Intermolecular interactions between gold clusters and selected amino acids cysteine and glycine: a DFT study

The intermolecular interactions between Au_n (n = 3–4) clusters and selected amino acids cysteine and glycine have been investigated by means of density functional theory (DFT). Present calculations show that the complexes possessing Au-NH₂ anchoring bond are found to be energetically favored. The results of NBO and frontier molecular orbitals analysis indicate that for the complex with anchoring bonds, lone pair electrons of sulfur, oxygen, and nitrogen atoms are transferred to the antibonding orbitals of gold, while for the complex with the nonconventional hydrogen bonds (Au···H–O), the lone pair electrons of gold are transferred to the antibonding orbitals of O-H bonds during the interaction. Furthermore, the interaction energy calculations show that the complexes with Au-NH₂ anchoring bond have relatively high intermolecular interaction energy, which is consistent with previous computational studies.     

The charge-transfer complex between protochlorophyllide and NADPH: an intermediate in protochlorophyllide photoreduction

A hypothesis describing the mechanism of photoactive protochlorophyllide (P) photoreduction in vivo, relating mainly to the molecular nature of the intermediates, is proposed. The hypothesis is compatible with currently published experimental data. After illumination of etiolated barley leaves at 143 to 153 K, the absorption of P remains essentially unchanged, but a new absorption band at 690 nm is observed. Appearance of this new intermediate enables to distinguish between light and dark stages of the photoconversion reaction. When returned to the higher temperature in the dark, the treated leaves begin accumulating chlorophyllide (Chlide), concomitant with the disappearance of the 690-nm band. The decay time of the excited P (P^*) is estimated at 300 ps, which approximates the time constant of photoinduced electron transfer (ET). It is suggested that the charge-transfer complex (CTC) in its ground state (GS) (ground state of CTC formed by the partial (δ) electron transfer), i.e. (P^δ−•••H–D^δ+), between P and NADPH – the electron and proton donor (H–D) – accumulates in the following sequence: P^* + H–D → (P^*•••H–D)→[(P^*•••H–D)←(P⁻•••H–D⁺)] → ¹(P⁻•••H–D⁺)] → ³(P⁻•••H–D⁺) → (P^δ−•••H–D ^δ+), where an equilibrium state (ES) – [(P^*•••H–D)←(P⁻•••H–D⁺)] – with a lifetime of about 1 to 2 ns, exists between the local excited (LE) and ET states. The existence of a triplet ET state – ³(P⁻•••H–D⁺) – is proposed because the time interval between recording of the ES and appearance of the CTC GS (35–250 ns) does not fit the lifetime of the singlet excited complex (exciplex). It is feasible that apart from NADPH, other intermediate proton carriers are contemporaneously involved in the dark reaction (P^δ−•••H–D^δ+) → Chlide, because proton binding to the C₇–C₈ bond in vivo takes place in the trans-configuration. The hydride ion may approach the C₇–C₈ bond from one side by heterolytic fission and an additional proton, donated by the protein group, may be simultaneously added to this bond from the opposite side of the porphyrin nucleus surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

Density functional theory study on (LiNH<Subscript>2</Subscript>)<Subscript>n</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–5) clusters

Geometrical structures and relative stabilities of (LiNH₂)_n (n = 1–5) clusters were studied using density functional theory (DFT) at the B3LYP/6-31G* and B3LYP/6-31++G* levels. The electronic structures, vibrational properties, N–H bond dissociation energies (BDE), thermodynamic properties, bond properties and ionization potentials were analyzed for the most stable isomers. The calculated results show that the Li–N and Li–Li bonds can be formed more easily than those of the Li–H or N–H bonds in the clusters, in which NH₂ is bound to the framework of Li atomic clusters with fused rings. The average binding energies for each LiNH₂ unit increase gradually from 142 kJ mol⁻¹ up to about 180 kJ mol⁻¹ with increasing n. Natural bond orbital (NBO) analysis suggests that the bonds between Li and NH₂ are of strong ionicity. Three-center–two-electron Li–N–Li bonding exists in the (LiNH₂)₂ dimer. The N–H BDE values indicate that the change in N–H BDE values from the monomer a1 to the singlet-state clusters is small. The N–H bonds in singlet state clusters are stable, while the N–H bonds in triplet clusters dissociate easily. A study of their thermodynamic properties suggests that monomer a1 forms clusters (b1, c1, d2 and e1) easily at low temperature, and clusters with fewer numbers of rings tend to transfer to ones with more rings at low temperature. E _g, E _HOMO and E _av decrease gradually, and become constant. Ring-like (LiNH₂)_3,4 clusters possess higher ionization energy (VIE) and E _g, but lower values of E _HOMO. Ring-like (LiNH₂)_3,4 clusters are more stable than other types. A comparison of structures and spectra between clusters and crystal showed that the NH₂ moiety in clusters has a structure and spectral features similar to those of the crystal.     

