Effect of temperature on the in vitro assembly of bacterial flagella

The temperature dependence for the rate of reconstitution or polymerization (k₊) at neutral pH of the protein, flagellin, to flagella was measured using Ostwald-type viscometers. Similarly, the kinetics for the reverse process, the thermally-induced depolymerization of flagella filaments to the flagellin monomer (k_?) was measured. The temperature at which k_? equals zero was used to define the thermal dissociation temperature or melting point of flagella filaments. The remarkable similarity of melting points obtained (36.8 ± 0.2 deg. C) for flagella isolated from three Salmonella strains (SJ670, SJ25 and SJ30 bearing H-antigen types i, 1.2 and e, n, x, respectively) suggests that the structural stability of these different protein filaments is also similar.On increasing the temperature between 12 and 28°C, k₊ increased smoothly and had a Q₁₀ of 1.8. Above 28.0, k₊ decreased rapidly and fell to zero at a temperature near 37°C, its precise value varying with the bacterial strain. This result supports the prior hypothesis (Gerber &; Noguchi, 1967) that on heating, a reversible co-operative transconformation occurs between different states of the protein; in one state, flagellin (M) can polymerize to flagella, whereas its conformational isomer(s) may do so with difficulty or not at all.For strains SJ25 and SJ30 the rates of polymerization and depolymerization both fall to zero near 37°C. Therefore, mixtures of monomer and flagella fragments (short polymers or “seeds”), in all ratios, appear to be in equilibrium at temperatures near this critical temperature, and neither polymerization of flagellin to flagella nor melting of polymers is apparent.Measurements made on flagella from strain SJ670 showed that k₊ and k_? approached zero at 45 and 37°C, respectively. Within this temperature range the conc entration of monomer in equilibrium with filaments was determined. By a null -point type experiment, solutions of monomer and seed were mixed to find the ratio that showed neither increases (polymerization) nor decreases (depolyme rization) in viscosity with time. An unexpected finding was that the temperature defines a critical monomer concentration, which exists in equilibrium with any concentration of filaments (and not the ratio of monomer-to-filament concentrations). Thus, the polymerization of fiagellin to flagella corresponds to a phase change akin to either crystallization or condensation.Application, of the Clapeyron-Clausius equation to the results obtained yields a heat of condensation of 70 kcal/mol of monomeric protein. The enthalpy change associated with M ? M_i is estimated as 110 kcal/mol of protein. Since the heat content of these various forms of flagella protein lies in the order M_i > F > M, by difference we estimate the enthalpy change for the conversion of monomers to polymers to be 40 kcal/mol of monomer.     

Biosynthesis of polyhydroxyalkanoate copolymers from methanol by Methylobacterium extorquens AM1 and the engineered strains under cobalt-deficient conditions

Methylobacterium extorquens AM1 has been shown to accumulate polyhydroxyalkanoate (PHA) composed solely of (R)-3-hydroxybutyrate (3HB) during methylotrophic growth. The present study demonstrated that the wild-type strain AM1 grown under Co²⁺-deficient conditions accumulated copolyesters of 3HB and a C₅-monomer, (R)-3-hydroxyvalerate (3HV), using methanol as the sole carbon source. The 3HV unit was supposed to be derived from propionyl-CoA, synthesized via the ethylmalonyl-CoA pathway impaired by Co²⁺ limitation. This assumption was strongly supported by the dominant incorporation of the 3HV unit into PHA when a strain lacking propionyl-CoA carboxylase was incubated with methanol. Further genetic engineering of M. extorquens AM1 was employed for the methylotrophic synthesis of PHA copolymers. A recombinant strain of M. extorquens AM1C_Ac in which the original PHA synthase gene phaC _Me had been replaced by phaC _Ac , encoding an enzyme with broad substrate specificity from Aeromonas caviae, produced a PHA terpolymer composed of 3HB, 3HV, and a C₆-monomer, (R)-3-hydroxyhexanoate, from methanol. The cellular content and molecular weight of the PHA accumulated in the strain AM1C_Ac were higher than those of PHA in the wild-type strain. The triple deletion of three PHA depolymerase genes in M. extorquens AM1C_Ac showed no significant effects on growth and PHA biosynthesis properties. Overexpression of the genes encoding β-ketothiolase and NADPH-acetoacetyl-CoA reductase increased the cellular PHA content and 3HV composition in PHA, although the cell growth on methanol was decreased. This study opens up the possibility of producing practical PHA copolymers with methylotrophic bacteria using methanol as a feedstock.     

Facile one-pot mono-dealkylation of H-phosphonate esters in high yield

The readily available dialkyl H-phosphonates (RO)₂P(O)H, where R = Me, Et, ⁱPr, ⁿBu and Bn can be mono-dealkylated by heating at reflux in excess tert-butylamine. Where R = Me or Bn then crystals of [(CH₃)₃CNH₃]⁺ [ROP(O)HO]⁻ form in the reaction vessel on overnight standing.     

