Binding of psoralen derivatives to DNA and chromatin: influence of the ionic environment on dark binding and photoreactivity.

The binding of two psoralen derivatives to DNA and to chromatin has been compared in some detail, particularly with respect to the influence of the ionic environment. The compounds used were the extremely hydrophobic 4,5’,8-trimethylpsoralen (TMP) and its extremely hydrophilic derivative 4’-aminomethyl-4,5’,8-trimethylpsoralen (AMT). Both the dark noncovalent interaction and the UV light induced covalent interaction with DNA were strongly reduced by increasing concentrations of N a + and Mg2+ ions, particularly in the range up to the physiological region. For any given ionic conditions, higher binding levels were observed for supercoiled relative to linear D N A and a t 4 O C relative to 20 OC. About 28% of the D N A in chromatin was accessible to either drug D e r i v a t i v e s of the furocoumarin, psoralen, have been used for many years in biological experiments relating to skin photosensitization (see Scott et al., 1976) and thus in the treatment of skin diseases such as psoriasis (Parrish et a]., 1974). Only recently have they been used as molecular probes for chromatin structure (Hanson et al., 1976; Wiesehahn et al., 1977), hairpin regions in viral D N A (Shen & Hearst, 1976), satellite D N A structure (Shen & Hearst, 1977), and bacterial repair mechanisms (Cole et al., 1976). The basis for all of these recent studies lies in the ability of psoralen derivatives to penetrate intact cells or isolated nuclei and covalently bind to the D N A upon irradiation with UV’ light in the region of 320-380 nm, to form monoadducts or cross-links (Musajo & Rodighiero, 1970; Cole, 1970). However, a wide variety of solution conditions were employed i n these experiments and, up to now, no systematic study of the effects of ionic environment on the dark binding and photoreaction of these drugs has been undertaken. In this work the influence of monovalent and divalent cations over a broad concentration range on psoralen binding to D N A has been examined, specifically to linear and supercoiled D N A and to D N A in chromatin where the effect of chromosomal protein dissociation at higher ionic strengths is superimposed upon the intrinsic effect of the metal ions on the DNA. Experiments have been carried out in parallel, using two psoralen derivatives of the same series which represent extreme