Vibrational Transition Moment Angles in Isolated Biomolecules: A Structural Tool
F. Dong,
R. E. Miller
Infrared spectroscopy is used extensively in the study of isolated
biomolecules, but it becomes less useful as it is applied to systems of
increasing complexity. Even if the individual vibrational bands can be
resolved spectroscopically, their assignment becomes problematic when
they are more closely spaced than can be determined using ab initio
methods. We describe a method that helps to alleviate this difficulty
by measuring the direction of the vibrational transition moment for
each vibrational band. The molecules of interest (adenine and cytosine)
are cooled to 0.37 kelvin in liquid helium nanodroplets and oriented in
a large dc electric field. A polarized infrared laser is then used to
determine the directions of the infrared transition moments relative to
the permanent dipole moment. Comparisons with ab initio calculations
provide detailed structural information, including experimental
evidence for nonplanarity of adenine and three tautomers of cytosine.
Department of Chemistry, University of North Carolina, Chapel
Hill, NC 27599, USA.