Methyl- and propylacetamidinates of lanthanides: Structures, catalytic and some physical properties

The acetamidinates {[MeNC(Me)NMe]₂Ln}₂[μ-η²-η²-MeNC(Me)NMe]₂ (Ln = Y (1), Dy (2)) and {[PrⁿNC(Me)NPrⁿ]₂Y}₂[μ-η²-η²-PrⁿNC(Me)NPrⁿ]₂ (3) have been prepared by the reactions of amides Ln[N(SiMe₃)₂]₃ with respective N,N′-disubstituted amidines MeNC(Me)NHMe or PrⁿNC(Me)NHPrⁿ. The reaction of Er[N(SiMe₃)₂]₃ with excess of monosubstituted amidine HNC(Me)NHPrⁱ or in a ratio of 1:2 resulted in the formation of compound {Er[NC(Me)NHPrⁱ]₃}_x (4). The same reaction with 1:1 ratio yielded heteroleptic complex {Er[N(SiMe₃)₂]₂[NC(Me)NHPrⁱ]}_x (5). The complexes 1, 2 and 3 have similar structures and contain four terminal and two μ-η²:η²-N,N-bridging amidinate groups binding the metal atoms. Volatility of 1, 2 and 3 is comparable to that of known monomeric La[PrⁱNC(R)NPrⁱ]₃. Compound 1 efficiently catalyzes the ring-opening polymerization of rac-lactide to give polylactide with M_n 53 085 and polydispersity 1.84.     

Ligand exchange and abstraction reactions involving titanium isopropoxide with tris(pentafluorophenyl)borane and -alane: ramifications for ring-opening polymerization of propylene oxide

In situ mixing of Ti(OⁱPr)₄ and B(C₆F₅)₃ generates a very efficient catalyst system for ring-opening polymerization (ROP) of propylene oxide (PO) with a turnover frequency (TOF) >1350/h, whereas the mixture of Ti(OⁱPr)₄ and Al(C₆F₅)₃ is inactive for the same polymerization. The inactivity of the Ti(OⁱPr)₄/Al(C₆F₅)₃ mixture is due to the formation of the stable isopropoxy-bridged bimetallic species Ti(OⁱPr)₃(μ-OⁱPr)Al(C₆F₅)₃ (1), the structure of which has been confirmed by X-ray diffraction. The products of the Ti(OⁱPr)₄ + B(C₆F₅)₃ reaction, however, depend on the Ti(OⁱPr)₄:B(C₆F₅)₃ ratio. The 1:1 ratio reaction in toluene at ambient temperature is rapid and produces the ligand exchange products: Ti(OⁱPr)₃C₆F₅ and ⁱPrOB(C₆F₅)₂ (2), along with a small amount of (ⁱPrO)₂BC₆F₅. The two resulting boranes are inseparable by recrystallization or vacuum distillation, and the formation of the undesired (ⁱPrO)₂BC₆F₅ is either significantly enhanced upon heating the reaction in toluene to 80 °C or nearly exclusive by carrying out the reaction in THF. By employing 1.2 equiv. of B(C₆F₅)₃ in the reaction with Ti(OⁱPr)₄, however, the formation of (ⁱPrO)₂BC₆F₅ is suppressed, enabling the isolation of the new borane 2 in its pure state. The excess of B(C₆F₅)₃ added to the reaction apparently slows down the exchange reaction by stabilizing the intermediate Ti(OⁱPr)₃(μ-OⁱPr)B(C₆F₅)₃ (4), as shown by the 1:2 Ti(OⁱPr)₄:B(C₆F₅)₃ reaction which initially forms the ligand abstraction product 4 followed by subsequent slow ligand exchange to give the final products Ti(OⁱPr)₃C₆F₅ and 2. The studies of these individual reactions, in combination with control polymerization runs, reveal that the active species responsible for the catalytic activity of the Ti(OⁱPr)₄/B(C₆F₅)₃ mixture is the isopropoxy borane 2. Thus, the isolated 2, in the absence or presence of a hydroxylic initiator, serves as a very effective catalyst for the ROP of PO, producing PPOs with M_n=2000-3000, M_w/M_n=1.30-1.43, and TOF >1400/h. The MALDI-TOF MS analyses of the PPOs formed show the linear PPO structures having the initiator and water molecules as end groups, demonstrating the control over the PPO structure.     

