Early
Earth’s atmosphere provided little shielding for ultraviolet light from
space, so many prebiotic molecules, bombarded by it and light of other
wavelengths, had a hard time surviving at all. But some molecules became
photostable—able to withstand the assault and thrive as building blocks
of life.
Five
of the many molecules that survived the bombardment from UV light were
the nucleic acid bases adenine, cytosine, guanine, thymine and uracil.
Now, in just published research, a University of Georgia physicist and a
collaborator in Germany have shown that one of these building blocks of
DNA and RNA, adenine, has an unexpectedly variable range of ionization
energies along its reaction pathways.
This
means that understanding experimental data on how adenine survives
exposure to UV light is much more complicated than previously thought.
It also has far-reaching implications for spectroscopic measurements of
heterocyclic compounds-those with atoms of at least two different
elements in their rings.
“Photoprotection
relies on the conversion of potentially harmful UV radiation into heat
and has to operate on ultrafast time scales to compete over pathways
that lead to the destruction of the biomolecule,” said Susanne Ullrich,
assistant professor in physics in the UGA department of physics and
astronomy, part of the Franklin College of Arts and Sciences.
“Disentangling these pathways and their time scales is challenging and
requires a very close collaboration between experimentalists and
theorists.”
The research is in the online journal Physical Chemistry Chemical Physics. Co-author of the paper is Mario Barbatti, a theorist at the Max-Planck Institute in Mulheim, Germany.
The
quantum-chemical calculations create for the first time a new baseline
on how time-resolved spectroscopic techniques based on photoionization
can be most reliably used to study this class of molecules.
“Photostable
organic molecules participated in the complex molecular evolution that
led to the formation of life,” said Ullrich. “Because of the
significance of nucleic acid bases as the genetic coding material, the
photophysics of nucleobases has received considerable theoretical and
experimental attention. This new work can help clarify inconsistencies
researchers have always found in studying photoionization and
photoelectron spectra of adenine.”
Ullrich
and her team used a technique called time-resolved photoionization with
femtosecond (a quadrillionth of a second) resolution to unravel the
mechanisms that protect adenine against UV damage. For the spectroscopic
measurements, they employ a state-of-the-art femtosecond laser and
custom-built photoelectron and photoion spectrometer.
Adenine
is vaporized and transported into the spectrometer in a supersonic jet
expansion. A pump pulse excites the sample of molecules, and finally a
probe pulse is used to examine the sample after an adjustable delay
time.
This
examination is based on the process of photoionization that removes an
electron from the molecule. The kinetic energy of the released
photoelectron is measured in the spectrometer and provides the
spectroscopic information needed to establish the photoprotection
mechanism of adenine. Interpretation, however, heavily relies on the
knowledge of ionization potentials (IP) along the relaxation pathways.
(Ionization potential is the energy needed to remove an electron from
the molecule.)
There
has been a longstanding divergence between theoretical and experimental
results when it comes to studying the IP of adenine and understanding
on which surface adenine “relaxes” after it is excited with UV light.
Understanding it more clearly could give new insights into how this
important building block of life has continued to exist with stability
in a world with millions of genetic threats.
“To
our surprise, we found there were significant variations in the
ionization energy between two different regions on this pathway,” said
Barbatti. “Due to the general character of the three pathways we
studied, we believe the IPs computed along them can be used as a general
guide for helping with setup and analysis for further experiments, not
only with adenine but other related compounds.”
Before
this work, little has been known about the behavior of ionization
potentials along the main reaction pathways for gaseous adenine in an
excited state. Calling that a “knowledge gap,” Barbatti and Ullrich say
the new findings have “implications for experimental setup and data
interpretation.”
Ionization potentials of adenine along the internal conversion pathways
