Orientation of non-spherical protonated water clusters revealed by infrared absorption dichroism

Jan O. Daldrop, Mattia Saita, Matthias Heyden, Victor A. Lorenz-Fonfria, Joachim Heberle () and Roland R. Netz ()
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Jan O. Daldrop: Freie Universität Berlin
Mattia Saita: Freie Universität Berlin
Matthias Heyden: Max-Planck-Institut für Kohlenforschung
Victor A. Lorenz-Fonfria: Universitat de València
Joachim Heberle: Freie Universität Berlin
Roland R. Netz: Freie Universität Berlin

Nature Communications, 2018, vol. 9, issue 1, 1-7

Abstract: Abstract Infrared continuum bands that extend over a broad frequency range are a key spectral signature of protonated water clusters. They are observed for many membrane proteins that contain internal water molecules, but their microscopic mechanism has remained unclear. Here we compute infrared spectra for protonated and unprotonated water chains, discs, and droplets from ab initio molecular dynamics simulations. The continuum bands of the protonated clusters exhibit significant anisotropy for chains and discs, with increased absorption along the direction of maximal cluster extension. We show that the continuum band arises from the nuclei motion near the excess charge, with a long-ranged amplification due to the electronic polarizability. Our experimental, polarization-resolved light–dark difference spectrum of the light-driven proton pump bacteriorhodopsin exhibits a pronounced dichroic continuum band. Our results suggest that the protonated water cluster responsible for the continuum band of bacteriorhodopsin is oriented perpendicularly to the membrane normal.

Date: 2018
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DOI: 10.1038/s41467-017-02669-9

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