Cyclic peptide-metal salt adducts. I. crystal structure of the hexaquocopper(II) perchlorate cyclosarcosylsarcosine 1:2 adducts

The crystal structure analysis of the 2:1 adduct of cyclosarcosylsarcosine with copper(II) perchlorate shows that the independent unit is composed of six water molecules octahedrally coordinated to the Cu(II) ion, two tetrahedral perchlorate ions and four independent halves of cyclosarcosylsarcosine molecules lying on crystallographic centers of symmetry. All available hydrogens of water molecules are involved in hydrogen bonding as donors and all carbonyl oxygen atoms of the cyclic peptide molecules function as acceptors. Two other oxygen atoms for each perchlorate anion participate in the hydrogen bonding scheme, which leads to the absence of the orientational disorder usually observed in these ions. Layers of inorganic material and layers of organic material, roughly parallel to the ab plane, pack alternatively with each other. Electrostatic and ion-dipole interactions together with hydrogen bonds are responsible for the building up of the crystals.     

Interactions of biotin with metal ions. X-ray crystal structure of the polymeric biotin--silver(I) nitrate complex: metal bonding to thioether and ureido carbonyl groups

X-ray structure analysis of the silver(I) complex of d-(+)-biotin, [Ag(biotin)(NO3)] X 0.5H2O, shows the complex to be polymeric with the silver ion coordinated tetrahedrally to nitrate and to three different biotin molecules. Binding to the latter involves two thioether sulfur atoms in the cis and trans direction with respect to the ureido ring, and one ureido carbonyl oxygen atom. The biotin carboxylate group is probably protonated and not coordinated with the silver ion.     

The structure of silver(I) iodide and bromide,and the silver(I) solvate in tetrahydrothiophene solution: An X-ray scattering and raman spectroscopic study

Structures of silver(I) iodide and bromide, and the solvated silver(I) ion are determined in tetrahydrothiophene solution with Large Angle X-ray Scattering (LAXS) technique. In a silver(I) perchlorate tetrahydrothiophene solution, silver(I) is solvated by four tetrahydrothiophene molecules in a regular tetrahedron. The main peak in the radial distribution function corresponds to four AgS distances at 2.526(7) Å. An SS distance at 2.65(2) Å in the solvent bulk is also included in the main peak. This shows that an internal structure exists in the tetrahydrothiophene bulk. Silver(I) iodide and bromide are tetrameric complexes with a stella quadrangula configuration, in saturated solution. The distances in the [AgI(SC₄H₈)]₄ complex are AgI 2.799(4); AgAg, 3.072(6) and II, 4.638(19) Å and in the [AgBr(SC₄H₈)]₄ complex they are AgBr, 2.592(3); AgAg, 2.866(5) and BrBr, 4.25(4) Å. The AgI bond distances in the [AgI(SC₄H₈)]₄ complex is shorter in solution than in the solid solvate. This is because bulk tetrahydrothiophene is a markedly weaker donor than free tetrahydrothiophene due to the sulfursulfur interactions in the bulk, shown to be around 2.65 Å. Raman spectroscopic studies on silver(I) and copper(I) iodide and silver(I) chloride tetrahydrothiophene solutions show that the polymetric structures predominate in concentrated solution and that they disintegrate upon dilution.     

Mono- and bimetallic gold(I) and silver(I) pentafluoropropionates and related compounds

A series of eight new carboxylate complexes of the general type (L)_nMOC(O)R (L=PMe₃; n=1; M=Ag, Au; R=C₂F₅. L=PPh₃; n=1-3; M=Ag; R=C₂F₅, t-Bu) have been prepared in high yields. Crystal and molecular structures have been determined for three representative examples. The crystal structure of (Ph₃P)AgOC(O)C₂F₅ contains dimers in which the silver atoms are bridged by the carboxylate oxygen atoms. This bridging resembles the structural motif found in silver carboxylates without ligand support. Usage of the smaller phosphine PMe₃ leads to the formation of a polymeric chain structure in (Me₃P)AgOC(O)C₂F₅ with bridging carboxylate anions and short Ag-Ag contacts holding the monomers together. The reaction of (4-Me₂N-C₆H₄)Ph₂ PAuCl with two equivalents of C₂F₅CO₂Ag leads to the formation of a mixed metal product containing both gold and silver. The crystal structure analysis of this compound revealed a tetranuclear complex containing a central dimeric silver pentafluoropropionate unit which is chelated by the (triarylphosphine)gold(I) pentafluoropropionate molecules via Ag-Au metallophilic contacts and Ag-O donor/acceptor interactions.     

