Low-barrier Hydrogen Bond Research Articles

Detecting metal ions by the color change in perylene diimide radical anion/b-PEI complex

The photophysical properties of the hydrogen bonded complexes consisting of low-barrier hydrogen bond (LBHB) depend on their environmental conditions. However, the stimuli responsive materials based on LBHB are still a challenging problem. Here, we report a novel material system which has a high color response to metal ions based on the position changes of the proton between the two heavy atoms involved in the low barrier hydrogen bond. Perylene diimide radical anion/branched polyethyleneimine hydrogen bonding complex (TFPDI-b-PEI) linked by low-barrier hydrogen bond ([O1-----H1---N1]−1) is obtained. The spectral responses of TFPDI-b-PEI to different metal ions are investigated in DMF and aqueous solutions by UV–Vis spectrophotometry. The results show that TFPDI-b-PEI can recognize Al3+, Ni2+ and Cd2+ by the blue shift of characteristic absorption peak (from 789 nm to 565 nm) and the change of absorbance. In essence, the geometry of LBHB in TFPDI-b-PEI can switch between two types of hydrogen bond ([O1----H1----N1]−1 and [O1-----H1---N1]−1) via adjusting the interaction between the surrounding molecules and amine groups in LBHB by interaction of metal ions with TFPDI-b-PEI. In addition, based on this hydrogen bonding dynamic property, TFPDI-b-PEI exhibits excellent colorimetric sensing performance for Al3+.

Colorimetric switching and sensing of CO2 based on reversible proton movement between the two heavy atoms in low barrier hydrogen bond in PDI radical anion/b-PEI hydrogen bonding complex

The photophysical properties of the hydrogen bond complexes consisting of low-barrier hydrogen bond (LBHB) depend on their environmental condition. However, the stimuli responsive materials based on LBHB are still a challenging problem. Here, we report a novel material system which has high color response to CO2 based on the position changes of the proton between the two heavy atoms in the low barrier hydrogen bond. Perylene diimide radical anion/branched polyethyleneimine hydrogen bonding complex (TFPDI-b-PEI) linked by low-barrier hydrogen bond ([O1-----H1---N1]−1) is obtained and it shows the maximum absorption of 789 nm. Meanwhile, the maximum absorption of TFPDI-b-PEI appears at 561 nm with purple when CO2 is adsorbed in the branched polyethyleneimine. In essence, the geometry of LBHB in TFPDI-b-PEI can switch between two types of hydrogen bond ([O1----H1----N1]−1 and [O1-----H1---N1]−1) via adjusting the interaction between the surrounding molecule and amine groups in LBHB by interaction of CO2 with TFPDI-b-PEI. Further the TFPDI-b-PEI shows excellent color switching and colorimetric sensing performance for CO2 aqueous solution and its gas based on this hydrogen bond dynamic characteristic.

Theoretical study on L-H+-L with identical donors: Short strong hydrogen bond or not?

Short strong hydrogen bonds (SSHBs) play a crucial role in many chemical processes. Recently, as the representative of SSHBs, [F-H-F]- was experimentally observed. [F-H-F]- has a symmetric structure, which can be described as a H+ acid shared by two terminal F- donors (F--H+-F-). To explore whether two identical donors are bound to result in SSHBs, we performed theoretical studies on a series of compounds (L-H+-L) with two identical electron donors (L corresponds to donors containing group 14, 15, 16, and 17 elements). The results show that identical donors do not definitely lead to SSHBs. Instead, typical hydrogen bonds also exist. Both electronegativity and basicity contribute to the patterns of hydrogen bonds, where more electronegative and weaker donors benefit to SSHBs. In addition, it was found that zero-point energies also respond to the hydrogen bonding systems. This systemic work is expected to provide more insights into SSHBs.

Small Molecules Promote Selective Denaturation and Degradation of Tubulin Heterodimers through a Low-Barrier Hydrogen Bond.

