8 research outputs found
Conformational Distributions and Hydrogen Bonding in Gel and Frozen Lipid Bilayers: A High Frequency Spin-Label ESR Study
The ESR parameters of PC spin labels in frozen membranes
do not
simply represent the membrane polarity or water penetration profile.
Instead, they show a distribution between hydrogen-bonded (HB) and
non-hydrogen-bonded (non-HB) states, which is affected by a number
of factors in the membrane composition. Similar to the exclusion of
solutes from crystallizing solvents, the pure bulk gel phase excludes
nitroxides, forcing acyl chains to take bent conformations. In these
conformations, the nitroxide is hydrogen-bonded. Furthermore, upon
gradual cooling in the supercooled gel, PC labels undergo slow lateral
aggregation, resulting in a broad background signal. However, if the
sample is instantly frozen, this background is replaced by the HB
component. In membranes with cholesterol, the observed HB/non-HB ratio
can best be described by a partition-like equilibrium between nitroxides
located in defects of lipid structure within the hydrophobic core
and those close to the membrane surface
Interaction of Spin-Labeled Lipid Membranes with Transition Metal Ions
The large values of spin relaxation
enhancement (RE) for PC spin-labels
in the phospholipid membrane induced by paramagnetic metal salts dissolved
in the aqueous phase can be explained by Heisenberg spin exchange
due to conformational fluctuations of the nitroxide group as a result
of membrane fluidity, flexibility of lipid chains, and, possibly,
amphiphilic nature of the nitroxide label. Whether the magnetic interaction
occurs predominantly via Heisenberg spin exchange (Ni) or by the dipole–dipole
(Gd) mechanism, it is essential for the paramagnetic ion to get into
close proximity to the nitroxide moiety for efficient RE. For different
salts of Ni the RE in phosphatidylcholine membranes follows the anionic
Hofmeister series and reflects anion adsorption followed by anion-driven
attraction of paramagnetic cations on the choline groups. This adsorption
is higher for chaotropic ions, e.g., perchlorate. (A chaotropic agent
is a molecule in water solution that can disrupt the hydrogen bonding
network between water molecules.) However, there is no anionic dependence
of RE for model membranes made from negatively charged lipids devoid
of choline groups. We used Ni-induced RE to study the thermodynamics
and electrostatics of ion/membrane interactions. We also studied the
effect of membrane composition and the phase state on the RE values.
In membranes with cholesterol a significant difference is observed
between PC labels with nitroxide tethers long enough vs not long enough
to reach deep into the membrane hydrophobic core behind the area of
fused cholesterol rings. This study indicates one must be cautious
in interpreting data obtained by PC labels in fluid membranes in terms
of probing membrane properties at different immersion depths when
it can be affected by paramagnetic species at the membrane surface
Magnetic Interaction of Transition Ion Salts with Spin Labeled Lipid Membranes: Interplay of Anion-Specific Adsorption, Electrostatics, and Membrane Fluidity
Magnetic Interaction of Transition Ion Salts with Spin Labeled Lipid
Membranes: Interplay of Anion-Specific Adsorption, Electrostatics,
and Membrane Fluidit
Controlled Dopant Migration in CdS/ZnS Core/Shell Quantum Dots
The physical properties of a doped
quantum dot (QD) are strongly
influenced by the dopant site inside the host lattice, which determines
the host–dopant coupling from the overlap between the dopant
and exciton wave functions of the host lattice. Although several synthetic
methodologies have been developed for introducing dopants inside the
size-confined semiconductor nanocrystals, the controlled dopant-host
lattice coupling by dopant migration is still unexplored. In this
work, the effect of lattice mismatch of CdS/ZnS core/shell QDs on
MnÂ(II) dopant behavior was studied. It was found that the dopant migration
toward the alloyed interface of core/shell QDs is a thermodynamically
driven process to minimize the lattice strain within the nanocrystals.
The dopant migration rate could be represented by the Arrhenius equation
and therefore can be controlled by the temperature and lattice mismatch.
Furthermore, the energy transfer between host CdS QDs and dopants
can be finely turned in a wide range by dopant migration toward the
alloyed interface during ZnS shell passivation, which provides an
efficient method to control both the number of the emission band and
the ratio of the emission from the host lattice and dopant ions
Substrate-Dependent Cleavage Site Selection by Unconventional Radical <i>S</i>‑Adenosylmethionine Enzymes in Diphthamide Biosynthesis
<i>S</i>-Adenosylmethionine (SAM) has a sulfonium ion
with three distinct C-S bonds. Conventional radical SAM enzymes use
a [4Fe-4S] cluster to cleave homolytically the C<sub>5′,adenosine</sub>-S bond of SAM to generate a 5′-deoxyadenosyl radical, which
catalyzes various downstream chemical reactions. Radical SAM enzymes
involved in diphthamide biosynthesis, such as Pyrococcus
horikoshii Dph2 (<i>Ph</i>Dph2) and yeast
Dph1-Dph2 instead cleave the C<sub>Îł,Met</sub>-S bond of methionine
to generate a 3-amino-3-carboxylpropyl radical. We here show radical
SAM enzymes can be tuned to cleave the third C-S bond to the sulfonium
sulfur by changing the structure of SAM. With a decarboxyl SAM analogue
(dc-SAM), <i>Ph</i>Dph2 cleaves the C<sub>methyl</sub>-S
bond, forming 5′-deoxy-5′-(3-aminopropylthio) adenosine
