Psoralens are naturally occurring linear furocoumarins and are used with long-wave UV light as an effective treatment for skin ailments such as psoriasis [1]. They intercalate into double stranded nucleic acids and in the presence of 365-nm UV light, undergo a photochemical reaction with the pyrimidine residues to form photoadducts. It has been experimentally confirmed that preferred base for the process is thymine [2].

Eight crystal and molecular structures of psoralens and their derivatives (both linear and angular, see Fig. 1) were determined by X-ray diffraction experiment. The extent of π-electron (de)localization in the rings depending on the topological environment was analyzed with use of geometry-based descriptor HOMA [3] and magnetism-based descriptor NICS [4]. For the latter, model analogues were optimized at B3LYP/6-311+G** level of theory. The comparison of the total energies led to the conclusion the isomers may differ at most by c.a. 18 kcal/mol, what is rationalized on the base of their topology.

Figure 1

[1] K. Wolff, T. B. Fitzpatrick, J. A. Parrish, F. Gschnait, B. Gilchrest, H. Höningsmann, M. A. Pathak, L. Tannenbaum, Arch. Dermatol. 1976, 112, 943.

[2] M. Ramaswamy, A. T. Yeung, Biochemistry 1994, 33, 5411-5413.

[3] T. M. Krygowski, J. Chem. Inf. Comput. Sci. 1993, 33, 70.

[4] P. v. R. Schleyer et al., J. Am. Chem. Soc. 1996, 118, 6317.

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