In this study, Szent-Gyorgyi and Isenberg examined the charge-transfer behavior of indole, and speculated about why indole and some indole derivatives (such as serotonin and tryptophan) readily form charge-transfer complexes with substances like riboflavin, even though indole's structure does not predict such reactivity.
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Szent‐Györgyi et al. (1960) studied this question.
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