Synthesis and structural studies on silvertin complex salts with cis-1,2-bis(diphenylphosphino)ethylene

Three new silver—tin complex salts with cis-1,2-bis(diphenylphosphino)ethylene have been synthesized. IR,³¹P and ¹¹⁹Sn NMR and conductivity data are reported and discussed together with the X-ray crystal structure of one of them, [Ag(cdppet)₂][SnPh₃(NO)₃)₂]. The silver atom is distorted tetrahedrally surrounded by the four phosphorus atoms of the two cdppet ligands. The tin atom is in a slightly distorted trigonal bipyramidal geometry with the phenyl carbon atoms in the equatorial plane and two oxygen atoms from monodentate nitrate groups at the apices.     

Biologically important ternary coordination complex. Crystal and molecular structures of adenine-glycylglycine-copper (II) complex

The crystal and molecular structures of an adenine-glycyl-glycine-copper(II) complex have been determined by X-ray diffraction. The chelating atoms, amino and amide nitrogen atoms, the carboxyl oxygen atom of the dipeptide, N(9) of adenine and one water molecule form a square-pyramid. The hydrogen-bonded adenine base-pairs stack with a distance of 3.8Å, while the dipeptides contact each other by NHO hydrogen bond to form a dimer.     

Stereoelectronic properties of metalloenzymes. 10. a refined model that mimics the type II copper(II) site in galactose oxidase2

A number of copper(II) complexes of tridentate ligands with various donor atoms have been studied in an attempt to duplicate the unusual reactivity patterns and accompanying spectral changes of the copper(II) center in galactose oxidase. Results indicate that in order to match the optical and electron spin resonance spectral change observed upon CN^? binding by the enzyme, an equatorial, negative ligand must be displaced in a small molecule model. The crystal and molecular structure of the best model complex was solved by a single crystal X-ray diffraction study. The compound, monoacetato-1,3-bis(2-(4-methyl-pyridyl)imino)isoindolatocopper(II), crystallizes in the centro-symmetric triclinic space group Pī with a = 7.392(3) Å, b = 13.782(5) Å, c = 23.422(12) Å, α = 92.08(3)°, β = 104.11(5)°, γ = 109.98(4)°, V = 2156(1) ų, d(obsd.)(calc.)=(1.43)(1.44) g/cm^?3 for mol wt of 466.7 and Z = 4. Diffraction data were collected with a Syntex Pl diffractometer using graphite-monochromatized Cu radiation (λ = 1.5418 Å). The copper atoms were located from a Patterson synthesis; all other nonhydrogen atoms were located via difference. Fourier techniques, and hydrogen atoms were placed in calculated positions. Final refinement resulted in discrepancy indices of R = 0.089 and “Goodness to Fit” = 3.68 for all 3608 reflections having (I) ? 3σ(I) (5°<2θ<100°). There are two unique molecules in the asymmetric unit that are monomeric and well separated. The geometry around the copper atom is approximately square pyramidal, with the coordination sphere derived from three nitrogens of the tridentate ligand, one oxygen from the acetate unit, and an oxygen atom of a water molecule occupying an axial position. The structure is surprising both in that an axial water molecule is present and that the remaining four ligand atoms to the copper atom are rather distorted from a planar configuration. The plane defined by the copper, N5, and N3 atoms intersects the plane defined by the copper, Nl, and Ol, atoms forming a “twist angle” of 25.0° (0.0° would be ideal for a planar inner coordination sphere). The stereoelectronics of the inner coordination spheres of the type II Cu(II) enzymes galactose oxidase and superoxide dismutase are discussed and appropriate comparisons are made with emphasis on the origin of spectral changes observed upon anion binding.     