Supramolecular synthon pattern in solid clioquinol and cloxiquine (APIs of antibacterial,antifungal, antiaging and antituberculosis drugs) studied by 35Cl NQR, 1H-17O and 1H-14N NQDR and DFT/QTAIM

The quinolinol derivatives clioquinol (5-chloro-7-iodo-8-quinolinol, Quinoform) and cloxiquine (5-chloro-8-quinolinol) were studied experimentally in the solid state via ³⁵Cl NQR, ¹H-¹⁷O and ¹H-¹⁴N NQDR spectroscopies, and theoretically by density functional theory (DFT). The supramolecular synthon pattern of O–H···N hydrogen bonds linking dimers and π–π stacking interactions were described within the QTAIM (quantum theory of atoms in molecules) /DFT (density functional theory) formalism. Both proton donor and acceptor sites in O–H···N bonds were characterized using ¹H-¹⁷O and ¹H-¹⁴N NQDR spectroscopies and QTAIM. The possibility of the existence of O–H···H–O dihydrogen bonds was excluded. The weak intermolecular interactions in the crystals of clioquinol and cloxiquine were detected and examined. The results obtained in this work suggest that considerable differences in the NQR parameters for the planar and twisted supramolecular synthons permit differentiation between specific polymorphic forms, and indicate that the more planar supramolecular synthons are accompanied by a greater number of weaker hydrogen bonds linking them and stronger π···π stacking interactions.     

A theoretical study of red-shifting and blue-shifting hydrogen bonds occurring between imidazolidine derivatives and PEG/PVP polymers

A theoretical study is presented with the aim to investigate the molecular properties of intermolecular complexes formed by the monomeric units of polyvinylpyrrolidone (PVP) or polyethyleneglycol (PEG) polymers and a set of four imidazolidine (hydantoine) derivatives. The substitution of the carbonyl groups for thiocarbonyl in the hydantoin scaffold was taken into account when analyzing the effect of the hydrogen bonds on imidazolidine derivatives. B3LYP/6-31G(d,p) calculations and topological integrations derived from the quantum theory of atoms in molecules (QTAIM) were applied with the purpose of examining the N–H⋯O hydrogen bond strengths formed between the amide group of the hydantoine ring and the oxygen atoms of PVP and PEG polymers. The effects caused by the N–H⋯O interaction fit the typical evidence for hydrogen bonds, which includes a variation in the stretch frequencies of the N–H bonds. These frequencies were identified as being vibrational red-shifts because their values decreased. Although the values of such calculated interaction energies are between 12 and 33 kJ mol⁻¹, secondary intermolecular interactions were also identified. One of these secondary interactions is formed through the interaction of the benzyl hydrogen atoms with the oxygen atoms of the PVP and PEG structures. As such, we have analyzed the stretch frequencies on the C–H bonds of the benzyl groups, and blue-shifts were identified on these bonds. In this sense, the intermolecular systems formed by hydantoine derivatives and PVP/PEG monomers were characterized as a mix of red-shifting and blue-shifting hydrogen-bonded complexes.     

