3 research outputs found
Artificial Carbonic Anhydrase-Ruthenium Enzyme for Photocatalytic Water Oxidation
Bovine carbonic anhydrase (BCA) is an enzyme that regulates
cellular
pH by catalyzing CO2 hydration. In this work, we used its
well-defined zinc-containing active site to host a series of four
sulfonamide-functionalized ruthenium-based water oxidation catalysts Ru1 to Ru4, thereby producing four BCA-Ru1 to BCA-Ru4 artificial metalloenzymes (ArMs). The four
ruthenium complexes differed either by the nature of the spectator
ligand (bda2– or tda2–) bound
to the catalytic center or by the length of the linker between the
axially ruthenium-bound pyridine moiety and the zinc-binding sulfonamide.
The two ArMs BCA-Ru1 and BCA-Ru2 were catalytically
active for photocatalytic water oxidation in aqueous solution in the
presence of [Ru(bpy)3](ClO4)2 as
a photosensitizer, Na2S2O8 as an
electron acceptor, and blue light (450 nm). The most active artificial
metalloenzyme, BCA-Ru1, could drive photocatalytic O2 production at particularly low ArM concentrations (5 μM),
yielding a turnover number (TON) of 348 and a turnover frequency (TOF)
of 9 min–1 that was 1 order of magnitude higher
than for the enzyme-free catalyst. A molecular dynamics study was
performed to model the interaction between the ruthenium catalyst
and the BCA protein. Overall, the protein scaffold modified
the second coordination sphere around the catalytic center, which
enhanced the activity and stability of two out of the four water oxidation
catalysts in aqueous solution, modifying their pH dependence and suppressing
the need for adding any organic solvents in solution. Altogether,
these results demonstrate how useful artificial metalloenzymes can
be for the making of artificial photosynthetic systems
Ultrafast Proton Shuttling in <i>Psammocora</i> Cyan Fluorescent Protein
Cyan, green, yellow, and red fluorescent
proteins (FPs) homologous
to green fluorescent protein (GFP) are used extensively as model systems
to study fundamental processes in photobiology, such as the capture
of light energy by protein-embedded chromophores, color tuning by
the protein matrix, energy conversion by Förster resonance
energy transfer (FRET), and excited-state proton transfer (ESPT) reactions.
Recently, a novel cyan fluorescent protein (CFP) termed psamFP488
was isolated from the genus <i>Psammocora</i> of reef building
corals. Within the cyan color class, psamFP488 is unusual because
it exhibits a significantly extended Stokes shift. Here, we applied
ultrafast transient absorption and pump–dump–probe spectroscopy
to investigate the mechanistic basis of psamFP488 fluorescence, complemented
with fluorescence quantum yield and dynamic light scattering measurements.
Transient absorption spectroscopy indicated that, upon excitation
at 410 nm, the stimulated cyan emission rises in 170 fs. With pump–dump–probe
spectroscopy, we observe a very short-lived (110 fs) ground-state
intermediate that we assign to the deprotonated, anionic chromophore.
In addition, a minor fraction (14%) decays with 3.5 ps to the ground
state. Structural analysis of homologous proteins indicates that Glu-167
is likely positioned in sufficiently close vicinity to the chromophore
to act as a proton acceptor. Our findings support a model where unusually
fast ESPT from the neutral chromophore to Glu-167 with a time constant
of 170 fs and resulting emission from the anionic chromophore forms
the basis of the large psamFP488 Stokes shift. When dumped to the
ground state, the proton on neutral Glu is very rapidly shuttled back
to the anionic chromophore in 110 fs. Proton shuttling in excited
and ground states is a factor of 20–4000 faster than in GFP,
which probably results from a favorable hydrogen-bonding geometry
between the chromophore phenolic oxygen and the glutamate acceptor,
possibly involving a short hydrogen bond. At any time in the reaction,
the proton is localized on either the chromophore or Glu-167, which
implies that most likely no low-barrier hydrogen bond exists between
these molecular groups. This work supports the notion that proton
transfer in biological systems, be it in an electronic excited or
ground state, can be an intrinsically fast process that occurs on
a 100 fs time scale. PsamFP488 represents an attractive model system
that poses an ultrafast proton transfer regime in discrete steps.
It constitutes a valuable model system in addition to wild type GFP,
where proton transfer is relatively slow, and the S65T/H148D GFP mutant,
where the effects of low-barrier hydrogen bonds dominate
Structure Determination of a Bio-Inspired Self-Assembled Light-Harvesting Antenna by Solid-State NMR and Molecular Modeling
The
molecular stacking of an artificial light-harvesting antenna
self-assembled from 3<sup>1</sup>-aminofunctionalized zinc-chlorins
was determined by solid-state NMR in combination with quantum-chemical
and molecular-mechanics modeling. A library of trial molecular stacking
arrangements was generated based on available structural data for
natural and semisynthetic homologues of the Zn-chlorins. NMR assignments
obtained for the monomer in solution were validated for self-assembled
aggregates and refined with <sup>1</sup>H–<sup>13</sup>C heteronuclear
correlation spectroscopy data collected from samples with <sup>13</sup>C at natural abundance. Solid-state ring-current shifts for the <sup>1</sup>H provided spatial constraints to determine the molecular
overlap. This procedure allows for a discrimination between different
self-assembled structures and a classification of the stacking mode
in terms of electric dipole alignment and π–π interactions,
parameters that determine the functional properties of light-harvesting
assemblies and conducting nanowires. The combination with quantum-mechanical
modeling then allowed building a low-resolution packing model in silico
from molecular stacks. The method allows for moderate disorder and
residual polymorphism at the stack or molecular level and is generally
applicable to determine molecular packing structures of aromatic molecules
with structural asymmetry, such as is commonly provided by functionalized
side chains that serve to tune the self-assembly process