Here, we report a novel mechanism to selectively degrade target proteins. 3-(3-Phenoxybenzyl)amino-β-carboline (PAC), a tubulin inhibitor, promotes selective degradation of αβ-tubulin heterodimers. Biochemical studies have revealed that PAC specifically denatures tubulin, making it prone to aggregation that predisposes it to ubiquitinylation and then degradation. The degradation is mediated by a single hydrogen bond formed between the pyridine nitrogen of PAC and βGlu198, which is identified as a low-barrier hydrogen bond (LBHB). In contrast, another two tubulin inhibitors that only form normal hydrogen bonds with βGlu198 exhibit no degradation effect. Thus, the LBHB accounts for the degradation. We then screened for compounds capable of forming an LBHB with βGlu198 and demonstrated that BML284, a Wnt signaling activator, also promotes tubulin heterodimer degradation through the LBHB. Our study provided a unique example of LBHB function and identified a novel approach to obtain tubulin degraders.

Release of a Proton and Formation of a Low-Barrier Hydrogen Bond between Tyrosine D and D2-His189 in Photosystem II.

In photosystem II (PSII), the second-lowest oxidation state (S1) of the oxygen-evolving Mn4CaO5 cluster is the most stable, as the radical form of the redox-active D2-Tyr160 is considered to be a candidate that accepts an electron from the lowest oxidation state (S0) in the dark. Using quantum mechanical/molecular mechanical calculations, we investigated the redox potential (E m) of TyrD and its H-bond partner, D2-His189. The potential energy profile indicates that the release of a proton from the TyrD...D2-His189 pair leads to the formation of a low-barrier H-bond. The E m depends on the H+ position along the low-barrier H-bond, e.g., 680 mV when the H+ is at the D2-His189 moiety and 800 mV when the H+ is at the TyrD moiety, which can explain why TyrD mediates both the S0 to S1 oxidation and the S2 to S1 reduction.

True-atomic-resolution insights into the structure and functional role of linear chains and low-barrier hydrogen bonds in proteins.

Hydrogen bonds are fundamental to the structure and function of biological macromolecules and have been explored in detail. The chains of hydrogen bonds (CHBs) and low-barrier hydrogen bonds (LBHBs) were proposed to play essential roles in enzyme catalysis and proton transport. However, high-resolution structural data from CHBs and LBHBs is limited. The challenge is that their 'visualization' requires ultrahigh-resolution structures of the ground and functionally important intermediate states to identify proton translocation events and perform their structural assignment. Our true-atomic-resolution structures of the light-driven proton pump bacteriorhodopsin, a model in studies of proton transport, show that CHBs and LBHBs not only serve as proton pathways, but also are indispensable for long-range communications, signaling and proton storage in proteins. The complete picture of CHBs and LBHBs discloses their multifunctional roles in providing protein functions and presents a consistent picture of proton transport and storage resolving long-standing debates and controversies.

Low-Barrier Hydrogen Bond in Fujikurin A-D: A Computational Study.

The compounds Fujikurin A, B, and D, recently isolated from Fusarium fujikuroi, possess intramolecular low-barrier hydrogen bonds (LBHBs), which are hydrogen bonds with a very low-energy barrier for proton transfer. The isolated compounds have a hydrogen-bonded proton that appears to rapidly switch between two equilibrium states via a transition state (TS). To understand the characteristics of these intramolecular LBHBs in detail, we performed path integral molecular dynamics (PIMD) simulations, which can consider nuclear quantum effects (NQEs) under a finite temperature. The PIMD simulations predicted that the NQE completely washed out the energy barrier for the proton transfer reaction. Consequently, a single-well shape emerged in the results, along with the effective free-energy potential surface for the hydrogen-bonded proton distribution. Thus, we conclude that the hydrogen-bonded proton in Fujikurin does not in fact transfer between two equilibrium structures but widely delocalizes around the global minimum structure involving the TS region.

Absorption wavelength along chromophore low-barrier hydrogen bonds

SummaryIn low-barrier hydrogen bonds (H-bonds), the pKa 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 pRCW 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.