Calixarene building block bis(2-hydroxyphenyl)methane (2HDPM) and hydrogen-bonded 2HDPM-H2O complex in electronic excited state

Intramolecular and intermolecular hydrogen bonding in electronic excited states of calixarene building blocks bis(2-hydroxyphenyl)methane (2HDPM) monomer and hydrogen-bonded 2HDPM-H₂O complex were studied theoretically using the time-dependent density functional theory (TDDFT). Twenty-four stable conformations (12 pairs of enantiomers) of 2HDPM monomer have been found in the ground state. From the calculation results, the conformations 1a and 1b which both have an intramolecular hydrogen bond are the most stable ones. The infrared spectra of 2HDPM monomer and 2HDPM-H₂O complex in ground state and S₁ state were calculated. The stretching vibrational absorption band of O₂???H₃ group in the monomer and complex disappeared in the S₁ state. At the same time, a new strong absorption band appeared at the C=O stretching region. From the calculation of bond lengths, it indicates that the O₂???H₃ bond is significantly lengthened in the S₁ state. However, the C₁???O₂ bond is drastically shortened upon electronic excitation to the S₁ state and has the characteristics of C=O band. Furthermore, the intramolecular hydrogen bond O₂???H₃?·?·?·?O₄ of the 2HDPM monomer and the intermolecular hydrogen bonds O₂???H₃?·?·?·?O₇ and O₇???H₉?·?·?·?O₄ of 2HDPM-H₂O complex are all shortened and strengthened in the S₁ state.

The reduced S<Subscript>−1</Subscript> and S<Subscript>−2</Subscript> oxidation states of the O<Superscript>2</Superscript>-evolving complex of photosystem II: An EPR microwave power saturation study

Progressive microwave power saturation (P_1/2) measurements have been performed on the tyrosine D radical (Y_D ^•) of photosystem II (PSII) in order to examine its relaxation enhancement by the oxygen-evolving complex (OEC) poised to the reduced S₋₁ and S₋₂ oxidation states by NO treatment. Analysis of the power saturation curves showed that the S₋₁ oxidation state of the OEC does not enhance the relaxation of Y_D ^•: it therefore possesses a diamagnetic ground state. In contrast, the Mn(II)-Mn(III) multiline electron paramagnetic resonance (EPR) signal characteristic of the S₋₂ oxidation state of the OEC was shown to provide a relaxation enhancement pathway for Y_D ^•, however less efficient relative to the one provided by the S₂-state multiline EPR signal. We also examined the Y_D ^• relaxation enhancement characteristics of the EPR-silent oxidation state produced after brief (1–5 min) dark incubation at 0°C of a PSII sample poised to the EPRactive S₋₂ state. This EPR-silent oxidation state denoted as “0°C incubation” state was shown to possess remarkably similar P_1/2 values with the EPR-active S₋₂ state in the overall examined temperature range (6–20 K). In addition, these values remained unchanged after successive cycles of the OEC between the EPR-active S₋₂ state and the “0°C incubation” state. The data presented in this work point to the conclusion that the “0°C incubation” state is indeed an S₋₂ oxidation state with half-integer spin.     

Deuterium isotope effects on <Superscript>15</Superscript>N backbone chemical shifts in proteins

Quantum mechanical calculations are presented that predict that one-bond deuterium isotope effects on the ¹⁵N chemical shift of backbone amides of proteins, ¹Δ¹⁵N(D), are sensitive to backbone conformation and hydrogen bonding. A quantitative empirical model for ¹Δ¹⁵N(D) including the backbone dihedral angles, Φ and Ψ, and the hydrogen bonding geometry is presented for glycine and amino acid residues with aliphatic side chains. The effect of hydrogen bonding is rationalized in part as an electric-field effect on the first derivative of the nuclear shielding with respect to N–H bond length. Another contributing factor is the effect of increased anharmonicity of the N–H stretching vibrational state upon hydrogen bonding, which results in an altered N–H/N–D equilibrium bond length ratio. The N–H stretching anharmonicity contribution falls off with the cosine of the N–H···O bond angle. For residues with uncharged side chains a very good prediction of isotope effects can be made. Thus, for proteins with known secondary structures, ¹Δ¹⁵N(D) can provide insights into hydrogen bonding geometries. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Theoretical characterization of SOME amides and esters DERIVATIVES of valproic acid

The equilibrium structures, the planarity of the C(=O)X linkage and the nature of the chemical bond in the Y−C(=O)−XR₁R₂ [where: Y= −CH−(CH₂−CH₂−CH₃)₂, X=N,O and R₁, R₂= H; alkyl and aryl groups and lone pair electrons (lp)] molecular fragment of derivates of Valproic acid (Vpa) with antiepileptic activity were studied systematically by means of B3LYP calculations and topological analysis of the electron localization function (ELF). The covariance parameter cov[Ω_i, Ω_j] reveals a dominating delocalization effect between the lone pair V(O₁), V(X) and the electron density of the H−C and H−X₁ bonds resulting from the existence of not only non-conventional intramolecular hydrogen bonding patterns as C−H...O/N but also a weak closed-shell stabilizing interaction type arising from a dihydrogen bonding as C−H...H−N, where H...H contacts at a significantly shorter distance than twice the hydrogen atom van der Waals radius. The analyzed data derived from ELF domains were found to be in agreement with the known features and properties of the hydrogen bonding interactions discussed in this work.     

