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_solv⟩ to be inversely proportional to the electrical conductivity σ₀ 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_solv⟩/ps) = 4.37 – 0.92 ln (σ₀/S m^–1) 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 ([Im₄₁]­[BF₄]) + 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 ([Im₄₁]­[BF₄]) + acetonitrile (CH₃CN) 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 S₀ ↔ S₁ transition of C153 are slightly (≤15%) larger in neat [Im₄₁]­[BF₄] than in CH₃CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (<i>D</i>) of Im₄₁⁺ and CH₃CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as <i>D</i>, τ ∝ η^<i>p</i> with values of <i>p</i> = −0.88, −0.77, and +0.90, respectively. Spectral/solvation response functions (<i>S</i>_ν(<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_solv⟩ = ∫₀^∞<i>S</i>_ν(<i>t</i>) d<i>t</i>) follow a power law on viscosity for mixture compositions 0.2 ≤ <i>x</i>_IL ≤ 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>_IL ∼ 1 predicted values of ⟨τ_solv⟩ are smaller than those observed by a factor of 2–3, but the two become approximately equal at <i>x</i>_IL = 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>_a (23 °C) = 8.8 × 10⁵ M^–1 s^–1 (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