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Protein dynamics have been suggested to have a crucial role in biomolecular recognition, but the precise molecular mechanisms remain unclear. Herein, we performed single-molecule fluorescence resonance energy transfer measurements for wild-type maltose-binding protein (MBP) and its variants to demonstrate the interplay of conformational dynamics and molecular recognition. Kinetic analysis provided direct evidence that MBP recognizes a ligand through an 'induced-fit' mechanism, not through the generally proposed selection mechanism for proteins with conformational dynamics such as MBP. Our results indicated that the mere presence of intrinsic dynamics is insufficient for a 'selection' mechanism. An energetic analysis of ligand binding implicated the critical role of conformational dynamics in facilitating a structural change that occurs upon ligand binding.

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:  Melatonin exhibits a wide variety of biological effects, including antioxidant and anti‐inflammatory functions. Its antioxidant role impedes the etiopathogenesis of pancreatitis, but little is known about the signaling pathway of melatonin in the induction of antioxidant enzymes in acute pancreatitis (AP). The aim of this study was to determine whether melatonin could prevent cerulein‐induced AP through nuclear factor erythroid 2‐related factor 2 (Nrf2) and curtail inflammation by inhibition of NF‐κB. AP was induced by two intraperitoneal (i.p.) injections of cerulein at 2 h intervals (50 μg/kg) in Sprague‐Dawley rats. Melatonin (10 or 50 mg/kg/daily, i.p.) was administered 24 h before each injection of cerulein. The rats were killed 12 h after the last injection. Acinar cell degeneration, pancreatic edema, and inflammatory infiltration were significantly different in cerulein‐ and melatonin‐treated rats. Melatonin significantly reduced amylase, lipase, MPO, and MDA levels, and increased antioxidant enzyme activities including SOD and GPx, which were decreased in AP (P < 0.05). Melatonin increased the expression of NQO1, HO‐1, and SOD2 when compared with the cerulein‐induced AP group (P < 0.05). In addition, melatonin increased Nrf2 expression, and reduced expressions of tumor necrosis factor‐alpha, IL‐1β, IL‐6, IL‐8, and iNOS. The elevated nuclear binding of NF‐κB in the cerulein‐induced pancreatitis group was inhibited by melatonin. These results show that melatonin increases antioxidant enzymes and Nrf2 expression, and limits inflammatory mediators in cerulein‐induced AP. It is proposed that melatonin may play an important role in oxidative stress via the Nrf2 pathway in parallel with reduction of inflammation by NF‐κB inhibition.

The tendency of a Hg(II) ion to strongly quench fluorescence of potential fluorescent sensors is explored. Fluorescence measurements show the expected order of the chelation-enhanced fluorescence (CHEF) effect of Zn(II) > Cd(II) >> Hg(II) ~ Cu(II), which is interpreted as (1) unpaired electrons causing the weak CHEF effect for Cu(II) and (2) the order Zn(II) > Cd(II) >> Hg(II) reflecting the "heavy atom" effect, which may be due to increasing spin-orbit coupling constants (ζ) for Zn(II) < Cd(II) << Hg(II). The structures of mercury(II) complexes of N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine (ADPA) are reported. [Hg(ADPA)Cl(2)HgCl(2)] (1) has one Hg(II) held by two bridging chlorides, while the other Hg(II) is coordinated to the ADPA ligand. The latter Hg(II) has a nearest π contact of 3.215 Å with a C atom from the anthracenyl group, which falls in the range of reported Hg-C π contacts with aromatic groups. This contact may be important in quenching the fluorescence of the Hg(II)/ADPA complex. A density functional theory study shows that the Hg-C interaction is strong enough to prevent a simple HOMO → LUMO transition of the fluorophore. In fact, the S(0) → S(1) and S(2) transitions in the Hg(II)/ADPA complex have significant charge-transfer character to mercury. An important aspect of the coordination geometry of Hg(II) is illustrated by 1, where Hg(II) tends to form a few (often only two) short bonds to the more covalently binding donor atoms present, with much longer bonds to other donor atoms. The Hg-N bonds to the two pyridyl N-donor atoms of ADPA in 1 are relatively short at 2.212(8) and 2.224(8) Å, while that to the central saturated N-donor atom of ADPA is long at 2.603(8) Å. The latter long Hg-N bond may allow a photoinduced electron-transfer (PET) effect, quenching the fluorescence of the anthracenyl fluorophore. The structure of [Hg(ADPA)Br(2)] (2) reflects the more covalent binding of the two bromine ligands compared to the clorine ligands of 1, with much longer Hg-C contacts with the anthracenyl fluorophore and a Hg-N contact with the saturated N atom of ADPA of 2.917 Å. The latter long Hg-N contact is related to the nearly negligible fluorescence of the ADPA complex in the presence of added Br(-). The addition of extra ligands to the Hg(II)/ADPA complex produces a weak increase in the fluorescence intensity for OH(-) ~ Cl(-) >> Br(-) > I(-), which is discussed in terms of an increasing PET effect, and to collisional quenching. The ligand design principles for generating turn-on sensors for mercury suggested by this work are discussed.

