PHYS 506 |
| Long-lived excited electronic states have been observed in a variety of DNA oligo- and polynucleotides. The yields and evolution of these states are postulated to depend on electronic coupling between proximal nucleobases. Here we report the study by femtosecond transient absorption spectroscopy of long-lived singlet excited states in the alternating oligonucleotide, d(GC)9. Under physiological conditions, d(GC)9 adopts the B-form double-helical conformation commonly found in genomic DNA. However, in the presence of high salt concentration, the sequence undergoes a structural transition to the left-handed Z-conformation. To investigate the difference in excited-state dynamics between the two secondary structures of d(GC)9, the transition between B- and Z- conformations was controlled by adjusting the salt concentration in solution and monitored by steady-state absorption and circular dichroism. The excited-state decays in both B- and Z- conformations of d(GC)9 were comprised of a single decay component with a lifetime of 5.9 ± 0.2 ps and 7.5 ± 0.3 ps, respectively. Despite notable changes in stacking geometries between adjacent bases in the different secondary structures, the excited states have similar lifetimes. On the other hand, markedly different dynamics are observed in D2O, suggesting that the rate-limiting deactivation step in d(GC)9 may involve interstrand proton transfer. These results will be discussed in light of recent models of DNA electronic structure. |
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Physical Chemistry Poster Session
7:00 PM-9:00 PM, Wednesday, August 22, 2007 BCEC -- Exhibit Hall - B2, Poster
Sci-Mix
Division of Physical Chemistry |