The intraduplex reaction of the alkylating reagent CIRCH2NHpd(TTCCCA) (X, CIR is p-(N-2-chloroethyl-N-methylaminophenyl) residue) with the target 26-mer d(TTGCCTTGAATGGGAAGAGGGTCATT) (P) in the presence of effectors was studied. The effectors used were Phn-L-pd(TTCAAGGC)p-L-Phn (E1) and Phn-L-pd(TGACCCTC)p-L-Phn (E2), where Phn is N-(2-hydroxyethyl)-phenazinium residue and L is NHCH2CH2NH spacer. The dependence of the alkylation extent of the target on the reagent concentration was treated using the equation derived earlier for the two-component system (reagent+target) to calculate association constants of X with P, PE1, PE2 and PE1E2. The latters were found to be K xe1 = 6.75 · 105 M−1, K xe2 = 4.15 · 104 M− andK xe12 = 5.87 · 106 106 M−1 as compared with the affinity of X to P K x = 2.16 · 104 M−1 in the absence of effectors. Taking into account the internal structure of the target, co-operativity parameters describing interactions in the tandem E1 · X · E2 arranged at the target were calculated as α 1 = 16, α 2 = 10 and α 12 = 139 for the duplexes PXE1, PXE2 and PXE1E2.
