Tryptamine-adenosine 5'-monophosphate interactions as studied by nuclear magnetic resonance and relaxation

The conformation of tryptamine-adenosine 5'-monosphate and of their 1:1 complex in neutral aqueous solution at 297 K has been investigated by proton NMR and relaxation. The dependences of the proton chemical shift as a function of the tryptamine and AMP concentrations yield an association constant of 6.5 +/- 0.5 1 . mol-1. The reorientation correlation time of the complex tau R = (2.5 +/- 0.1) . 10(-10) s has been determined from the deuteron and ESR linewidth measurements on specifically labelled AMP. The proton longitudinal relaxation shows that the adenine and indole rings are head-to-head stacked 0.31 +/- 0.01 nm apart as confirmed by proton chemical shift measurements. In this complex, the AMP ribose ring takes the 3'-endo (N) conformation and the orientation of the adenine base is anti, whereas the tryptamine aminoethyl residue, in the gauche conformation, is most likely bound to the phosphate by coulombic interactions.