Why are dimethyl sulfoxide and dimethyl sulfone such good solvents?

We have carried out B3PW91 and MP2-FC computational studies of dimethyl sulfoxide, (CH₃)₂SO, and dimethyl sulfone, (CH₃)₂SO₂. The objective was to establish quantitatively the basis for their high polarities and boiling points, and their strong solvent powers for a variety of solutes. Natural bond order analyses show that the sulfur–oxygen linkages are not double bonds, as widely believed, but rather are coordinate covalent single S⁺→O⁻ bonds. The calculated electrostatic potentials on the molecular surfaces reveal several strongly positive and negative sites (the former including σ-holes on the sulfurs) through which a variety of simultaneous intermolecular electrostatic interactions can occur. A series of examples is given. In terms of these features the striking properties of dimethyl sulfoxide and dimethyl sulfone, their large dipole moments and dielectric constants, their high boiling points and why they are such good solvents, can readily be understood. Figure Dimers of dimethyl sulfoxide (DMSO; left) and dimethyl sulfone (DMSO2; right) showing O S—O -hole bonding and C H—O hydrogen bonding. Sulfur atoms are yellow, oxygens are red, carbons are gray and hydrogens are white     

The cadmium binding domains in the metallothionein isoform Cd7-MT10 from Mytilus galloprovincialis revealed by NMR spectroscopy

The metal–thiolate connectivity of recombinant Cd₇-MT10 metallothionein from the sea mussel Mytilus galloprovincialis has been investigated for the first time by means of multinuclear, multidimensional NMR spectroscopy. The internal backbone dynamics of the protein have been assessed by the analysis of ¹⁵N T ₁ and T ₂ relaxation times and steady state {¹H}–¹⁵N heteronuclear NOEs. The ¹¹³Cd NMR spectrum of mussel MT10 shows unique features, with a remarkably wide dispersion (210 ppm) of ¹¹³Cd NMR signals. The complete assignment of cysteine Hα and Hβ proton resonances and the analysis of 2D ¹¹³Cd–¹¹³Cd COSY and ¹H–¹¹³Cd HMQC type spectra allowed us to identify a four metal–thiolate cluster (α-domain) and a three metal–thiolate cluster (β-domain), located at the N-terminal and the C-terminal, respectively. With respect to vertebrate MTs, the mussel MT10 displays an inversion of the α and β domains inside the chain, similar to what observed in the echinoderm MT-A. Moreover, unlike the MTs characterized so far, the α-domain of mussel Cd₇-MT10 is of the form M₄S₁₂ instead of M₄S₁₁, and has a novel topology. The β-domain has a metal–thiolate binding pattern similar to other vertebrate MTs, but it is conformationally more rigid. This feature is quite unusual for MTs, in which the β-domain displays a more disordered conformation than the α-domain. It is concluded that in mussel Cd₇-MT10, the spacing of cysteine residues and the plasticity of the protein backbone (due to the high number of glycine residues) increase the adaptability of the protein backbone towards enfolding around the metal–thiolate clusters, resulting in minimal alterations of the ideal tetrahedral geometry around the metal centres.     

Probing the ground state of the purple mixed valence CuA center in nitrous oxide reductase: a CW ENDOR (X-band) study of the 65Cu, 15N-histidine labeled enzyme and interpretation of hyperfine couplings by molecular orbital calculations