Synthesis, X-ray structure and antimycobacterial activity of silver complexes with alpha-hydroxycarboxylic acids

In this paper, synthesis, characterization and antimycobacterial properties of a new water-soluble complex identified as silver-mandelate are described. Elemental and thermal analyses are consistent with the formula [Ag(C(6)H(5)C(OH)COO)](n). The polymeric structure was determined by single X-ray diffraction and the two-dimensional structure is based on the bis(carboxylate-O,O') dimer [Ag-O, 2.237(3), 2.222(3) Angstrom]. The structure is extended along both the b and c axes through two oxygen atoms of a bidentate alpha-hydroxyl-carboxylate residue [Ag-OH(hydroxyl), 2.477(3) Angstrom; Ag-O(carboxylate), 2.502(3) Angstrom; O-Ag-O, 63.94(9) degrees]. A strong d(10)-d(10) interaction was observed between two silver atoms. The Ag - Ag distance is 2.8307(15) Angstrom. The NMR (13)C spectrum in D(2)O shows that coordination of the ligand to Ag(I) occurs through the carboxylate group in solution. Potentiometric titration shows that only species with a molar metal:ligand ratio of 2:2 are formed in aqueous solution. The mandelate complex and the silver-glycolate, silver-malate and silver-hydrogen-tartarate complexes were tested against three types of mycobacteria, Mycobacterium avium, Mycobacterium tuberculosis and Mycobacterium kansasii, and their minimal inhibitory concentration (MIC) values were determined. The results show that the four complexes are potential candidates for antiseptic or disinfectant drugs for discharged secretions of patients affected with tuberculosis.     

Structure of cobalt carbonic anhydrase complexed with bicarbonate.

The three-dimensional structure of a complex between catalytically active cobalt(II) substituted human carbonic anhydrase II and its substrate bicarbonate was determined by X-ray crystallography (1.9 A). One water molecule and two bicarbonate oxygen atoms are found at distances between 2.3 and 2.5 A from the cobalt ion in addition to the three histidyl ligands contributed by the peptide chain. The tetrahedral geometry around the metal ion in the native enzyme with a single water molecule 2.0 A from the metal is therefore lost. The geometry is difficult to classify but might best be described as distorted octahedral. The structure is suggested to represent a water-bicarbonate exchange state relevant also for native carbonic anhydrase, where the two unprotonized oxygen atoms of the substrate are bound in a carboxylate binding site and the hydroxyl group is free to move closer to the metal thereby replacing the metal-bound water molecule. A reaction mechanism based on crystallographically determined enzyme-ligand complexes is represented.     

Pt(II) and Ag(I) complexes with acesulfame: Crystal structure and a study of their antitumoral, antimicrobial and antiviral activities

Two new complexes of platinum(II) and silver(I) with acesulfame were synthesized. Acesulfame is in the anionic form acesulfamate (ace). The structures of both complexes were determined by X-ray crystallography. For K₂[PtCl₂(ace)₂] the platinum atom is coordinated to two Cl⁻ and two N-acesulfamate atoms forming a trans-square planar geometry. Each K⁺ ion interacts with two oxygen atoms of the S(O)₂ group of each acesulfamate. For the polymeric complex [Ag(ace)]_n the water molecule bridges between two crystallographic equivalent Ag1 atoms which are related each other by a twofold symmetry axis. Two Ag1 atoms, related to each other by a symmetry centre, make bond contact with two equivalent oxygen atoms. These bonds give rise to infinite chains along the unit cell diagonal in the ac plane. The in vitro cytotoxic analyses for the platinum complex using HeLa (human cervix cancer) cells show its low activity when compared to the vehicle-treated cells. The Ag(I) complex submitted to in vitro antimycobacterial tests, using the Microplate Alamar Blue (MABA) method, showed a good activity against Mycobacterium tuberculosis, responsible for tuberculosis, with a minimal inhibitory concentration (MIC) value of 11.6 μM. The Ag(I) complex also presented a promising activity against Gram negative (Escherichia coli and Pseudomonas aeruginosa) and Gram positive (Enterococcus faecalis) microorganisms. The complex K₂[PtCl₂(ace)₂] was also evaluated for antiviral properties against dengue virus type 2 (New Guinea C strain) in Vero cells and showed a good inhibition of dengue virus type 2 (New Guinea C strain) replication at 200 μM, when compared to vehicle-treated cells.     

