Sandra Rifai - Dalhousie University

Placement: University of Alberta
Supervisor: Dr. Glen Loppnow
Current Position:  Graduate student, University of Ottawa

Psoralens

Psoralens are a class of three ring, heterocyclic, planar compounds that occur naturally in a variety of plants. Presently, fourty different psoralens have been identified, three of which I have worked with: 8-MOP, 5-MOP, and TMP. It was observed in 1959 that the 8-MOP and TMP of a particular celery pink rot were causing skin damage in workers when handling them in sunlight. Presently, it has been observed that under a controlled environment and in specific doses psoralens can help to cure skin damage rather than to cause it.

It was first suggested in 1971 that psoralens photoreact with DNA. The cycloaddition of psoralen to DNA involves several steps, the first of which is the 'dark binding', or intercalation of psoralen into the DNA. If the sample is then irradiated with long wavelength UV light from 320-410 nm either a furan-side or pyrone-side monoadduct of cis-syn configuration is formed. If a second photon is absorbed by the furan-side monoadduct, a diadduct also of cis-syn configuration is formed. The pyrone-side monoadduct cannot be driven to interstrand crosslink because it cannot absorb light of wavelength longer than 320 nm.

In the above process it has been observed that psoralen preferentially photoreacts at the thymine base of DNA, either as a result of preferential binding to thymine sites and/or an inherently more reactive thymine. I attempted to determine whether psoralen preferentially binds at thymine sites. I began by studying the electronic structure of psoralens, dissolved in several different solvents, using UV absorption, Raman, and Resonance Raman spectroscopies. I complexed 8-MOP with DNA of random sequences of adenine and thymine (poly(dA-dT)), as well as with DNA strands of random sequences of guanine and cytosine (poly(dG-dC)). Two stretches in particular were affected when the psoralen was complexed with poly(dA-dT) DNA. There was a shift evident in the C-O str as well as intensity changes in the C=C str. Both of these results are supported by solvent effect studies which indicate that the C-O str is weakly sensitive to polar, hydrogen bonding solvents whereas the C=C appears to be insensitive. These results imply that psoralen has a preferential binding to the thymine sites in DNA.