Neutron crystallography reveals mechanisms used by Pseudomonas aeruginosa for host-cell binding

The opportunistic pathogen Pseudomonas aeruginosa, a major cause of nosocomial infections, uses carbohydrate-binding proteins (lectins) as part of its binding to host cells. The fucose-binding lectin, LecB, displays a unique carbohydrate-binding site that incorporates two closely located calcium ions bridging between the ligand and protein, providing specificity and unusually high affinity. Here, we investigate the mechanisms involved in binding based on neutron crystallography studies of a fully deuterated LecB/fucose/calcium complex. The neutron structure, which includes the positions of all the hydrogen atoms, reveals that the high affinity of binding may be related to the occurrence of a low-barrier hydrogen bond induced by the proximity of the two calcium ions, the presence of coordination rings between the sugar, calcium and LecB, and the dynamic behaviour of bridging water molecules at room temperature. These key structural details may assist in the design of anti-adhesive compounds to combat multi-resistance bacterial infections.

Hydrogen bond networks in gas-phase complex anions.

Hydrogen bond networks (HBNs) have piqued the interest of the scientific community due to their crucial roles in nature. However, HBNs that are isolated from complicated backgrounds for unraveling their characteristics are still scarce. Herein, we propose that HBNs exist in complex anions formed between α-cyclodextrin (α-CD) and four benzoic acids (RBAs) in the gas phase. The complex anions are facilely extracted from solutions via the electrospray ionization technique, and subsequently activated through collision for the investigation of their transition dynamics. It is revealed that the generation of deprotonated α-CD and neutral RBAs is the unexpected dominant dissociation pathway for all the four complex anions, and the complex anions formed from more acidic RBAs exhibit higher stabilities. These dissociation results are successfully explained by the cooperative stretching dynamics of the proposed HBNs that are formed involving the intramolecular HBN of α-CD and the intermolecular hydrogen bonds (HBs) between α-CD and RBAs. Furthermore, the rarely observed low barrier HBs (LBHBs) are suggested to be present in the HBNs. It is believed that the present complex anions can serve as a facilely accessible and informative model for studying HBNs in the future.

An N⋯H⋯N low-barrier hydrogen bond preorganizes the catalytic site of aspartate aminotransferase to facilitate the second half-reaction.

Pyridoxal 5′-phosphate (PLP)-dependent enzymes have been extensively studied for their ability to fine-tune PLP cofactor electronics to promote a wide array of chemistries. Neutron crystallography offers a straightforward approach to studying the electronic states of PLP and the electrostatics of enzyme active sites, responsible for the reaction specificities, by enabling direct visualization of hydrogen atom positions. Here we report a room-temperature joint X-ray/neutron structure of aspartate aminotransferase (AAT) with pyridoxamine 5′-phosphate (PMP), the cofactor product of the first half reaction catalyzed by the enzyme. Between PMP NSB and catalytic Lys258 Nζ amino groups an equally shared deuterium is observed in an apparent low-barrier hydrogen bond (LBHB). Density functional theory calculations were performed to provide further evidence of this LBHB interaction. The structural arrangement and the juxtaposition of PMP and Lys258, facilitated by the LBHB, suggests active site preorganization for the incoming ketoacid substrate that initiates the second half-reaction.

Hyperbilirubinemia induced oxidative stress resulting in glomerular injury in rats