Homo- and heterochiral coordination polymers of silver with diaminocyclohexane as bridging ligand: Trends in alkylbenzoates

Reaction of Ag₂CO₃ with four methyl-substituted derivatives of benzoic acid afforded silver benzoates; no additional anions are involved in these solids. One of the silver carboxylates was studied by X-ray diffraction: in the crystal, silver 3,5-dimethylbenzoate monohydrate consists of carboxylato-bridged discrete dinuclear molecules with a short Ag?Ag separation of 2.7719(5) Å and one weakly bonded hydrate water molecule per cation. The binary silver carboxylates were combined with either racemic or enantiopure trans-1,2-diaminocyclohexane and resulted in four homochiral and four heterochiral crystalline solids. All eight structures feature cationic chain polymers, carboxylate anions and hydrate water. In three of the solids derived from the racemic ligand, the individual cationic chains are homochiral. In all structures, the primary coordination of the Ag(I) centers by nitrogen is essentially linear. Despite the chemical similarity in the anions, the backbone of the polymers proved to be conformationally soft with variable Ag-N-C-C torsion angles. In the resulting structures, the diamine ligand may bridge two cations in a wide distance range between ca. 3.0 and ca. 7.2 Å. Both the chirality of the trans-1,2-diaminocyclohexane ligand and the substitution pattern of the benzoate anion have strong impact on the nature of secondary interactions perpendicular to the polymer strands: either weak coordination by carboxylato or hydrate water oxygen atoms or argentophilic interactions are encountered. The Ag?Ag contacts increase the dimensionality of the solids from chain polymers to layer structures.     

Synthesis, crystal structures and in vitro anti-fungal activities of two silver(I) coordination polymers with fluconazole

Two new polymeric silver(I)-fluconazole complexes: [Ag(HFlu)(NO₃)]_n (1) and {[Ag(HFlu)₂](ClO₄)}_n (2), have been synthesized and structurally characterized. The crystal structure of 1 consists of infinite 1D single strand helical coordination arrays with alternative …PMPM… arrangements, which are interlinked through hydrogen bonding interactions to generate a 3D network. The shortest intrachain Ag?Ag distance bridged by HFlu ligand is 8.287(1) Å. In 2, each Ag(I) ion is coordinated by four triazole N atoms from four HFlu ligands to form a 2D coordination layer, which has a helical arrangement along the [1 0 0] direction. The results of anti-fungal studies demonstrate that both silver(I) complexes are more active in comparison to the fluconazole drug.     

Topography of cyclodextrin inclusion complexes : Part II. The iodine-cyclohexa-amylose tetrahydrate complex; its molecular geometry and cage-type crystal structure

Iodine-cyclohexa-amylose tetrahydrate [(C₆H₁₀O₅)₆ ·I₂·d4H₂O] crystallizes in the orthorhombic space-group P2₁2₁2₁, a  14.240 Å, b  36.014 Å, c  9.558 Å. The structure was solved by heavy-atom techniques and refined by least-squares methods to a conventional discrepancy index R  0.148 for the 2872 observed data. The six d-glucose residues are in the C1 chair conformation; the conformational angles vary in magnitude from 45 to 66°, the angles O(5)-C(5)-C(6)-O(6) are close to · 70°, and the six O(4) atoms are almost coplanar (r.m. s. displacement 0.13 Å). Only four of the six O(2) ?O(3) intramolecular hydrogen bonds have formed, which renders the molecule less symmetrical and more conical-shaped than in the previously determined α-cyclodextrin-potassium acetate complex. The iodine molecule is coaxial with the cyclohexa-amylose molecule. The I-I distance is a conventional 2.677 Å. Close interactions between the iodine atoms and the host molecule comprise carbon atoms C(5) and C(6) and oxygen atoms O(4), with interatomic distances all equal to or greater than van der Waals contacts. Intermolecular, almost-linear, short contacts O ? I-I?O with I?O distances of 3.22 and 3.07 Å indicate attractive interaction.The molecules are arranged in herring-bone “cage-type” fashion, with the four water molecules as space-filling mediators; the structure is held together by an intricate network of hydrogen bonds.     

