Comparison of solution conformational preferences for the hallucinogens bufotenin and psilocin using 360-MHz proton...

The 360-MHz 1H NMR spectra of bufotenin and psilocin were obtained, both as the free bases in CDCl3 and as protonated salts in D2O. Coupling constants for the side-chain methylenes were derived using the LAOCN3 program. These time-averaged coupling constants indicate that the trans and gauche rotamers of both compounds have about equal energy in D2O. There is a slight excess of the trans rotamer of bufotenin in CDCl3. For psilocin, in contrast, the gauche form is highly favored in CDCl3. The magnitude of this stabilization was estimated at about 1 kcal/mol using rotamer populations and free energy of transfer from published partitioning studies. It is suggested that this could result from a very weak hydrogen bond. On the other hand, the difference in partitioning between bufotenin and psilocin, which seems to be a major determinant of biological activity, is largely due to a difference in the basicity of the two compounds. The pKa values for the amino group of psilocin and bufotenin were determined to be 8.47 and 9.67, respectively.