To observe the effect of hyperbilirubinemia on glomerulus of rats, and to explore its dose-response and mechanism. Twenty-four adult male Sprague-Dawley (SD) rats were divided into four groups according to the random number table method, with 6 rats in each group. Hyperbilirubinemia rat model was reproduced by intraperitoneal injection of bilirubin once every 12 hours for 4 times, at doses of 50, 100, and 200 mg/kg in low, medium, and high dose bilirubin group (LB group, MB group, HB group), respectively. The rats in negative control group (NC group) were given the same solvent without bilirubin powder. Urine was collected 24 hours after administration, and total protein (TP) level was detected. Then the rats were sacrificed, the blood was collected by cardiac puncture, and the total bilirubin (TBil) and direct bilirubin (DBil) levels were measured by automatic biochemical analyzer. The renal tissue was collected and stained with periodic acid-Schiff (PAS) staine, the glomerular morphology was observed under light microscope, and the glomerular injury score was performed. Podocyte morphology was observed by transmission electron microscopy after uranium acetate and lead citrate double staining. The activity of superoxide dismutase (SOD) and content of malondialdehyde (MDA) were determined by colorimetric method. The expression level of podocyte specific marker Wilms tumor protein-1 (WT-1) was determined by Western blotting. With the increase of bilirubin dose, the contents of 24-hour urine TP, blood TBil, blood DBil and MDA content in kidney tissue were gradually increased, and the SOD activity and WT-1 expression in kidney tissue were gradually decreased. The differences between LB group, MB group, HB group and NC group were statistically significant [24-hour urine TP (mg): 24.85±2.22, 52.57±3.66, 56.84±3.49 vs. 7.50±1.33; blood TBil (μmol/L): 37.75±2.19, 81.37±2.13, 125.13±9.96 vs. 5.53±0.41; blood DBil (μmol/L): 15.50±1.96, 37.88±1.05, 64.53±2.89 vs. 2.38±0.35; kidney MDA (μmol/g): 3.14±0.65, 5.01±0.28, 7.50±1.08 vs. 2.30±0.20; kidney SOD (kU/g): 95.91±10.43, 57.06±15.90, 37.12±11.72 vs. 113.91±12.16; kidney WT-1 protein (WT-1/GAPDH): 0.280±0.006, 0.239±0.006, 0.198±0.001 vs. 0.361±0.005; all P < 0.05]. It was shown under light microscope that uneven thickness of mesangial membrane and basement membrane of the glomerulus, and some of them were accompanied by hyperplasia and widening. The glomerular injury score increased with the increase in bilirubin dose. The differences between LB group, MB group, HB group and NC group were statistically significant (17.50±1.05, 25.00±1.41, 34.00±1.41 vs. 11.67±0.82, all P < 0.05). Transmission electron microscopy showed that with the increase of bilirubin dose, the damage of glomerular podocytes was aggravated. Hyperbilirubinemia induced damage to glomerulus in a dose-dependent manner. In the lethal dose range, the higher the dose, the stronger the damage, which might be related to the oxidative stress promoted by bilirubin and the damage of glomerular podocytes.

Alkali metal salts of 4-hydroxybenzoic acid: a structural and educational study.

As part of an educational exercise designed to introduce school students to the technique of single-crystal X-ray diffraction and enhance their understanding of primary and secondary bonding, a group of nine secondary school students was given the opportunity to prepare new compounds and to solve and refine data collected on the crystalline materials they had prepared. Their investigation of the alkali metal salts of 4-hydroxybenzoic acid (H2hba) yielded nine new compounds and their structures are described in this article. Whilst the salts might be expected to have similar atomic arrangements, there are significant differences in their structures. Although H2hba is a relatively simple organic molecule, it displays remarkable coordinative flexibility, forming ionic solids containing the uncharged molecule, the monoanion Hhba- or the dianion hba2-. A common feature of the structures is their layered arrangement: alternating hydrophilic layers made up of closely packed metal-oxygen polyhedra separated by the hydrophobic component of the hydroxybenzoate linking units. Close packing of these units seems to be a dominant influence in determining the overall structure. The hydroxybenzoate units are usually both parallel and antiparallel with their immediate neighbours, with packing that can be edge-to-face, face-to-face or a mixture of the two. Hydrogen bonding plays a key role in the structure of most compounds and a short strong hydrogen bond (SSHB) is observed in two of the networks. The compounds of 4-hydroxybenzoic acid, C7H6O3, described here are: poly[di-μ-aqua-μ-4-oxidobenzoato-dilithium], [Li2(C7H4O3)(H2O)2]n, 1, poly[triaqua-μ-4-oxidobenzoato-dilithium], [Li2(C7H4O3)(H2O)3]n, 2, poly[μ-4-hydroxybenzoato-lithium], [Li(C7H5O3)]n, 3, catena-poly[4-hydroxybenzoate [[diaquasodium]-di-μ-aqua]], {[Na(H2O)4](C7H5O3)}n, 4, poly[di-μ-aqua-aqua-μ-4-hydroxybenzoato-potassium], [K(C7H5O3)(H2O)3]n, 5, poly[μ-aqua-μ-4-hydroxybenzoato-potassium], [K(C7H5O3)(H2O)]n, 6, poly[aqua-μ-4-hydroxybenzoato-rubidium], [Rb(C7H5O3)(H2O)]n, 7, poly[aqua-μ-4-hydroxybenzoato-caesium], [Cs(C7H5O3)(H2O)]n, 8, poly[[μ-aqua-aqua(μ-4-hydroxybenzoato)(4-hydroxybenzoic acid)sodium] monohydrate], {[Na(C7H5O3)(C7H6O3)(H2O)2]·H2O}n, 9, poly[[(μ-4-hydroxybenzoato)(μ-4-hydroxybenzoic acid)rubidium] monohydrate], {[K(C7H5O3)(C7H6O3)]·H2O}n, 10, and poly[[(μ-4-hydroxybenzoato)(μ-4-hydroxybenzoic acid)rubidium] monohydrate], {[Rb(C7H5O3)(C7H6O3)]·H2O}n, 11.

