Current Research - Biophysics

 

 

Photostability of Biomolecular Building Blocks

Early life on this planet may have developed under hostile conditions, without the presence of a significant stratospheric ozone layer, and was potentially exposed to harmful ultra-violet (UV) radiation from space. Evolution chose certain, relatively complex organic molecules as the building blocks of life. These must undoubtedly have had their own photoprotective properties. To be effective, their excited state electronic relaxation mechanisms must operate on ultrafast time scales in order to dominate over competing photochemical processes that potentially lead to destruction of the biomolecule. Our intention is to discern in detail the photoprotective mechanisms inherent to individual biomolecules, and, hence, their suitability as the building material for life.

A case of fundamental interest, DNA bases and DNA base pairs have been the focus of our recent work. In a wavelengths dependant TRPES study, we successfully disentangled the non-adiabatic electronic relaxation dynamics in isolated Adenine. Based on the spectral and dynamical information obtained in our study we proposed the following model for radiationless decay pathways in Adenine. Excitation at 250 and 267nm prepares the optically bright S2(ππ*) state which shows strong coupling to the S1(nπ*). Rapid internal conversion (t<50fs) populates the lower lying S1(nπ*) state which has a lifetime of 750fs. We observed only minor signals from long-lived (ns) triplet states with decreasing yields at higher excitation energies, which indicates relaxation of the S1(nπ*) state predominantly to the ground state. At 267nm, we found evidence for an additional channel which is consistent with the dissociative S3(πσ*) state proposed as an additional ultrafast relaxation pathway from S2(ππ*) by ab initio quantum chemical calculations. TRPES measurements of Thymine, Uracil and Cytosine also indicate ultrafast relaxation for these bases. We therefore conclude that DNA bases possess inherent photoprotective properties, which convert potentially harmful electronic energy into vibrational energy (heat). Only a minor fraction of population remains trapped in an electronically excited state.
Rounded Rectangle:   Left graph: TRPES of adenine excited with 250nm (267nm data not shown) and ionized with 200nm. Center: Color coded map of photoelectron intensity as a function of time delay and kinetic energy. Left: Evolution of each photoelectron channel as a function of time delay. Bottom: Time-integrated photoelectron spectra. Right graph: Time-integrated photoelectron spectra of S2(ππ*) with a <50fs lifetime (top) and S1(nπ*) with a 750fs lifetime (bottom). The identification and assignment of the different ionization channels are based on ab initio ionization correlations (TD-B3LYP/6-31++G**): S1, the lowest nπ* state, preferentially ionizes into the D1 (n-1) cation excited state, whereas S2, the lowest ππ* state, and S3, a πσ* state, both preferentially ionize into the D0 (π-1) cation ground state. The stars denote the He(I) vertical ionization potential for D0 (π-1) and D1 (n-1) (in black) and the vertical IPs expected for the vibrationally excited states at 267nm (red) and 250nm (blue). At 267nm, an additional ionization channel contributes to the nominal S2(ππ*) band but not the S1(nπ*) band, as is apparent from the energy-corrected difference spectra (dotted line). (These spectra were recorded by Susanne Ullrich as part of a Feodor-Lynen fellowship at NRC Ottawa, Femtosecond Science group of Albert Stolow.)

Department of Physics and Astronomy

Ullrich Group