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.
Highlights
Infrared continuum bands that extend over a broad frequency range are a key spectral signature of protonated water clusters
Because a proton moves from one water molecule to a neighboring one, which is a consequence of low barriers in the proton energy landscape[10,11,12,13], the infrared (IR) absorption spectrum of an excess proton in water is not characterized by sharp bands but rather by very broad, so-called continuum bands
For bR, light-induced broad IR absorption bands have been observed to rise and decay during the photocycle[19,20,21,22]. The interpretation of these broad bands is subtle due to several complications: (i) Three water clusters exist in bR, so it is a priori not clear which water cluster gives rise to which spectral feature22. (ii) The translocation of a proton from the cytoplasmic to the extracellular side involves a number of transient states. (iii) More than one excess proton presumably is present at a time. (iv) In the wavenumber range 2500–3000 cm−1, broad bands have been assigned to strongly hydrogen-bonded water molecules that do not contain an excess proton23. (v) The energy dissipated from the retinal excitation might be absorbed by bulk water molecules, generating transient broad bands[20]
Summary
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. Polarization-resolved light–dark difference spectrum of the light-driven proton pump bacteriorhodopsin exhibits a pronounced dichroic continuum band. (iv) In the wavenumber range 2500–3000 cm−1, broad bands have been assigned to strongly hydrogen-bonded water molecules that do not contain an excess proton. The continuum band of bR observed in the 1800–2200 cm−1 range, devoid of any other spectral contributions, has been interpreted in terms of protons that are delocalized over a hydrogen-bonded network of amino acid side chains and water molecules. An alternative scenario involves a proton that is shared between glutamate residues[26], see ref. 22 for a recent overview
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