Neutron structures of Leishmania mexicana triosephosphate isomerase in complex with reaction-intermediate mimics shed light on the proton-shuttling steps.

Triosephosphate isomerase (TIM) is a key enzyme in glycolysis that catalyses the interconversion of glyceraldehyde 3-phosphate and dihydroxy-acetone phosphate. This simple reaction involves the shuttling of protons mediated by protolysable side chains. The catalytic power of TIM is thought to stem from its ability to facilitate the deprotonation of a carbon next to a carbonyl group to generate an enediolate intermediate. The enediolate intermediate is believed to be mimicked by the inhibitor 2-phosphoglycolate (PGA) and the subsequent enediol intermediate by phosphoglycolohydroxamate (PGH). Here, neutron structures of Leishmania mexicana TIM have been determined with both inhibitors, and joint neutron/X-ray refinement followed by quantum refinement has been performed. The structures show that in the PGA complex the postulated general base Glu167 is protonated, while in the PGH complex it remains deprotonated. The deuteron is clearly localized on Glu167 in the PGA-TIM structure, suggesting an asymmetric hydrogen bond instead of a low-barrier hydrogen bond. The full picture of the active-site protonation states allowed an investigation of the reaction mechanism using density-functional theory calculations.

Is Deprotonation of the Oxygen-Evolving Complex of Photosystem II during the S1 → S2 Transition Suppressed by Proton Quantum Delocalization?

We address the protonation state of the water-derived ligands in the oxygen-evolving complex (OEC) of photosystem II (PSII), prepared in the S2 state of the Kok cycle. We perform quantum mechanics/molecular mechanics calculations of isotropic proton hyperfine coupling constants, with direct comparisons to experimental data from two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopy and extended X-ray absorption fine structure (EXAFS). We find a low-barrier hydrogen bond with significant delocalization of the proton shared by the water-derived ligand, W1, and the aspartic acid residue D1-D61 of the D1 polypeptide. The lowering of the zero-point energy of a shared proton due to quantum delocalization precludes its release to the lumen during the S1→ S2 transition. Retention of the proton facilitates the shuttling of a proton during the isomerization of the tetranuclear manganese-calcium-oxo (Mn4Ca-oxo) cluster, from the "open" to "closed" conformation, a step suggested to be necessary for oxygen evolution from previous studies. Our findings suggest that quantum-delocalized protons, stabilized by low-barrier hydrogen bonds in model catalytic systems, can facilitate the accumulation of multiple oxidizing equivalents at low overpotentials.

Direct detection of coupled proton and electron transfers in human manganese superoxide dismutase

Human manganese superoxide dismutase is a critical oxidoreductase found in the mitochondrial matrix. Concerted proton and electron transfers are used by the enzyme to rid the mitochondria of O2•−. The mechanisms of concerted transfer enzymes are typically unknown due to the difficulties in detecting the protonation states of specific residues and solvent molecules at particular redox states. Here, neutron diffraction of two redox-controlled manganese superoxide dismutase crystals reveal the all-atom structures of Mn3+ and Mn2+ enzyme forms. The structures deliver direct data on protonation changes between oxidation states of the metal. Observations include glutamine deprotonation, the involvement of tyrosine and histidine with altered pKas, and four unusual strong-short hydrogen bonds, including a low barrier hydrogen bond. We report a concerted proton and electron transfer mechanism for human manganese superoxide dismutase from the direct visualization of active site protons in Mn3+ and Mn2+ redox states.

Protonation Equilibrium in the Active Site of the Photoactive Yellow Protein.

