4 research outputs found
Impact of Nanoparticle Aggregation on Protein Recovery through a Pentadentate Chelate Ligand on Magnetic Carriers
The
growing need for more efficient separation techniques still
dominates downstream processing of biomolecules, thus encouraging
the continuous development of advanced nanomaterials. In this paper
we present an improved process for recovering recombinant histidine
tagged green fluorescent protein from an <i>E. coli</i> cell
lysate. Superparamagnetic coreāshell nanocarriers are functionalized
with a pentadentate chelate affinity ligand and then loaded with metal
ions (Cu<sup>2+</sup>, Ni<sup>2+</sup>, or Zn<sup>2+</sup>). The separation
process yields high binding capacity (250 mg/g), good selectivity,
purity >98%, good recyclability with 90% capacity after 9 cycles,
and long-term stability. We determined the main physical properties
of the magnetite-based nanoparticles such as saturation magnetization
(59 A m<sup>2</sup>/kg), primary particle diameter (22 Ā± 4 nm),
and specific surface area (89 m<sup>2</sup>/g). Our results show that
this material is a promising tool for bioseparation applications.
One special focus of the work includes analyzing the changes in the
hydrodynamic size distribution using dynamic light scattering and
transmission electron microscopy. We relate these effects to different
interaction levels in the system and discuss how the stronger aggregation
of the magnetite core is the main limiting factor for the separation
yield, leading to a considerable decrease in the number of metal ions
available for biomolecular capture. Otherwise weaker interactions
lead instead to agglomeration effects that have no impact on the binding
capacity of the system. The simple relation between the size of the
aggregated units and the size of the primary particles corresponds
approximately to the relation between the number of existing binding
sites and the actual protein binding in the separation process. Compared
with that, the effect of steric hindrance among proteins is of less
significance
Direct Observation of Photoinduced Tautomerization in Single Molecules at a Metal Surface
Molecular switches are of fundamental
importance in nature, and light is an important stimulus to selectively
drive the switching process. However, the local dynamics of a conformational
change in these molecules remain far from being completely understood
at the single-molecule level. Here, we report the direct observation
of photoinduced tautomerization in single porphycene molecules on
a Cu(111) surface by using a combination of low-temperature scanning
tunneling microscopy and laser excitation in the near-infrared to
ultraviolet regime. It is found that the thermodynamically stable
trans configuration of porphycene can be converted to the metastable
cis configuration in a unidirectional fashion by photoirradiation.
The wavelength dependence of the tautomerization cross section exhibits
a steep increase around 2 eV and demonstrates that excitation of the
Cu d-band electrons and the resulting hot carriers play a dominant
role in the photochemical process. Additionally, a pronounced isotope
effect in the cross section (ā¼100) is observed when the transferred
hydrogen atoms are substituted with deuterium, indicating a significant
contribution of zero-point energy in the reaction. Combined with the
study of inelastic tunneling electron-induced tautomerization with
the STM, we propose that tautomerization occurs via excitation of
molecular vibrations after photoexcitation. Interestingly, the observed
cross section of ā¼10<sup>ā19</sup> cm<sup>2</sup> in
the visibleāultraviolet region is much higher than that of
previously studied molecular switches on a metal surface, for example,
azobenzene derivatives (10<sup>ā23</sup>ā10<sup>ā22</sup> cm<sup>2</sup>). Furthermore, we examined a local environmental
impact on the photoinduced tautomerization by varying molecular density
on the surface and find substantial changes in the cross section and
quenching of the process due to the intermolecular interaction at
high density
Threshold Energies for Single-Carbon Knockout from Polycyclic Aromatic Hydrocarbons
We
have measured absolute cross sections for ultrafast (femtosecond)
single-carbon knockout from polycyclic aromatic hydrocarbon (PAH)
cations as functions of HeāPAH
center-of-mass collision energy in the 10ā200 eV range. Classical
molecular dynamics (MD) simulations cover this range and extend up
to 10<sup>5</sup> eV. The shapes of the knockout cross sections are
well-described by a simple analytical expression yielding experimental
and MD threshold energies of <i>E</i><sub>th</sub><sup>Exp</sup> = 32.5 Ā± 0.4 eV and <i>E</i><sub>th</sub><sup>MD</sup> = 41.0 Ā± 0.3 eV, respectively. These are the first measurements
of knockout threshold energies for molecules isolated in vacuo. We
further deduce semiempirical (SE) and MD displacement energies, i.e.,
the energy transfers to the PAH molecules at the threshold energies
for knockout, of <i>T</i><sub>disp</sub><sup>SE</sup> = 23.3 Ā± 0.3 eV and <i>T</i><sub>disp</sub><sup>MD</sup> = 27.0
Ā± 0.3 eV. The semiempirical results compare favorably with measured
displacement energies for graphene (<i>T</i><sub>disp</sub> = 23.6 eV)
Halide-Free Synthesis and Tribological Performance of Oil-Miscible Ammonium and Phosphonium-Based Ionic Liquids
Due to their low vapor pressures,
nonflammability, high thermal
stabilities, and excellent tribological properties ionic liquids (ILs)
are highly attractive lubricant base oils and additives. However,
for practical applications of ILs in lubrication, two requirements
are often limiting, the required miscibility with standard mineral
oils (ā„5 wt %) and the complete absence of corrosive halide
ions in the ionic liquid. Moreover, the need for full compatibility
with standard oil additives reduces the number of potential IL-based
lubricant additives even further. In this contribution, an economic
halide-free synthesis route to oil-miscible ionic liquids is presented,
and very promising tribological properties of such ILs as base oil
or additive are demonstrated. Therefore, sliding tests on bearing
steel and XPS analysis of the formed surface films are shown. Corrosion
test results of different bearing metals in contact with our halide-free
ILs and (salt) water prove their applicability as real life lubricants.
In the sustainable chemistry and engineering context, we present a
halide-free design approach for ionic performance chemicals that may
contribute to significant energy savings due to their enhanced lubrication
properties