 CW ENDOR (X-band) spectra for the purple mixed-valence [Cu(1.5+)...Cu(1.5+)], S = 1/2, Cu_A site in nitrous oxide reductase were obtained after insertion of ⁶⁵Cu or both ⁶⁵Cu and ¹⁵N-histidine. The ¹⁴N/¹⁵N isotopic substitution allowed for an unambiguous deconvolution of proton and nitrogen hyperfine couplings in the spectra. A single nitrogen coupling with a value of 12.9 ± 0.4 MHz for ¹⁴N was detected. Its anisotropy was characteristic for imidazole bound to copper. A spin density of 3–5% was estimated for the nitrogen donors to Cu_A, indicating that the ground state is ²B_3u. Proton hyperfine structure was detected from four C_β protons of coordinating cysteine residues. Their isotropic and anisotropic parts were deconvoluted by spectral simulation. From the anisotropic couplings a spin density of 16–24% was estimated for each of the cysteine thiolate donors of Cu_A. The [N_HisCu(RS)₂CuN_His]⁺ core structure of Cu_A in nitrous oxide reductase from Pseudomonas stutzeri is predicted to be similar to the crystallographically determined Cu_A* structure (Wilmanns M, Lappalainen P, Kelly M, Sauer-Eriksson E, Saraste M (1995) Proc Natl Acad Sci USA 92 : 11955–11959), but distinct from the Cu_A structure of Paracoccus denitrificans cytochrome c oxidase (Iwata S, Ostermeier C, Ludwig B, Michel H (1995) Nature 376 : 660–669). The angular dependence of the isotropic couplings as a function of the electronic ground state was calculated by the INDO/S method. The Mulliken atomic-spin populations calculated by a gradient-corrected density functional method and the semiempirical INDO/S method were compared with experimentally derived spin populations, and good agreement between theory and experiment was found for both calculations. The ground state of Cu_A is best represented by the resonance structures of the form [Cu^IS^–S^–Cu^II↔ Cu^IS^•S^–Cu^I↔ Cu^IS^–S^•Cu^I↔ Cu^IIS^–S^–Cu^I]. It is proposed that the Cu 4s,p as well as sulfur 3d orbitals play a role in the stabilization of this novel type of cluster. Received: 17 September 1997 / Accepted: 28 October 1997     

Degradation of methamidophos by <Emphasis Type="Italic">Hyphomicrobium</Emphasis> species MAP-1 and the biochemical degradation pathway

Methamidophos is one of the most widely used organophosphorus insecticides usually detectable in the environment. A facultative methylotroph, Hyphomicrobium sp. MAP-1, capable of high efficiently degrading methamidophos, was isolated from methamidophos-contaminated soil in China. It was found that the addition of methanol significantly promoted the growth of strain MAP-1 and enhanced its degradation of methamidophos. Further, this strain could utilize methamidophos as its sole carbon, nitrogen and phosphorus source for growth and could completely degrade 3,000 mg l⁻¹ methamidophos in 84 h under optimal conditions (pH 7.0, 30°C). The enzyme responsible for methamidophos degradation was mainly located on the cell inner membrane (90.4%). During methamidophos degradation, three metabolites were detected and identified based on tandem mass spectrometry (MS/MS) and gas chromatography-mass spectrometry (GC–MS) analysis. Using this information, a biochemical degradation pathway of methamidophos by Hyphomicrobium sp. MAP-1 was proposed for the first time. Methamidophos is first cleaved at the P–N bond to form O,S-dimethyl hydrogen thiophosphate and NH₃. Subsequently, O,S-dimethyl hydrogen thiophosphate is hydrolyzed at the P–O bond to release –OCH₃ and form S-methyl dihydrogen thiophosphate. O,S-dimethyl hydrogen thiophosphate can also be hydrolyzed at the P–S bond to release –SCH₃ and form methyl dihydrogen phosphate. Finally, S-methyl dihydrogen thiophosphate and methyl dihydrogen phosphate are likely transformed into phosphoric acid.     

All-electron scalar relativistic calculation of water molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation was performed on Au _n H₂O (n = 1–13) clusters using density functional theory (DFT) with the generalized gradient approximation at PW91 level. The calculation results reveal that, after adsorption, the small gold cluster would like to bond with oxygen and the H₂O molecule prefers to occupy the single fold coordination site. Reflecting the strong scalar relativistic effect, Au _n geometries are distorted slightly but still maintain a planar structure. The Au–Au bond is strengthened and the H–O bond is weakened, as manifested by the shortening of the Au–Au bond-length and the lengthening of the H–O bond-length. The H–O–H bond angle becomes slightly larger. The enhancement of reactivity of the H₂O molecule is obvious. The Au–O bond-lengths, adsorption energies, VIPs, HLGs, HOMO (LUMO) energy levels, charge transfers and the highest vibrational frequencies of the Au–O mode for Au _n H₂O clusters exhibit an obvious odd-even oscillation. The most favorable adsorption between small gold clusters and the H₂O molecule takes place when the H₂O molecule is adsorbed onto an even-numbered Au _n cluster and becomes an Au _n H₂O cluster with an even number of valence electrons. The odd–even alteration of magnetic moments is observed in Au _n H₂O clusters and may serve as material with a tunable code capacity of “0” and “1” by adsorbing a H₂O molecule onto an odd or even-numbered small gold cluster.     