The role and existence of low-barrier hydrogen bonds (LBHBs) in enzymatic and protein activity has been largely debated. An interesting case is that of the photoactive yellow protein (PYP). In this protein, two short HBs adjacent to the chromophore, p-coumaric acid (pCA), have been identified by X-ray and neutron diffraction experiments. However, there is a lack of agreement on the chemical nature of these H-bond interactions. Additionally, no consensus has been reached on the presence of LBHBs in the active site of the protein, despite various experimental and theoretical studies having been carried out to investigate this issue. In this work, we perform a computational study that combines classical and density functional theory (DFT)-based quantum mechanical/molecular mechanical (QM/MM) simulations to shed light onto this controversy. Furthermore, we aim to deepen our understanding of the chemical nature and dynamics of the protons involved in the two short hydrogen bonds that, in the dark state of PYP, connect pCA with the two binding pocket residues (E46 and Y42). Our results support the existence of a strong LBHB between pCA and E46, with the H fully delocalized and shared between both the carboxylic oxygen of E46 and the phenolic oxygen of pCA. Additionally, our findings suggest that the pCA interaction with Y42 can be suitably described as a typical short ionic H-bond of moderate strength that is fully localized on the phenolic oxygen of Y42.

How enzymes harness highly unfavorable proton transfer reactions.

Acid-base reactions that are exceedingly unfavorable under standard conditions can be catalytically important at enzyme active sites. For example, in triose phosphate isomerase, a glutamate side chain (nominal pKa ≈ 4 in solution) can in fact deprotonate a CH group that is vicinal to a carbonyl (pKa ≈ 18 in solution). This is true because of three distinct interactions: (a) ground state pKa shifts due to environment polarity and electrostatics; (b) dramatic increases in effective molarity due to optimization of proximity and orientation; and (c) transition state pKa shifts due to binding interactions and the formation of strong low barrier hydrogen bonds. In this report, we review the literature showing that the sum of these three effects supplies more than enough free energy to push forward proton transfer reactions that under standard conditions are exceedingly nonspontaneous and slow.

Short Strong Hydrogen Bonds can Hinder Complex Formation: A Stability and Structure Study of Copper(II) Alkyl-N-iminodiacetic Acid Complexes in Aqueous Systems and Solid State

In the present study the solution and coordination chemistry of copper(II)–alkyl-N-iminodiacetate systems are studied in aqueous solution by potentiometry, using ion selective copper and pH electrodes, EXAFS (extended X-ray absorption fine structure) and dye probe molecular absorption spectrophotometry. Alkyl-N-iminodiacetates with varying alkyl chain length, methyl (CH3–), n-hexyl (C6H13–), n-dodecyl (C12H25–) and n-octadecyl (C18H37–) were used to tune the amphiphilic properties of the ligands. The polar head groups have both oxygen (hard Lewis base) and nitrogen donor (border-line Lewis base) atoms. This means that metal ions with different bonding characteristics may bind these ligands differently. Furthermore, the chelating properties of the polar head group may be regulated by pH as the acid–base properties of the imine and carboxylic acid groups are different. Copper(II) forms two stable complexes with alkyl-N-iminodiacetates with short alkyl chains, present as monomers in aqueous solution, log10β1 = 11.10(2), log10β2 = 19.5(2) for methyl-N-iminodiacetate, and log10β1 = 12.22(4), log10β2 = 21.9(2) for n-hexyl-N-iminodiacetate. n-Octadecyl-N-iminodiacetic acid, present as large aggregates in acidic aqueous solution, has short strong hydrogen bonds between carboxylic acid and carboxylate groups in the surface of the aggregates, which hinder complex formation at pH values below 4, obstructs it in the pH region 4–7, while the complex formation behaves as for short-chained alkyl-N-iminodiacetates at pH > 7. The structure around copper in copper(II)–alkyl-N-iminodiacetate complexes in aqueous solution and solid state formed at different pH values and copper(II):alkyl-N-iminodiacetate ratios has been determined by EXAFS. The coordination chemistry of copper(II) shows four strong bonds in the equatorial plane, and two different Cu–O/N bond distances, ca. 0.2 Å apart, in the axial positions of a non-centrosymmetric tetragonally elongated octahedron.

Interplay between spin crossover and proton migration along short strong hydrogen bonds.

The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T1(LIESST) = 68 K and T2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.