Solid-state nuclear magnetic resonance of rhodopsin and its photointermediates

Abstract

Photoisomerization of the membrane-bound light receptor protein rhodopsin leads
to a highly energetic species called bathorhodopsin, which is stable at temperatures
below 125 K. Bathorhodopsin stores about 2/3 of the absorbed photon energy but
the mechanisms with which this energy is stored is not completely understood. A
new insight into these mechanisms by means of low-temperature solid-state NMR
is both subject and aim of this Ph.D. thesis. The issue of the energy storage has
been investigated by a solid state magic angle spinning technique which combines
modern symmetry-based recoupling techniques with in situ cooling of the sample.
Production of bathorhodopsin is also done in situ in a customized NMR probe.
Three kind of experiments are discussed: chemical shift, distance and torsional
angle measurements. The first kind of experiments led to carbon chemical shifts
values for almost all the carbons along the retinylidene chain of the retinal chromophore
of bathorhodopsin. Our measurements show a significant perturbations of
the 13C chemical shifts in bathorhodopsin which is interpreted in terms of charge
delocalization along the chain and therefore indicates a participation of an electrostatic
mechanism to the energy storage. This is at variance with an earlier solid
state NMR study where only minor perturbations of the electronic structure in the
isomerized retinylidene chain were observed. We believe that these data incorrectly
refer to bathorhodopsin because of the incorrect conditions of temperature and illumination
applied. To sample for other local mechanisms that may contribute to the
energy storage, the C-C distance of the last two carbons of the retinylidene chain,
at the link with the protein opsin, was also measured but no significant differences
with rhodopsin have been found. Finally, the H-C=C-H torsional angle at the double
bound where the isomerization takes place was measured in a double-quantum
heteronuclear local field spectroscopy (2Q-HLF) experiment. Results indicate a
deviation from planarity of at least 40? about this double bond in bathorhodopsin
suggesting an unquantified amount of torsional strain acting as a further energy
storage mechanism. In addition to these very interesting results, this thesis reports
methods, equipment and procedures ready to be used for the study of other similar
light-triggered processes.

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Identifiers

ORCID for Maria Concistre: orcid.org/0000-0001-7111-0782

ORCID for Malcolm H. Levitt: orcid.org/0000-0001-9878-1180

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