Time-dependent density functional theory study on the electronic excited-state hydrogen bonding of the chromophore coumarin 153 in a room-temperature ionic liquid

In the present work, in order to investigate the electronic excited-state intermolecular hydrogen bonding between the chromophore coumarin 153 (C153) and the room-temperature ionic liquid N,N-dimethylethanolammonium formate (DAF), both the geometric structures and the infrared spectra of the hydrogen-bonded complex C153–DAF⁺ in the excited state were studied by a time-dependent density functional theory (TDDFT) method. We theoretically demonstrated that the intermolecular hydrogen bond C₁?=?O₁···H₁–O₃ in the hydrogen-bonded C153–DAF⁺ complex is significantly strengthened in the S₁ state by monitoring the spectral shifts of the C=O group and O–H group involved in the hydrogen bond C₁?=?O₁···H₁–O₃. Moreover, the length of the hydrogen bond C₁?=?O₁···H₁–O₃ between the oxygen atom and hydrogen atom decreased from 1.693 Å to 1.633 Å upon photoexcitation. This was also confirmed by the increase in the hydrogen-bond binding energy from 69.92 kJ mol^?1 in the ground state to 90.17 kJ mol^?1 in the excited state. Thus, the excited-state hydrogen-bond strengthening of the coumarin chromophore in an ionic liquid has been demonstrated theoretically for the first time.     

Density functional theory study of the potassium complexation of an unsymmetrical 1,3-alternate calix[4]-crown-5-N-azacrown-5 bearing two different crown rings

Theoretical studies of an unsymmetrical calix[4]-crown-5-N-azacrown-5 (1) in a fixed 1,3-alternate conformation and the complexes 1·K⁺(a), 1·K⁺(b), 1·K⁺(c) and 1·K⁺K⁺ were performed using density functional theory (DFT) at the B3LYP/6-31G* level. The fully optimized geometric structures of the free macroligand and its 1:1 and 1:2 complexes, as obtained from DFT calculations, were used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal–ligand and cation–π interactions were investigated. NBO analysis indicated that the stabilization interaction energies (E ₂) for O…K⁺ and N…K⁺ are larger than the other intermolecular interactions in each complex. The significant increase in electron density in the RY* or LP* orbitals of K⁺ results in strong host–guest interactions. In addition, the intermolecular interaction thermal energies (ΔE, ΔH, ΔG) were calculated by frequency analysis at the B3LYP/6-31G* level. For all structures, the most pronounced changes in the geometric parameters upon interaction are observed in the calix[4]arene molecule. The results indicate that both the intermolecular electrostatic interactions and the cation–π interactions between the metal ion and π orbitals of the two pairs that face the inverted benzene rings play a significant role.     

Crystal structure and conformational analysis of s-cis-(acetylacetonato)(ethylenediamine-N,N′-diacetato)-chromium(III)

The crystal structure of [Cr(edda)(acac)] (edda = ethylediamine-N,N′-diacetate; acac = acetylacetonato) has been determined by a single crystal X-ray diffraction study at 150 K. The chromium ion is in a distorted octahedral environment coordinated by two N and two O atoms of chelating edda and two O atoms of acac, resulting in s-cis configuration. The complex crystallizes in the space group P2₁/c of the monoclinic system in a cell of dimensions a = 10.2588(9), b = 15.801(3), c = 8.7015(11) ?, β =101.201(9)° and Z = 4. The mean Cr-N(edda), Cr-O(edda) and Cr-O(acac) bond distances are 2.0829(14), 1.9678(11) and 1.9477(11) ? while the angles O-Cr-O of edda and O-Cr-O of acac are 171.47(5) and 92.72(5)°, respectively. The crystal structure is stabilized by N–H⋯O hydrogen bonds linking [Cr(edda)(acac)] molecules in distinct linear strands. The visible electronic and IR spectroscopic properties are also discussed. An improved, physically more realistic force field, Vibrationally Optimized Force Field (VOFF), capable of reproducing structural and vibrational properties of [Cr(edda)(acac)] was developed and its transferability demonstrated on selected chromium(III) complexes with similar ligands.     