Molecular self-assembly is the spontaneous association of molecules into structured aggregates by which nature builds complex functional systems. While numerous examples have focused on 2D self-assembly to understand the underlying mechanism and mimic this process to create artificial nano- and microstructures, limited progress has been made toward 3D self-assembly on the molecular level. Here we show that a helical β-peptide foldamer, an artificial protein fragment, with well-defined secondary structure self-assembles to form an unprecedented 3D molecular architecture with a molar tooth shape in a controlled manner in aqueous solution. Powder X-ray diffraction analysis, combined with global optimization and Rietveld refinement, allowed us to propose its molecular arrangement. We found that four individual left-handed helical monomers constitute a right-handed superhelix in a unit cell of the assembly, similar to that found in the supercoiled structure of collagen.

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The idea that M···C π contacts between diamagnetic heavy metal ions such as Pb(II), Ag(I), Pd(II), or Hg(II) and the anthracenyl fluorophore of adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) are responsible for quenching the fluorescence of the complexes of these metal ions with adpa is explored crystallographically. The structures of [Pb(adpa)(NO3)2] (1), [Ag(adpa)NO3] (2), [Pd(adpa)NO3]NO3 (3), [Zn(adpa)(NO3)2] (4), and [Cd(adpa)Br2] (5) are reported. The π contacts with the fluorophore are for 1 are a Pb···C π contact of 3.178 Å; for 2, an Ag···C π contact of 3.016 Å; and for 3, a Pd···C π contact of 2.954 Å on the axial site of the Pd(II) ion. The Zn(II) ion in 4 has no Zn···C π contact, with the anthracenyl fluorophore rotated completely away from the Zn(II) ion. These structures confirm that in the Pb(II), Ag(I), and Pd(II) complexes of adpa, which experience strong quenching of fluorescence, there are strong M···C π contacts, as expected if it is the π contacts that quench fluorescence. In contrast, for the Zn(II) adpa complex, which forms no π contact, there is a strong increase in fluorescence intensity. The structure of 5 shows a long Cd···C π contact at 3.369 Å, in contrast to a previously reported structure with two coordinated nitrates where the Cd···C π contact is 3.097 Å. The long Cd···C π contact in [Cd(adpa)Br2] suggests how coordination of Br(-), as well as other more covalently bound ligands such as Cl(-), SCN(-), and S2O3(2-), cause an increase in fluorescence intensity, reported for the Cd(II)adpa complex in 50% CH3OH/H2O. Coordination of covalently bound ligands to the Cd(II) weakens the Cd···C π contact and so enhances fluorescence, whereas more ionically bound ligands such as SO4(2-), NO3(-), or H2O produce a strong Cd···C π contact and weakened fluorescence. Complexes of the Cd(II)/adpa type may form the basis for a new type of anion/small molecule sensor. The tendency of metal ions to form π contacts with aromatic groups is analyzed in terms of the frequency of occurrence of π contacted structures in the literature, as well as by DFT calculations on the adpa complexes.