(dAPTA, <b>1</b>). The methyl cleavage activity, like the cleavage
of the other two C-S bonds, is dependent on the presence of a [4Fe-4S]<sup>+</sup> cluster. Electron-nuclear double resonance and mass spectroscopy
data suggests that mechanistically one of the S atoms in the [4Fe-4S]
cluster captures the methyl group from dc-SAM, forming a distinct
EPR-active intermediate, which can transfer the methyl group to nucleophiles
such as dithiothreitol. This reveals the [4Fe-4S] cluster in a radical
SAM enzyme can be tuned to cleave any one of the three bonds to the
sulfonium sulfur of SAM or analogues, and is the first demonstration
a radical SAM enzyme could switch from an Fe-based one electron transfer
reaction to a S-based two electron transfer reaction in a substrate-dependent
manner. This study provides an illustration of the versatile reactivity
of Fe-S clusters
Interface Engineering of Mn-Doped ZnSe-Based Core/Shell Nanowires for Tunable Host–Dopant Coupling
Transition
metal ion doped one-dimensional (1-D) nanocrystals (NCs)
have advantages of larger absorption cross sections and polarized
absorption and emissions in comparison to 0-D NCs. However, direct
synthesis of doped 1-D nanorods (NRs) or nanowires (NWs) has proven
challenging. In this study, we report the synthesis of 1-D Mn-doped
ZnSe NWs using a colloidal hot-injection method and shell passivation
for core/shell NWs with tunable optical properties. Experimental results
show optical properties of the NWs are controlled by the composition
and thickness of the shell lattice. It was found that both the host–Mn
energy transfer and Mn–Mn coupling are strongly dependent on
the type of alloy at the interface of doped core/shell NWs. For Mn-doped
type I ZnSe/ZnS core/shell NWs, the ZnS shell passivation can enhance
florescence quantum yield with little effect on the location of the
incorporated Mn dopant due to the identical cationic Zn<sup>2+</sup> site available for Mn dopants throughout the core/shell NWs. However,
for Mn-doped quasi type II ZnSe/CdS NWs and ZnSe/CdS/ZnS core/shell
NWs, the cation alloying (Zn<sub>1–<i>x</i></sub>Cd<sub><i>x</i></sub>SÂ(e)) can lead to metal dopant migration
from the core to the alloyed interface and tunable host–dopant
energy transfer efficiencies and Mn–Mn coupling. As a result,
a tunable dual-band emission can be achieved for the doped NWs with
the cation-alloyed interface. The interfacial alloying mediated energy
transfer and Mn–Mn coupling provides a method to control the
optical properties of the doped 1-D core/shell NWs
Dph3 Is an Electron Donor for Dph1-Dph2 in the First Step of Eukaryotic Diphthamide Biosynthesis
Diphthamide, the
target of diphtheria toxin, is a unique posttranslational
modification on translation elongation factor 2 (EF2) in archaea and
eukaryotes. The biosynthesis of diphthamide was proposed to involve
three steps. The first step is the transfer of the 3-amino-3-carboxypropyl
group from <i>S</i>-adenosyl-l-methionine (SAM)
to the histidine residue of EF2, forming a C–C bond. Previous
genetic studies showed this step requires four proteins in eukaryotes,
Dph1–Dph4. However, the exact molecular functions for the four
proteins are unknown. Previous study showed that Pyrococcus
horikoshii Dph2 (PhDph2), a novel iron-sulfur cluster-containing
enzyme, forms a homodimer and is sufficient for the first step of
diphthamide biosynthesis <i>in vitro</i>. Here we demonstrate
by <i>in vitro</i> reconstitution that yeast Dph1 and Dph2
form a complex (Dph1-Dph2) that is equivalent to the homodimer of
PhDph2 and is sufficient to catalyze the first step <i>in vitro</i> in the presence of dithionite as the reductant. We further demonstrate
that yeast Dph3 (also known as KTI11), a CSL-type zinc finger protein,
can bind iron and in the reduced state can serve as an electron donor
to reduce the Fe-S cluster in Dph1-Dph2. Our study thus firmly establishes
the functions for three of the proteins involved in eukaryotic diphthamide
biosynthesis. For most radical SAM enzymes in bacteria, flavodoxins
and flavodoxin reductases are believed to serve as electron donors
for the Fe-S clusters. The finding that Dph3 is an electron donor
for the Fe-S clusters in Dph1-Dph2 is thus interesting and opens up
new avenues of research on electron transfer to Fe-S proteins in eukaryotic
cells
Organometallic Complex Formed by an Unconventional Radical <i>S</i>‑Adenosylmethionine Enzyme
<i>Pyrococcus horikoshii</i> Dph2 (<i>Ph</i>Dph2) is
an unusual radical <i>S</i>-adenosylmethionine
(SAM) enzyme involved in the first step of diphthamide biosynthesis.
It catalyzes the reaction by cleaving SAM to generate a 3-amino-3-carboxypropyl
(ACP) radical. To probe the reaction mechanism, we synthesized a SAM
analogue (SAM<sub>CA</sub>), in which the ACP group of SAM is replaced
with a 3-carboxyÂallyl group. SAM<sub>CA</sub> is cleaved by <i>Ph</i>Dph2, yielding a paramagnetic (<i>S</i> = 1/2)
species, which is assigned to a complex formed between the reaction
product, α-sulfinyl-3-butenoic acid, and the [4Fe-4S] cluster.
Electron–nuclear double resonance (ENDOR) measurements with <sup>13</sup>C and <sup>2</sup>H isotopically labeled SAM<sub>CA</sub> support a π-complex between the CC double bond of
α-sulfinyl-3-butenoic acid and the unique iron of the [4Fe-4S]
cluster. This is the first example of a radical SAM-related [4Fe-4S]<sup>+</sup> cluster forming an organometallic complex with an alkene,
shedding additional light on the mechanism of <i>Ph</i>Dph2
and expanding our current notions for the reactivity of [4Fe-4S] clusters
in radical SAM enzymes