3 research outputs found
Excited-State Proton Transfer of Fluorescein Anion as an Ionic Liquid Component
Fluorescent
ionic liquids (FILs) incorporating the fluorescein
anion have been prepared by anion exchange of the parent quaternary
ammonium chloride (Quat<sup>+</sup>Cl<sup>ā</sup>) ionic liquid.
By controlling the molar ratio of fluorescein to Quat<sup>+</sup>Cl<sup>ā</sup>, ionic liquids incorporating different prototropic
forms of fluorescein were prepared. The 1:1 molar ratio ionic liquid
(FIL1) is essentially composed of monoanionic fluorescein, while dianionic
fluorecein is predominant in the FIL with a 1:2 molar ratio (FIL2).
The fluorescence excitation spectrum of FIL2 is markedly different
from its absorption spectrum. Absorption features the fluorescein
dianion, while the excitation spectrum is exclusively due to the monoanion.
In FIL1, the absorption and excitation spectra are both characteristic
of the monoanion. In both FILs, emission of the dianion is observed
upon excitation of the monoanion. This unusual behavior is interpreted
in the context of a fast deprotonation of the monoanion in the excited
state. The presence of residual water in the ionic liquid is important
for the proton transfer process. By lowering the pH of FIL1, the transient
proton transfer is inhibited, and the emission of the monoanion could
be observed. The FILs have completely different spectroscopic properties
from solvated fluorescein in Quat<sup>+</sup>Cl<sup>ā</sup>, where the prototropic equilibrium is shifted toward the neutral
forms
Influence of Intracellular Membrane pH on Sphingolipid Organization and Membrane Biophysical Properties
Glucosylceramide
(GlcCer) is a signaling lipid involved in the
regulation of several cellular processes. It is present in different
organelles, including the plasma membrane, Golgi apparatus, endoplasmic
reticulum, and lysosomes. Accordingly, GlcCer is exposed to different
pH environments in each organelle, which may lead to alterations in
its properties and lateral organization and subsequent biological
outcome. In this study, we addressed the effect of pH on the biophysical
behavior of this lipid and other structurally related sphingolipids
(SLs). Membranes composed of POPC (1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine) and C16-GlcCer, sphingomyelin, and different
acyl chain ceramides were characterized by fluorescence spectroscopy,
confocal microscopy, and surface pressureāarea measurements
under neutral and acidic conditions. The results show that changing
the pH from 7.4 to 5.5 has a larger impact on C16-GlcCer-containing
membranes compared to other SLs. In addition, acidification mainly
affects the organization and packing properties of the GlcCer-enriched
gel phase, suggesting that the interactions established by the glucose
moiety, in the GlcCer molecule, are those most affected by the increase
in the acidity. These results further highlight the role of GlcCer
as a modulator of membrane biophysical properties and will possibly
contribute to the understanding of its biological function in different
organelles
The Chemistry of Imperfections in NāGraphene
Many propositions have been already
put forth for the practical
use of N-graphene in various devices, such as batteries, sensors,
ultracapacitors, and next generation electronics. However, the chemistry
of nitrogen imperfections in this material still remains an enigma.
Here we demonstrate a method to handle N-impurities in graphene, which
allows efficient conversion of pyridinic N to graphitic N and therefore
precise tuning of the charge carrier concentration. By applying photoemission
spectroscopy and density functional calculations, we show that the
electron doping effect of graphitic N is strongly suppressed by pyridinic
N. As the latter is converted into the graphitic configuration, the
efficiency of doping rises up to half of electron charge per N atom