Structure of bovine pancreatic phospholipase A2 at 1.7 Å resolution

The crystal structure of bovine pancreatic phospholipase A2 has been refined to 1.7 Å resolution. The starting model for this refinement was the previously published structure at a resolution of 2.4 Å (Dijkstra et al., 1978). This model was adjusted to the multiple isomorphous replacement map with Diamond's real space refinement program (Diamond, 1971,1974) and subsequently refined using Agarwal's least-squares method (Agarwal, 1978). The final crystallographic R-factor is 17.1% and the estimated root-mean-square error in the positional parameters is 0.12 Å. The refined model allowed a detailed survey of the hydrogen-bonding pattern in the molecule. The essential calcium ion is located in the active site and is stabilized by one carboxyl group as well as by a peptide loop with many residues unvaried in all known phospholipase A2 sequences. Five of the oxygen ligands octahedrally surround the ion. The sixth octahedral position is shared between one of the carboxylate oxygens of Asp49 and a water molecule. The entrance to the active site is surrounded by residues involved in the binding of micelle substrates. The N-terminal region plays an important role here. Its α-NH₊³ group is buried and interacts with Gln4, the carbonyl oxygen of Asn71 and a fully enclosed water molecule, which provides a link between the N terminus and several active site residues. A total of 106 water molecules was located in the final structure, most of them in a two-layer shell around the protein molecule. The mobility in the structure was derived from the individual atomic temperature factors. Minimum mobility is found for the main chain atoms in the central part of the two long α-helices. The active site is rather rigid.     

Modeling the effect of H-bonding interactions and molecular packing on the molecular structure of [Ag(ethylnicotinate)2]NO3 complex

The gas phase molecular structure of a single isolated molecule of [Ag(Etnic)₂NO₃];1 where Etnic = Ethylnicotinate was calculated using B3LYP method. The H-bonding interaction between 1 with one (complex 2) and two (complex 3) water molecules together with the dimeric formula [Ag(Etnic)₂NO₃]₂;4 and the tetrameric formula [Ag(Etnic)₂NO₃]₄;5 were calculated using the same level of theory to model the effect of intermolecular interactions and molecular packing on the molecular structure of the titled complex. The H-bond dissociation energies of complexes 2 and 3 were calculated to be in the range of 12.220–14.253 and 30.106–31.055 kcal?mol^?1, respectively, indicating the formation of relatively strong H-bonds between 1 and water molecules. The calculations predict bidentate nitrate ligand in the case of 1 and 2, leading to distorted tetrahedral geometry around the silver ion with longer Ag–O distances in case of 2 compared to 1, while 3 has a unidentate nitrate ligand leading to a distorted trigonal planar geometry. The packing of two [Ag(Etnic)₂NO₃] complex units; 4 does not affect the molecular geometry around Ag(I) ion compared to 1. In the case of 5, the two asymmetric units of the formula [Ag(Etnic)₂NO₃] differ in the bonding mode of the nitrate group, where the geometry around the silver ion is distorted tetrahedral in one unit and trigonal planar in the other. The calculations predicted almost no change in the charge densities at the different atomic sites except at the sites involved in the C–H?O interactions as well as at the coordinated nitrogen of the pyridine ring.

Crystal Structure of Organolanthanide Complexes [Li(THF)2]2 (μt-Cl)4(η5-C5 H5 )Ln·THF (Ln = La,Nd)

The crystal and molecular structures of the title compounds have been determined from single crystal X-ray diffraction. The two complexes crystallize in the orthorhombic space group Pna2₁ with Z = 4. Lattice parameters are: a = 10.504(2) [10.522(2)], ¹b = 16.816(4) [16.927(3)] and c = 18.931(4) [18.969(3)] Å. The two crystals are isomorphous. The structures were solved by Patterson and Fourier techniques and refined by least-squares techniques to R = 0.0430 for 1508 reflections. The Nd³⁺. ion is eight-coordinate, being bonded to five carbons of the cyclopentadiene ring, to four chloride atoms and to the one oxygen atom of the THF ring. The NdC distances are in the range 2.67–2.85 Å (average: 2.77 Å) and Nd-Cl distances are in the range 2.76–2.80 Å (average: 2.78 Å). The Nd-O distance is 2.52 A. The Li⁺ ion is four-coordinate, being bonded to the two chloride atoms and to the two oxygen atoms of the two THF rings. The other Li⁺ ion is the same as the above. The Li-Cl distances are in the range 2.17– 2.55 Å (average: 2.35 A) and LiO distances are in the range 1.89–1.98 Å (average: 1.91 Å). The Nd atom and the two Li atoms are bridged asymmetrically by the chloride ions, respectively.     

Structure of acidic phospholipase A2 for the venom of Agkistrodon halys blomhoffii at 2.8 A resolution.

