Thermodynamic and Kinetic Parameters of Oligonucleotide—Oligopeptide Interactions

It is demonstrated that field‐jump techniques provide a very useful method for the investigation of protein—nucleic acid interactions. Electric field pulses induce the dissociation of nucleotide—peptide complexes by a dissociation field effect. Amplitudes and time constants of this effect can be used to determine both thermodynamic and kinetic parameters. This method, as well as conventional ultraviolet absorbance and circular dichroism titrations, is applied to study the interaction of various combinations of the oligonucleotides (A) 4 , (A) 5 , (A) 6 , (dA) 6 , (I) 6 , (U) 6 and (C) 6 (all lacking terminal phosphates) with the oligopeptides (Lys) 2 , (Lys) 3 , (Arg) 2 , (Arg) 3 , Lys‐Gly‐Lys, Lys‐Phe‐Lys, Lys‐Tyr‐Lys and Lys‐Trp‐Lys. As demonstrated by chain length dependences the affinity increases with an increasing number of negative charges on the oligonucleotide and of positive charges on the oligopeptide. In addition to these electrostatic effects there are some more specific interactions. This is demonstrated by the fact that the stability constant for the interaction of (I) 6 with (Arg) 3 is higher by a factor of 4 than the corresponding constant obtained with (Lys) 3 . This effect is attributed to hydrogen bonding between the guanidino group of arginine and the hypoxanthine base. The data obtained with peptides containing aromatic residues reveal only relatively weak interactions of these residues with the bases of the oligonucleotides. The dynamics of the oligonucleotide—oligopeptide interactions are characterised by very high rates. The complex formation is a diffusion‐controlled reaction with some acceleration due to electrostatic attraction effects. The lifetime of the complexes investigated is in the microsecond time range.