Darren McMurtrie - University of Victoria

Placement: University of Alberta
Supervisor: Dr. Glen Loppnow

Characterizing the Reverse Reaction of the Photodimerization of Thymidylyl-(3'® 5')-Thymidine (TpT) using Absorption and Raman Spectroscopy

The formation of pyrimidine dimers has been linked to many of the physiological effects of UV irradiation. Acetophenone sensitized irradiation of thymidylyl-(3^/® 5^/)-thymidine (TpT) yields a variety of photoproducts. The major product, the cis-syn stereoisomer, is studied here using absorption and Raman spectroscopy.

UV-VIS absorption spectra of thymine and TpT were recorded which identified the two major chromophoric transitions. The electronic transition at 208 nm represents the C=O p ® p *, and the electronic transition at 266 nm represents the C=C p ® p *. Raman spectra were recorded of thymine and TpT at excitation wavelengths within these bands, at 234 nm and 266 nm, using a titanium doped sapphire laser (Ti:Sapphire) and a neodymium doped YAG laser (Nd:YAG) respectively. The purpose of recording these spectra was to observe and assign the various resonance-enhanced Raman active bands for the pyrimidine base.

Since a Raman spectrum of the cis-syn photodimer was not actually obtained at 266 nm using the Nd:YAG laser, a Raman spectrum at 355 nm will be attempted. A wavelength of 355 nm is far from the absorption maximum at 200 nm, thus photoreversion to TpT could be prevented at this wavelength. A potential problem in obtaining these spectra could be identifying and assigning individual bands present. Resonance Raman scattering occurs when the sample is irradiated with an exciting line, whose energy corresponds to that of an electronic transition of a particular chromophoric group. Thus, a wavelength such as 355 nm could be too far from the absorption band at 200 nm to have much resonance enhancement, so no selectivity of chromophores will be obtained and a very concentrated solution of the cis-syn photodimer will have to be prepared.

Good quality Raman spectra were obtained of TpT at 266 nm using a high power of approximately 10 mW, but this power caused photoreversion of the cis-syn photodimer to TpT. Reducing the power of the 266 nm line enabled the kinetics of the photoreaction to be monitored by UV-VIS absorption spectroscopy, but the resultant Raman spectra were of poor quality. The resulting kinetics and photophysics will be discussed in the context of the states and potential energy surfaces involved in the reaction.