Role of D-Valine Residues in the Antitumor Drug Actinomycin D:Replacement of D-Valines with Other D-Amino Acids Changes the DNA Binding Characteristics and Transcription Inhibitory Activities

D-valine analogues of the antitumor drug actinomycin D, in which D-valine residues were replaced with D-threonine, D-tyrosine, D-phenylalanine, and D-O-methyltyrosine residues, have been totally synthesized. The crystal structure of the D-O-methyltyrosine analogue has been determined (a = b = 21.352(6), c = 44.525(9) angstrom; space group P4 sub(1)2 sub(1)2; R = 0.19 for 803 out of 1114 reflections at 1.8 angstrom resolution data). Replacements of D-valines did not change the overall conformation of the molecule, and the substituted groups were located on the side opposite to the DNA binding site, suggesting that the analogues can bind intercalatively at 5'-GC-3' sequences of DNA like actinomycin D does. In the crystals, the analogue molecules constitute a tight dimer, and a pair of stacked chromophores of the dimer was further sandwiched by two methoxyphenyl groups of neighboring molecules. These strong aromatic-aromatic stacking forces among the molecules appear to reduce very much the water solubility of the aromatic analogues. The characteristics of binding of the analogues to various DNA's including d(GAAGCTTC) sub(2), d(GTTGCAAC) sub(2), poly(dA-dT), poly(dG-dC), and calf thymus DNA have been examined by using the visible spectrum methods. Difference spectra of actinomycin D and the analogues with oligonucleotides indicated that the analogues bind intercalatively to the DNA, as actinomycin D does, but the association constants were reduced to approximately one-half that of actinomycin D. The spectra of the aromatic analogues titrated with calf thymus DNA indicated that the aromatic analogues bound somehow differently to the longer DNA's. A simple profile analysis of the spectra suggested that the aromatic analogues bound to calf thymus DNA not only with intercalation, as actinomycin D does, but also with side binding. Nevertheless, the association constants of the aromatic analogues to calf thymus DNA with the intercalation mode were found to be quite similar to those of the short oligonucleotides. This conclusion has been supported by the melting behaviors of the DNA with the aromatic analogues, in which the melting curves of the analogues were superimposable on the melting curve of DNA with actinomycin D, suggesting that the aromatic analogue molecules were intercalated into the DNA. The inhibitory activities of actinomycin D and analogues on RNA polymerase in vitro were examined using calf thymus DNA and E. coli RNA polymerase. All actinomycin D a