Abstract
Recent crystallographic results revealed conformational changes of zwitterionic ectoine upon hydration. By means of confocal Raman spectroscopy and density functional theory calculations, we present a detailed study of this transformation process as part of a Fermi resonance analysis. The corresponding findings highlight that all resonant couplings are lifted upon exposure to water vapor as a consequence of molecular binding processes. The importance of the involved molecular groups for water binding and conformational changes upon hydration is discussed. Our approach further shows that the underlying rapid process can be reversed by carbon dioxide saturated atmospheres. For the first time, we also confirm that the conformational state of ectoine in aqueous bulk solution coincides with crystalline ectoine in its dihydrate state, thereby highlighting the important role of a few bound water molecules.
Highlights
Raman spectroscopy and density functional theory calculations, we present a detailed study of this transformation process as part of a Fermi resonance analysis
In addition to this remarkable water binding behaviour and the resulting hygroscopicity,[2] it was assumed that the properties of the first hydration shell around ectoine are responsible for further effects, ranging from the stabilization of proteins[6,7,8,9,10,1] to the protection of DNA from ultraviolet[11] and ionizing radiation damage.[12,13]
The mechanistic aspects of this reaction are not known, we propose by reasons of chemical intuition that the transiently formed unstable H2CO3 species within the nanometer size channels in the ectoine dihydrate crystal decompose under water removal
Summary
Fermi resonance is a very common mechanism in vibrational spectra of polyatomic molecules with complex structure.[22] It appears when a fundamental vibrational frequency lies closely to an overtone or combination frequency. The anhydrate-to-dihydrate ectoine transformation under the influence of water vapour took a few minutes to start and few tens of seconds to reach the final stable hydration state. Ground-state energies were calculated for single zwitterionic ectoine molecules with axial as well as equatorial carboxylate group conformations, both in continuum solvent[35] and in gas phase approximation. Before the calculation of spectra, a minimization of the total energy (geometry optimization) was performed for 100 cycles
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