STRUCTURE OF FIVE GUANIDINE DERIVATIVES AND A STUDY ON BINDING OF THESE MOLECULES WITH CALF THYMUS DNA. Kulandavelu Subramanian, Sudha Lakshmanan, Dept. Of Physics, Anna University, Madras-600025, India, Gertraud Koellner and Thomas Steiner, Institute for Crystallography, Freie University Takustrasse 6, D-14159, Berlin, Germany

Spectrophotometric and spectrofluorometric methods were used to study the interaction of five Guanidine derivatives with Calf thymus DNA. The variation of the interaction with different concentration of DNA,keeping the Guanidine concentration constant, was examined. Based on these studies an effort has been made to elucidate the relationship between molecular structure and DNA binding efficiency of these compounds. The half reciprocal plot of the absorption titration data resulted in an intrinsic binding constant(K) of 1.5x10⁴, 0.5x10⁴, 0.8x10⁴, 0.7x10⁴ M^-1 in base pairs for Guanidine(I), Guanidine(II), Guanidine(III), Guanidine(IV) and Guanidine(V) respectively. Hypochromism was suggested to be due to strong interaction between the electronic sites of the binding chromophore and that of the DNA bases. In addition to the decrease in intensity, a small red shift, extensive broadening were also observed in the spectra. The fluorescence data were analysed to construct the binding isotherms and from these isotherms, binding constants were estimated. A plot of r/C_f Vs r gave the binding constant of 4.6x10⁴, 5.14x10⁴ M^-l for Guanidine(II) and guanidine(III). For Guanidine(I), Guanidine(IV) and Guanidine(V) the Scatchard plots are not linear and this shows that the binding is co-operative. Guanidine(I): P2₁/c, a=7.970(2), b=14.544(3), c=16.502(4)Å, ß= 100.58(2)deg., Z=4, R=0.0453, wR=0.1607 Guanidine(II):P1, a=7.96(1), b=9.979(2), c=12.033(2), [[alpha]]=86.48(2), ß=80.29(2), [[gamma]]=77.44(2)deg., Z=2, R=0.0583, wR=0.1885. Guanidine(III): P2₁/c, a=9.211(4), b=14.671(4), c=11.533(4)Å, ß=111.14(4)deg., Z=4, R=0.0652, wR=0.075. Guanidine(IV): Pbca, a=14.928(3), b=14.999(2), c=18.952(2)Å, Z=8, R=0.068, wR=0.073. Guanidine(V):Aba2, a=14.978(3), b=16.709(3), c=18.847(3)Å, Z=8, R=0.0529, wR=0.0592.