Stabilization of the native tertiary structure of yeast tRNA^Leu₃ by cationic metal complexes

Complexes of inert (slowly exchanging) multivalent transition metal cations and neutral ligands were found to effectively replace Mg²⁺ as stabilizers of the biologically active "native" form of yeast tRNA^Leu₃. From an analysis of the sigmoidal dependence of the denatured native conformational equilibrium on complex cation concentration at 25°C in 0·15 m-KCl at pH 7·4, it is calculated that about five complex cations must be tightly bound to stabilize the native conformer. Since this number is larger than the three binding sites previously observed for alkaline earth metal ions and polyamines, there must be sites of high negative charge on the tRNA accessible to Mg²⁺ and spermidine³⁺ but not to the more bulky complex cations. In that case, the complex cations must occupy additional sites in order to stabilize the native conformer. The relative effectiveness with which the cations stabilize the native conformation is: Co(NH₃)₆³⁺ ≳ Pt(en)₂(en-H)³⁺ > Co(en)₃³⁺ > Ir(NH₃)₅Cl²⁺ {reversed tilde equals} Pt(NH₃)₄²⁺. This order is consistent with general electrostatic considerations. Hydrogen bonding of the co-ordinated amines to tRNA phosphates could enhance the affinity of Co(NH₃)₆³⁺. Since the complex cation-tRNA association constants are even larger than those of the cobalt complexes with multivalent inorganic anions, a cluster of at least three phosphates could constitute the binding site of each tightly bound cation in tRNA.