Structures of Biomolecules
in the gas phase
-JPSGroup-
Circular Dichroism spectroscopy
Introduction
Circular dichroism spectroscopy
(see Fig. 1)
is widely used to study biological systems, in particular, to probe changes
in the conformation of a molecule. The molecules concerned are usually
very large, and therefore detailed structural analysis of their CD data
is difficult. Most of the data analysis is therefore qualitative, or empirical
in nature. The advantages, however, are that the experiments can be done
in the solution phase (a more 'natural' environment than the crystalline/solid
or gas phase), they are quick and easy to perform, and that they are uniquely
sensitive to the asymmetry of the system. This last aspectespecially is
interesting, since asymmetry can be a key feature of the interaction of
a drug with its receptor.
In the JPSimons Group
conformational assignments of the drugs ephedrine and pseudoephedrine,
and of adrenaline-related neurotransmitters have been carried out
in
the gas phase, not only of the monomeric structures, but also of the
singly
and multiply hydrated complexes. A clear picture of the conformational
landscapes of these (hydrated) molecules has emerged. These systems are
now the ideal subjects to be studied with CD, in order to achieve, for
the first time, a detailed structural comparison between gas-phase conformations
and CD data.
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Figure 1. The principle
of CD spectroscopy: the absorption spectra from left and right circularly
polarised light of an (optically active) sample are recorded; the CD spectrum
is the difference in intensity between them.
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Current research projects
Structural information from gas phase experiments
and ab initio computation led to a re-appraisal of the ‘sector rule’
for the Cotton effect associated with absorption into the 1Lb
state of the aromatic ring (see Fig. 2). This
empirical rule relates the sign of the observed CD signal to the absolute
configuration of the chiral centre which generates it.
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Figure 2. The 'old sector
rule', based on wrong structural assumptions of (R)-1-phenyl ethanol, and
the 'revised sector rule', which is based on the absolute, spectroscopically
determined structure of the molecule, and which predicts the correct sign
of the CD signal.
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Our current systems of interest, the monomers
and hydrated clusters of (1R, 2S) ephedrine and its diastereoisomer (1S,
2S) pseudoephedrine (see Fig. 3) have been
studied under free-jet expansion conditions and by UV circular dichroism
spectroscopy in aprotic and H-bonding solvents. Absolute cluster configurations
have been determined in the gas phase by comparing their mass-selected
R2PI spectra, their partially resolved UV rotational band contours and/or
infra-red ion depletion spectra with predictions from ab initio
calculations.
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Figure 3. Revised sector
rule predictions for the sign of the (UV) CD spectra of ephedrine and pseudoephedrine.
Note the large conformational change around C2
upon hydration in the most stable gas phase conformer of pseudoephedrine,
in contrast to the unaltered structure of ephedrine.
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The CD spectra in aprotic and H-bonding solvents
as shown in Fig. 4 can be discussed in terms
of solvent-induced conformational change and/or changes in the relative
orientation of the electronic and magnetic transition dipoles (indeed,
the two possible changes may be correlated).
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Figure 4. CD and absorption
spectra of (1R,2S) ephedrine and (1S,2S) pseudoephedrine in cyclohexane
(blue) and acetonitrile (orange), and in the H-bonding solvents methanol
(green) and water (magenta).
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Collaborations
All condensed phase experiments
are carried out in the biospectroscopy laboratory at the Imperial
College of Science, Technology and Medicine,
London, in the group of Professor
George Tranter.
References
Molecular conformation in the
gas phase and in solution, P. Butz, G.E. Tranter and J.P. Simons, Phys.
Chem. Comm., 2002, 5 (14), 91–93.
Neurotransmitters
| Sugars | Amino
acids, peptides | Protonated
species | Circular Dichroism spectroscopy
Last updated 08/10/2002
by
Lavina Snoek