6 research outputs found
Identification of Binding Sites in Copper(II)-Peptide Complexes Using Infrared Spectroscopy
Complex
formation of the copper(II) ion (CuII) with
histidine (H) and H-containing peptides plays a crucial role in various
metallo-enzymatic reactions. To elucidate the nature of coordinate
bonding in CuII complexes, Fourier-transform infrared spectroscopy
and 2D IR spectroscopy were employed to investigate the coordination
geometries of CuII with diglycine, l-histidylglycine
(HG), glycyl-l-histidine (GH), and glycylglycyl-l-histidine. The coordination of CuII to different peptide
groups, including the peptide N- and C-termini, the amide group, and
the imidazole of the H side chain, exhibits distinct spectral features.
The derived molecular structure of the CuII–HG complex
based on these spectral features significantly differs from that of
CuII–GH, suggesting a preference of the N-terminus
and the steric hindrance of the H side chain in CuII chelation
Conductivity and Solvation Dynamics in Ionic Liquids
It
was shown recently that a simple dielectric continuum model predicts
the integral solvation time of a dipolar solute ⟨τ<sub>solv</sub>⟩ to be inversely proportional to the electrical
conductivity σ<sub>0</sub> of an ionic solvent or solution.
In this Letter, we provide a more general derivation of this connection
and show that available data on coumarin 153 (C153) in ionic liquids
generally support this prediction. The relationship between solvation
time and conductivity can be expressed by ln(⟨τ<sub>solv</sub>⟩/ps) = 4.37 – 0.92 ln (σ<sub>0</sub>/S m<sup>–1</sup>) in 34 common ionic liquids
Complete Solvation Response of Coumarin 153 in Ionic Liquids
The dynamic Stokes shift of coumarin 153, measured with
a combination
of broad-band fluorescence upconversion (80 fs resolution) and time-correlated
single photon counting (to 20 ns), is used to determine the complete
solvation response of 21 imidazolium, pyrrolidinium, and assorted
other ionic liquids. The response functions so obtained show a clearly
bimodal character consisting of a subpicosecond component, which accounts
for 10–40% of the response, and a much slower component relaxing
over a broad range of times. The times associated with the fast component
correlate with ion mass, confirming its origins in inertial solvent
motions. Consistent with many previous studies, the slower component
is correlated to solvent viscosity, indicating that its origins lie
in diffusive, structural reorganization of the solvent. Comparisons
of observed response functions to the predictions of a simple dielectric
continuum model show that, as in dipolar solvents, solvation and dielectric
relaxation involve closely related molecular dynamics. However, in
contrast to dipolar solvents, dielectric continuum predictions systematically
underestimate solvation times by factors of at least 2–4
Dielectric Relaxation and Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Protic Liquid Mixture: 1‑Butyl-3-Methylimidazolium Tetrafluoroborate + Water
Dielectric and solvation data on
mixtures of 1-butyl-3-methylimidazilium
tetrafluoroborate ([Im<sub>41</sub>][BF<sub>4</sub>]) + water are
reported and used to examine the utility of dielectric solvation models.
