MICHAELS, H. B., AND HUNT, J. W. Reactions of the Hydroxyl Radical with Polynucleotides. Radiat. Res. 56, 57-70 (1973). Reactions of the hydroxyl radical with various monomeric components of nucleic acid, and with polynucleotides, were studied by observing the transient absorption of the -OH adduct species. At neutral pH, the rate constants for the attack of -OH on monomers were of the order of 5 X 109 M-l sec-l. The rate constants, per base, for OH attack on single-stranded polynucleotides were approximately a factor of 5 lower: these were 1.2 X 109 M-1 sec-l, 1.25 X 10W M-1 sec-1, and 0.9 X 109 M-1 sec-1 for poly C, poly U, and poly A, respectively. The double-stranded polynucleotides poly (A + U) and DNA had rate constants, per base, for reaction with -OH at pH 7, of 0.5 and 0.4 X 109 M-1 sec-l, respectively. Absorption spectra and values of extinction coefficients are presented for the OH adducts. It is shown that the absorbing transient product of *OH attack is the same species in the polymers poly C, poly U, and poly A, as in the corresponding nucleotides CMP, UMP, and AMP. Kinetic and spectral data indicate that -OH attacks a double-stranded polynucleotide at random base sites; the -OH adduct spectrum of poly (A + U) is identical in shape and magnitude to a composite average of the poly U and poly A adduct spectra. Secondary and tertiary structure appears to have little effect on *OH reactivity with polynucleotides.