Structures of Biomolecules
in the gas phase
-JPSGroup-
Protonated species in
the gas phase
Introduction
Under physiological conditions,
important molecular groups are generally charged: basic groups can be protonated,
acidic groups can be deprotonated, amino acids in neutral solution are
predominantly zwitterions, peptides and proteins can be highly charged.
Mass-spectrometric studies based on electrospray ionisation (ESI) observe
multiply protonated, charged ions, [M+nH]n+, resulting
from the addition of protons to basic sites of the biological sample. In
contrast, all laser-based spectroscopic gas phase experiments to date,
have only been able to focus on neutral species, or singly charged
ions of non-protonated character, because of the nature and limitations
of the sample evaporation techniques, necessarily employed.
Current research projects
In recent years, a number
of studies have focused on the photochemical reactions of phenol-ammonia
clusters. Excitation of the phenol chromophore to the first excited singlet
state results in the formation of protonated ammonia clusters ((NH3)nH+).
As a first step in producing protonated biomolecules in the gas-phase we
have studied clusters of phenol with ethanolamine, the essential backbone
of a number of neurotransmitters and pharmaceuticals.(see Fig.
1).
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Figure 1. Calculated structures
of ethanolamine (neutral MP2/6-311+G**, protonated HF/6-31G*).
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The time-of-flight
mass spectrum (Fig. 2)
for the 1:1 cluster shows signal in both the cluster ion mass channel (Phenol-Ethanolamine+)
and in a mass channel corresponding to the mass of ethanolamine plus one
(i.e. Ethanolamine-H+); little or no signal
is observed in the phenol+ mass channel. Closer examination
of the TOF mass peaks indicates a distinct asymmetric shape to the ethanolamine-H+
feature; evidence that proton transfer (and/or cluster dissociation) takes
place during the passage along the flight-tube and hence after ionisation.
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Figure 2. Mass-spectrum
of the (protonated) products of the 1:1 complex phenol:ethanolamine.
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Its small size makes
it computationally and experimentally tractable; calculated structures
of both the neutral and protonated forms are shown in Fig.
3. The neutral structure is typical of that
found for a variety of related species; a hydrogen bond of the type OH…N
stabilises the conformer. In contrast, the protonated form has a reversed
hydrogen bond of the type NH…O; the alcohol OH group is essentially
free. Current work is focused on recording the infra-red spectrum of the
protonated species and to extending the process to larger, more biologically
relevant molecules such as ephedrine.
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Figure 3. Calculated structures (B3LYP/6-31+G*) of ephedrine and
protonated ephedrine. Note the switch in the OH behaviour from donor (neutral
ephedrine) to acceptor (protonated molecule).
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Neurotransmitters
| Sugars | Amino
acids, peptides | Protonated species | Circular
Dichroism spectroscopy
Last updated 08/10/2002
by
Lavina Snoek