In the formation of serotonin-picrate crystals, serotonin is the donor molecule and picrate is the electron acceptor. A red-colored charge-transfer complex is formed when serotonin is added to picric acid.
The geometry of charge-transfer electronic transitions can be studied in serotonin-picrate crystals. The nitro groups of picric acid interact with C2 and C3 of the indole ring, suggesting that the nitro group of the electron acceptor associates with the pi electron cloud of 5-HT.
"It is significant that the observed geometry is such that charge-transfer electronic transitions apparently can occur and impart color to the [red serotonin picrate] crystals." (Bugg,C.E. 1970)Indoles in general form charge-transfer complexes. The electron-donating ability of the indole nucleus is related to a high-lying pi electron on the carbon atom at position-3, or C3, of the indole donor.
Tryptophan is an indole derivative, and it is a better electron donor than most aromatic amino acids, thus proteins are known to participate in charge-transfer reactions via their tryptophan residues. When tryptophan is mixed with riboflavin and cooled to -78 C, a strong red color is observed. Tryptophan also forms a visible charge-transfer complex with electron acceptors DPN+ or TPN+. At the temperature of dry ice, tryptophan-DPN+ and tryptophan-TPN+ complexes had a yellow color, with absorption in the region of 400 nm.
Overall, serotonin is a better electron donor than tryptophan. This has been shown theoretically by calculating the kHOMO energy of 5-HT and tryptophan, and experimentally by mixing 5-HT or tryptophan with the same electron acceptor, riboflavin. 5-HT and tryptophan both form charge-transfer complexes with riboflavin but serotonin complexes much more strongly, thus verifying that it is a better electron donor than tryptophan. It has been suggested that the physiological properties of 5-HT might be related to the electron donor capabilities of the hydroxyindole moiety. For example, serotonin can pair with electron acceptor molecules of biological importance, such as nicotinamide or DPN.
Isenberg I. and A. Szent-Gyorgyi (1959). On Charge Transfer Complexes between Substances of Biochemical Interest. Proceedings of the National Academy of Sciences U. S. A. 45, 1229-1231. 10.1073/pnas.45.8.1229
SZENT-GYORGYI A., I. ISENBERG and J. McLAUGHLIN (1961). Local and pi-pi interactions in charge transfer. Proceedings of the National Academy of Sciences U. S. A. 47, 1089-1094. 10.1073/pnas.47.8.1089
Bugg C. E. and U. Thewalt (1970). Crystal structure of serotonin picrate, a donor-acceptor complex. Science 170, 852-854. 10.1126/science.170.3960.852