Theoretical study of the hydrogen-bonded complexes serotonin–water/hydrogen peroxide

Eight H-bonded complexes between serotonin (5-hydroxy-tryptamine) and water/hydrogen peroxide were studied at the B3LYP and HF levels of theory, using the 6-31+G(d) basis set. A thermodynamic analysis was performed in order to find the most stable complex. The calculated bonding parameters showed that the most stable H-bonded complex is formed between serotonin and hydrogen peroxide by means of the intermolecular H-bond –H₂N...H–OOH. Fig. a Theoretical study of the hydrogen-bonded supersystems serotonin-water/hydrogen peroxide     

Effect of alkyl substitution on H-bond strength of substituted amide-alcohol complexes

The effect of alkyl substitution (CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, and t-C₄H₉) on the hydrogen bond strengths (H-bond) of substituted amide-alcohol complexes has been systematically explored. B3LYP/aug-cc-pVDZ method was applied to a total of 215 alkyl substituted amide-alcohol complexes to delineate the effect of substitution on the H-bond strength; formamide-water complex is taken as reference point. Complexes are classified into five types depending on the hydrogen donor, acceptor and the site of alkyl substitution (Type-IA, Type-IIA, Type-IB, Type-IIB and Type-III). The strength of H-bond was correlated with geometrical parameters such as proton-acceptor (H∙∙∙∙Y) distance, the length of proton donating bond (X–H). In all the complexes N–H and O–H stretching frequencies are red-shifted. The effect of alkyl substitution on N–H and O–H stretching frequencies were analyzed. Topological parameters like electron density at H∙∙∙∙Y and X–H bond critical points as derived from atom in molecules (AIM) theory was also evaluated. When C = O group is participating in H-bond, the strength of H-bond decreases with increasing size of alcohols except for methanol (Type-IA, Type-III and Type-IB complexes). But it increases with increasing size of alkyl groups on amide and decreases with bulky groups. In the case of N–H group as H-bond donor, the strength of H-bond increases with increasing size of alcohols (Type-IIA and Type-IIB complexes) whereas decreases with increasing size of alkyl groups on amide. Type-IA, IIA, IB and IIB complexes exhibit good correlations among IE, H-bond distance and electron density at bcp. In Type-III complexes, average H-bond distance and sum of electron densities shows better correlation with IEs than the corresponding individuals. The correlation of IE less with electron density at RCP compared to sum of electron densities.     

Electron spin-lattice relaxation studies of different forms of the S2 state multiline EPR signal of the Photosystem II oxygen-evolving complex

The pulsed EPR inversion recovery sequence has been utilized to monitor the temperature dependence of the electron spin-lattice relaxation rate of the Mn cluster of the Photosystem II oxygen evolving complex poised in a variety of S ₂ state forms giving rise to g = 2 multiline EPR signals. A previous study (Lorigan and Britt (1994) Biochemistry 33: 12072–12076) showed that for PS II membranes treated with 5% ethanol, the S ₂ state Mn cluster relaxes via the Orbach spin-lattice relaxation mechanism, where the relaxation is enhanced via phonon scattering off an excited state spin manifold, in this case at an energy of Δ = 36.5 cm⁻¹ above the S = 1/2 ground state giving rise to the multiline EPR signal. Parallel experiments are reported for PS II membranes with 5% methanol, treated with ammonia, and following short and long term dark adaptation. In each case, the temperature dependence of the electron spin-lattice relaxation rate is consistent with Orbach relaxation, and the range of excited state energies is relatively narrow (33.8 cm⁻¹ ≤ Δ ≤ 39.7 cm⁻¹). In addition, short term dark adapted (6 min, ‘active state’) PS II membranes show biphasic recovery traces which indicate that a minority fraction of the oxygen evolving complexes are trapped in a form with greatly slowed spin-lattice relaxation. This revised version was published online in June 2006 with corrections to the Cover Date.