methyl group is also the same for heroin and morphine. And for caffeine and theobromine.
The extra methyl group in methamphetamine makes it more stable than amphetamine, meaning that it lasts a lot longer in the body before being broken down by the monoamine oxidase enzyme. The same is also true for the aforementioned substances; heroin and caffeine has one more methyl group then their counterparts, morphine and theobromine. That’s why both heroin and caffeine have amplified effects over their counterparts and are therefore generally more addictive.