In 1958, Isenberg and Szent-Gyorgyi mixed LSD-25 with riboflavin-5'-phosphate and observed a red color at –78 C. They discovered that the riboflavin molecule had taken up one electron from LSD. LSD was functioning as an electron donor in the formation of riboflavin charge-transfer complexes; the transferred electron (e.g. to reduce riboflavin) comes from the pi-electron pool of the LSD indole system. In the same publication, the authors reported on tryptopan-riboflavin, 5-HT-riboflavin, and 1-methyl-medmain-riboflavin complexes, all of which led to the appearance of a red color at -78 C. The results are shown in Table 1 below.
|From Hallucinogens HOMO, charge-transfer|
Histidine-riboflavin or tyrosine-riboflavin did not give a red color, indicating that no charge-transfer complex had been formed, and that histidine and tyrosine are inferior in electron-donating ability compared to drugs such as tryptophan and LSD. There were variations among the drugs which formed a charge-transfer complex with riboflavin. For example, the 5-HT-riboflavin complex was 7X stronger than the tryptophan-riboflavin complex indicating that serotonin was a better electron donor than tryptophan.
Some tissues such as liver have a great quantity of strongly bound riboflavin. The brown color of the liver can be attributed to the flavin radical formed in a charge-transfer interaction with liver protein. If liver tissue becomes cancerous, it takes on a different balance of reductive and oxidative processes and changes color.
Isenberg I. and A. Szent-Gyorgyi. (1958). FREE RADICAL FORMATION IN RIBOFLAVIN COMPLEXES. Proc.Natl.Acad.Sci.U.S.A. 44, 857-862. doi:10.1073/pnas.44.9.857