Interactions between polyamine analogs with antiproliferative effects and tRNA: a 15N NMR analysis

N-Bisalkylpolyamine analogs have been shown to exert antiproliferative effects in many tumor models, with the bisethyl derivatives exerting the greatest activities. 15N NMR spectroscopy was used to explore the interactions between these analogs and tRNA. When tRNA was added to solutions of 15N-enriched homospermine (4-4-4), bisethylhomospermine (BE-4-4-4), bismethylhomospermine (BM-4-4-4), bisethylspermine (BE-3-4-3) and 1,19-bis(ethylamino)-5,10,15-triazanonadecane (BE-4-4-4-4), the spin-lattice relaxation times T1 of the nitrogens were strongly reduced. From the temperature dependence of these T1's we calculated the rotational activation energies (Ea) of the correlation times of the amino groups in the presence and absence of tRNA. These data indicate that: i) the N-bisethyl derivatives bind strongly to tRNA through their-NH2(+)-groups (most likely, through hydrogen bonding); ii) the binding is weakest in the N-bismethyl derivative and iii) homospermine binds very weakly and mainly through its -NH3(+)-group (most likely, through electrostatic binding). The binding of the polyamine analogs to tRNA was also estimated by the increase of the half-line widths (D1/2) of the -NH2(+)-groups, derived from the effects that tRNA has on the spin-spin relaxation time T2. The decrease of the V1/2 values of the -NH2(+)-groups in the (15N-polyamine)-tRNA complexes when the analogs were chased away by an excess of spermine confirmed the stronger binding of the bisethyl- with respect to the bismethyl derivatives, as well as the weak binding of homospermine to tRNA. A correlation was also found between the binding strengths of the analyzed polyamine analogs and their antiproliferative activities.