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Medical Term:

Hückel rule

Pronunciation: heck'el

Definition: the number of depolarized electrons in an aromatic ring is equal to 4n + 2 where n is 0 or any positive integer; l-tyrosine, l-phenylalanine, l-tryptophan, and l-histidine (when the imidazole ring is deprotonated) obey this rule.

[Erich Hückel]

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