Dielectric permittivity and loss spectra (25 °C) were recorded
over the frequency range 200 MHz to 89 GHz at 17 compositions and
fit to a 4-Debye form. Dynamic Stokes shift measurements on the solute
coumarin 153 (C153), made by combining fluorescence upconversion (80
fs resolution) and time-correlated single photon counting data (20
ns range), were used to determine the solvation response at 7 compositions
(20.5 °C). All properties measured here were found to depend
upon mixture composition in a simple continuous manner, especially
when viewed in terms of volume fraction. Solvation response functions
predicted by a simple dielectric continuum model are similar to but
∼7-fold faster than the spectral response functions measured
with C153. The solvation data are in better agreement with the recently
published predictions of a semimolecular model of Biswas and co-workers
[<i>J. Phys. Chem. B</i> <b>2011</b>, <i>115</i>, 4011], but these latter predictions are systematically slow by
a factor of ∼3
Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic Liquid Mixture: 1‑Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile
Solvation energies, rotation times,
and 100 fs to 20 ns solvation
response functions of the solute coumarin 153 (C153) in mixtures of
1-butyl-3-methylimidazolium tetrafluoroborate ([Im<sub>41</sub>][BF<sub>4</sub>]) + acetonitrile (CH<sub>3</sub>CN) at room temperature (20.5
°C) are reported. Available density, shear viscosity, and electrical
conductivity data at 25 °C are also collected and parametrized,
and new data on refractive indices and component diffusion coefficients
presented. Solvation free energies and reorganization energies associated
with the S<sub>0</sub> ↔ S<sub>1</sub> transition of C153 are
slightly (≤15%) larger in neat [Im<sub>41</sub>][BF<sub>4</sub>] than in CH<sub>3</sub>CN. No clear evidence for preferential solvation
of C153 in these mixtures is found. Composition-dependent diffusion
coefficients (<i>D</i>) of Im<sub>41</sub><sup>+</sup> and
CH<sub>3</sub>CN as well as C153 rotation times (τ) are approximately
related to solution viscosity (η) as <i>D</i>, τ
∝ η<sup><i>p</i></sup> with values of <i>p</i> = −0.88, −0.77, and +0.90, respectively.
Spectral/solvation response functions (<i>S</i><sub>ν</sub>(<i>t</i>)) are bimodal at all compositions, consisting
of a subpicosecond fast component followed by a broadly distributed
slower component extending over ps-ns times. Integral solvation times
(⟨τ<sub>solv</sub>⟩ = ∫<sub>0</sub><sup>∞</sup><i>S</i><sub>ν</sub>(<i>t</i>) d<i>t</i>) follow
a power law on viscosity for mixture compositions 0.2 ≤ <i>x</i><sub>IL</sub> ≤ 1 with <i>p</i> = 0.79.
With recent broad-band dielectric measurements [<i>J</i>. Phys. Chem. B 2012, 116, 7509] as input, a simple dielectric continuum model provides predictions
for solvation response functions that correctly capture the distinctive
bimodal character of the observed response. At <i>x</i><sub>IL</sub> ∼ 1 predicted values of ⟨τ<sub>solv</sub>⟩ are smaller than those observed by a factor of 2–3,
but the two become approximately equal at <i>x</i><sub>IL</sub> = 0.2. Predictions of a recent semimolecular theory [J. Phys. Chem. B 2011, 115, 4011] are less
accurate, being uniformly slower than the observed solvation dynamics
Studying Protein–Protein Binding through T‑Jump Induced Dissociation: Transient 2D IR Spectroscopy of Insulin Dimer
Insulin homodimer associates through
the coupled folding and binding
of two partially disordered monomers. We aim to understand this dynamics
by observing insulin dimer dissociation initiated with a nanosecond
temperature jump using transient two-dimensional infrared spectroscopy
(2D IR) of amide I vibrations. With the help of equilibrium FTIR and
2D IR spectra, and through a systematic study of the dependence of
dissociation kinetics on temperature and insulin concentration, we
are able to decompose and analyze the spectral evolution associated
with different secondary structures. We find that the dissociation
under all conditions is characterized by two processes whose influence
on the kinetics varies with temperature: the unfolding of the β
sheet at the dimer interface observed as exponential kinetics between
250 and 1000 μs and nonexponential kinetics between 5 and 150
μs that we attribute to monomer disordering. Microscopic reversibility
arguments lead us to conclude that dimer association requires significant
conformational changes within the monomer in concert with the folding
of the interfacial β sheet. While our data indicates a more
complex kinetics, we apply a two-state model to the β-sheet
unfolding kinetics to extract thermodynamic parameters and kinetic
rate constants. The association rate constant, <i>k</i><sub>a</sub> (23 °C) = 8.8 × 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup> (pH 0, 20% EtOD), is approximately 3 orders
of magnitude slower than the calculated diffusion limited association
rate, which is explained by the significant destabilizing effect of
ethanol on the dimer state and the highly positive charge of the monomers
at this pH