Radiation Chemistry Department. Biocybernetics Department, Institute of Nuclear Medicine & Allied Sciences, Brig. S. K. Mazumdar Road, Delhi-110 054, India E-mail: snu@inmas.org Fax: 91-11-23919509 Manuscript received 20 July 2004, revised 22 September 2004. accepted 9 December 2004 This study was undertaken in order to find out the effect of solvent composition on mode of interaction of H1 histone with DNA, suitability of DNA. H1 histone and DNA-H1 complex as in vitro biological dosimeters and protection possibility of DNA by Ht histone against ionizing radiation-induced damage. Analytical techniques were used for the study of complex formation. Gamma irradiation was used to deliver doses to the biopolymers and their complexes. Equilibrium dialysis method was used for complex formation. FBX dosimetry was done for the delivery of total dose and dose rate determination. DNA : H1 complex formation ratio were : 4.17 x 109 : 4.652 x 10-6, ML-1 in 10-3 M phosphate buffer; 4.17 x 10-9 : 1.16.1 x 10-6 ML-1 in 0.1 M phosphate buffer and 2.085 X 109 :9.304 X 10-7 ML-1 in 0.15 M SSC buffer. Linearity in response with absorbed dose up to 50 Gy (dose rate 1.50 Gy/min) was observed with (i) DNA at 207 nm and 259 nm at 4.17 x 10-9 ML-1, (ii) with H1 histone at 202 nm and 270 nm at 1.163 x 10-6 ML-1 in 0.1 M phosphate buffer, pH 7.0, (iii) with DNA : H1 at 206 nm and 260 nm in 0.1 M phosphate buffer, (iv) with DNA : H1 at 226 nm and 260 nm in SSC buffer. A constant decrease in fluorescent intensity (Ex 280 nm, Em 338 nm) was observed for H1 histone with increase in dose. The average percentage hypcrchromicily in 0.1 M phosphate buffer was 20% for DNA, 11% for H1 histone and 17% for DNA : H1 complex. In 0.15 M SSC buffer, for the complex it was 14% for a dose of 50 Gy at a dose rate of 1.5 Gy/min. The degrees of unfolding of the chromophores for this complex with dose were different in different solvents. Complex formations arc due to electrostatic, minor groove binding and intercalation modes and are solvent dependent. DNA, H1 and DNA : H1 complex can be used as in vitro biological dosimetes. Binding modes arc responsible for protection of DNA by H1 histone up to a dose of 50 Gy.