Combined NMR and UV-Vis Spectroscopic Studies of Models for the Hydrogen Bond System in the Active Site of Photoactive Yellow Protein: H-Bond Cooperativity and Medium Effects

Charge Relay Without Proton Transfer: Coupling of Two Short Hydrogen Bonds via Imidazole in Models of Catalytic Triad of Serine Protease Active Site

A homologous series of 20 substituted alcohol-imidazole-acetate model complexes imitating the charge relay system in Ser-His-Asp catalytic triad of serine proteases is considered quantum-chemically. We show qualitatively that the geometries of alcohol-imidazole and imidazole-acetate short hydrogen bonds are strongly coupled via the central imidazole and such complexes are capable of effectively relaying the charge from acetate to alcohol moiety upon relatively small concerted proton displacements. We hypothesize an alternative catalytic mechanism of serine proteases that does not require two complete proton transfers or hydrogen bond breakage between Ser and His residues.

Absorption wavelength along chromophore low-barrier hydrogen bonds

In low-barrier hydrogen bonds (H-bonds), the pK_a values for the H-bond donor and acceptor moieties are nearly equal, whereas the redox potential values depend on the H⁺ position. Spectroscopic details of low-barrier H-bonds remain unclear. Here, we report the absorption wavelength along low-barrier H-bonds in protein environments, using a quantum mechanical/molecular mechanical approach. Low-barrier H-bonds form between Glu46 and p-coumaric acid (pCA) in the intermediate pR_CW state of photoactive yellow protein and between Asp116 and the retinal Schiff base in the intermediate M-state of the sodium-pumping rhodopsin KR2. The H⁺ displacement of only ∼0.4 Å, which does not easily occur without low-barrier H-bonds, is responsible for the ∼50 nm-shift in the absorption wavelength. This may be a basis of how photoreceptor proteins have evolved to proceed photocycles using abundant protons.

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The low-barrier H-bond formation is a prerequisite for proton transfer

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How the absorption wavelength changes as H⁺ moves is an open question

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The H⁺ displacement of ∼0.4 Å leads to the absorption wavelength shift of ∼50 nm

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The localization of the molecular orbitals plays a key role in the wavelength shift

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects

Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d₈ and in CD₂Cl₂. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d₈ and CD₂Cl₂, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD₂Cl₂. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model.