The crystal structure of acidic phospholipase A2 from the venom of Agkistrodon halys blomhoffii has been determined by molecular replacement methods based on the known structure of Crotalus atrox PLA2, a same group II enzyme. The overall structures, except the calcium-binding regions, are very similar to each other. A calcium ion is pentagonally ligated to two carboxylate oxygen atoms of Asp-49 and each carbonyl oxygen atoms of Tyr-28, Gly-30 and Ala-31. A reason why the former enzyme functions as monomeric form, while the latter one does as dimer, could be presumed by the structural comparison of these calcium-binding regions. Although Gly-32 is usually participated as a ligand in the coordination with calcium ion in group I PLA2, it is characteristically replaced to Ala-31 in the present structure, and thus the coordination geometry of calcium ion is rather different from the usually observed one.     

Chlorotriphenyl(quinolinium-2-carboxylato)tin(IV) monohydrate

The crystal structure of chlorotriphenyl(quinolinium-2-carboxylato)tin(IV) monohydrate is reported. The crystals are monoclinic, space group C2/c with cell parameters a = 20.048(3) Å, b = 11.724(1) Å, c = 23.291(3) Å, ]gb = 113.42(1), Z = 8, refined to R_F = 0.034 on 3331 observed reflections. The tin(IV) atom is five-coordinate, being found to three phenyl groups, the chlorine atom and an oxygen from the quinaldic acid. The geometry around the tin atom is trigonal bipyramidal, with the three phenyl groups occupying the equatorial positions, and the chlorine and quinaldic acid oxygen, the apical ones. The acidic proton of quinaldic acid has shifted position in the complex, and is bound to the heterocyclic nitrogen atom.The acid is thus coordinated in the form of a zwitterion. These trigonal bipyramidal units are linked together as dimers by pairs of water molecules, each of which hydrogen-bonds to the non-coordinated carboxylate oxygen atoms of both quinaldic acid molecules, plus the heterocyclic nitrogen atom of one quinaldic acid molecule. For complex formation with the protonated acid, the heterocyclic nitrogen should be alpha to the carboxylic acid group.     

Copper(II) halide complexes of 2-aminobenzophenone. Crystal and molecular structure of bis(2-aminobenzophenone)dichlorocopper(II)

The complexes CuX₂L₂ (X = Cl, Br; L = 2-aminobenzophenone) were prepared and characterized by means of magnetic and spectroscopic measurements. For the Cl compound the crystal structure was also determined. Crystals are triclinic, space group P$\text{1}$, with a = 13.397(3), b = 10.752(2), c = 9.205(2) Å, α = 72.26(1)°, β = 91.58(1)°, γ = 106.86(1)°, and Z = 2. The structure was solved by the heavy-atom method and refined by least-squares calculations to R = 0.034 for 2581 counter data. It consists of discrete CuX₂L₂ monomers showing distorted trigonal bipyramidal coordination geometry about the copper ion. The amino nitrogens are axial ligands, with the equatorial positions occupied by two chlorine atoms and a carbonyl oxygen from one L molecule acting as a bidentate ligand. Infrared and ligand field spectroscopies and magnetic measurements, interpreted on the basis of the known crystal structure, also suggest a similar structure for the related Br compound.     

Crystal structures of the methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath): Implications for substrate gating and component interactions

The crystal structure of the nonheme iron-containing hydroxylase component of methane monooxygenase hydroxylase (MMOH) from Methylococcus capsulatus (Bath) has been solved in two crystal forms, one of which was refined to 1.7 Å resolution. The enzyme is composed of two copies each of three subunits (α₂β₂γ₂), and all three subunits are almost completely α-helical, with the exception of two β hairpin structures in the α subunit. The active site of each α subunit contains one dinuclear iron center, housed in a four-helix bundle. The two iron atoms are octahedrally coordinated by 2 histidine and 4 glutamic acid residues as well as by a bridging hydroxide ion, a terminal water molecule, and at 4°C, a bridging acetate ion, which is replaced at −160°C with a bridging water molecule. Comparison of the results for two crystal forms demonstrates overall conservation and relative orientation of the domain structures. The most prominent structural difference identified between the two crystal forms is in an altered side chain conformation for Leu 110 at the active site cavity. We suggest that this residue serves as one component of a hydrophobic gate controlling access of substrates to and products from the active site. The leucine gate may be responsible for the effect of the B protein component on the reactivity of the reduced hydroxylase with dioxygen. A potential reductase binding site has been assigned based on an analysis of crystal packing in the two forms and corroborated by inhibition studies with a synthetic peptide corresponding to the proposed docking position. Proteins 29:141–152, 1997. © 1997 Wiley-Liss, Inc.