The proton-bound
dimer of hydrogen sulfate and formate is an archetypal
structure for ionic hydrogen-bonding complexes that contribute to
biogenic aerosol nucleation. Of central importance for the structure
and properties of this complex is the location of the bridging proton
connecting the two conjugate base moieties. The potential energy surface
for bridging proton translocation features two local minima, with
the proton localized at either the formate or hydrogen sulfate moiety.
However, electronic structure methods reveal a shallow potential energy
surface governing proton translocation, with a barrier on the order
of the zero-point energy. This shallow potential complicates structural
assignment and necessitates a consideration of nuclear quantum effects.
In this work, we probe the structure of this complex and its isotopologues,
utilizing infrared (IR) action spectroscopy of ions captured in helium
nanodroplets. The IR spectra indicate a structure in which a proton
is shared between the hydrogen sulfate and formate moieties, HSO
4
–
···H
+
···
–
OOCH. However, because of the nuclear quantum effects
and vibrational anharmonicities associated with the shallow potential
for proton translocation, the extent of proton displacement from the
formate moiety remains unclear, requiring further experiments or more
advanced theoretical treatments for additional insight.
Biological membrane fluidity and thus the local viscosity in lipid membranes are of vital importance for many life processes and implicated in various diseases. Here, we introduce a novel viscosity sensor design for lipid membranes based on a reporting nanoparticle, a sulfated dendritic polyglycerol (dPGS), conjugated to a fluorescent molecular rotor, indocarbocyanine (ICC). We show that dPGS-ICC provides high affinity to lipid bilayers, enabling viscosity sensing in the lipid tail region. The systematic characterization of viscosity-and temperature-dependent photoisomerization properties of ICC and dPGS-ICC allowed us to determine membrane viscosities in different model systems and in living cells using fluorescence lifetime imaging (FLIM). dPGS-ICC distinguishes between ordered lipids and the onset of membrane defects in small unilamellar single lipid vesicles and is highly sensitive in the fluid phase to small changes in viscosity introduced by cholesterol. In microscopy-based viscosity measurements of large multilamellar vesicles, we observed an order of magnitude more viscous environments by dPGS-ICC, lending support to the hypothesis of heterogeneous nanoviscosity environments even in single lipid bilayers. The existence of such complex viscosity structures could explain the large variation in the apparent membrane viscosity values found in the literature, depending on technique and probe, both for model membranes and live cells. In HeLa cells, a tumor-derived cell line, our nanoparticle-based viscosity sensor detects a membrane viscosity of ∼190 cP and is able to discriminate between cell membrane and intracellular vesicle localization. Thus, our results show the versatility of the dPGS-ICC nano-conjugate in physicochemical and biomedical applications by adding a new analytical functionality to its medical properties.
Hydrogen bonding interactions are essential in the structural stabilization and physicochemical properties of complex molecular systems, and carboxylic acid functional groups are common participants in these motifs. Consequently, the neutral...
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