Light microscope study of mixed helices in reconstituted Salmonella flagella

In this morphological study of bacterial flagella, single flagellar filaments in solutions were photographed by dark-field light microscopy to determine parameters to describe their intact shapes. First, I measured pitches of helices I, II and III assumed by copolymers of flagellins from Salmonella strains SJ25 and SJ814 (Asakura & Iino, 1972), and combined the data with electron microscopic data of the contour length per period of each filament to calculate the pitch angles of the three helices. (The pitch angle is the angle between a tangent to the filament and the helical axis.)Secondly, filaments which consisted of two blocks assuming different helices were prepared by two-step copolymerization of SJ25 and SJ814 flagellins and the configurations of these mixed-type filaments were examined. In filaments of any mixed type, the axes of the constituent blocks were oriented at the same angle, called the “block angle” Ψ. This angle was found to be approximated by Ψ = 180 ° ? ¦θ₁ ? θ₂¦, where θ₁ and θ₂ are the pitch angles of the mixed helices. On the basis of this finding, the morphology of mixed-type filaments is discussed.     

Synthesis and structural characterization of mixed-ligand silver(I) complexes containing diphosphazanes and bidentate N-donor ligands

The reactions of a self-assembled silver(I) coordination polymer, [Ag₂{μ-PrⁱN(PPh₂)₂}(μ-NO₃)₂]_n (1) with various bidentate N-donor ligands such as DABCO, 2,2′-bipyridyl and 1,10-phenanthroline yield 1-D helices or π-π stacked polymers, depending on the chelate vector of the N-donor ligand. The molecular structures of the resultant complexes, [Ag₂{μ-PrⁱN(PPh₂)₂}(DABCO)(NO₃)₂]_n (2), [Ag₂{μ-PrⁱN(PPh₂)₂}(2,2′-bipy)₂(NO₃)₂] (3) and [Ag₂{μ-PrⁱN(PPh₂)₂}(1,10-phen)₂](NO₃)₂ (4) have been confirmed by single-crystal X-ray diffraction. Complex 2 exists as an infinite helical polymer because of the exo-bidentate nature of DABCO. Complex 3 assumes a 2D grid motif as a result of intermolecular π-π stacking among adjacent bipyridine moieties. The phenanthroline complex 4 exhibits strong inter- and intramolecular π-π stacking interactions.     

Rhodium and iridium complexes bearing an alkyloxazoline-substituted indenyl ligand (Indox ligand) and their stereoselective construction of metal-centered chirality

The Indox ligands, [{(S)-(ⁱPr)Indox}_n]H (1) [n=2 (a), 3 (b)] and [{(H)Indox}_n=3]H (2), in which an indenyl group and an oxazoline ring are connected by an ethylene or propylene spacer, have been prepared. Reaction of [Ir(coe)₂Cl]₂ or [RhCl(C₂H₄)₂]₂ with the potassium salt of 1 afforded η⁵-[{(S)-(ⁱPr)Indox}_n]Ir(coe)₂ (3) or η⁵-[{(S)-(ⁱPr)Indox}_n]Rh(C₂H₄)₂ (6) as a 1:1 mixture of two diastereomers. The oxazoline ring in 3 and 6 did not coordinate to the metal center. When the complexes 3 or 6 reacted with iodine in diethyl ether, oxidative addition proceeded and the oxazoline ring coordinated to the metal center to give diiodoiridium(III) or rhodium(III) complexes, η⁵:η¹-[{(S)-(ⁱPr)Indox}_n]M(I)₂ [M=Ir (4), Rh (7)]. The corresponding diiodoiridium(III) complex bearing the Indox ligand 2, η⁵:η¹-[{(H)Indox}_n=2]Ir(I)₂ (5), was also prepared by a similar method. Reaction of 4 or 7 with PPh₃ in THF afforded diiodo-phosphine complexes, η⁵-[{(S)-(ⁱPr)Indox}_n]M(PPh₃)(I)₂ [M=Ir (8), Rh (9)] as a 1:1 mixture of two diastereomers in which the oxazoline ring dissociated from the metal center. The related reaction of 8 or 9 with more than 2 equiv. of AgOTf afforded the cationic complexes, [η⁵:η¹-[{(S)-(ⁱPr)Indox}_n]M(PPh₃)(OTf)]OTf [M=Ir (10), Rh (11)], having a stereogenic center at the metal center as a mixture of only two diastereomers. From ¹H and ³¹P NMR analyses, each diastereomer of 8 or 9 afforded only a single isomer of 10 or 11. The corresponding iridium(III) complex bearing the Indox ligand 2, [η⁵:η¹-[{(H)Indox}_n=3]Ir(PPh₃)(OTf)]OTf (12) was also prepared. The coordinated triflate ligand of 12 was slowly replaced by water in CDCl₃ to afford the dicationic aquo complex, (S^*_pl,R^*_Ir)-[η⁵:η¹-[{(H)Indox}_n=3]Ir(PPh₃)(H₂O)](OTf)₂ (13). The monocationic complex, [η⁵:η¹-[{(S)-(ⁱPr)Indox}_n=2]Ir(PPh₃)(I)]OTf (14a), having metal-centered chirality, was observed as a mixture of only two diastereomers in the reaction of 10a (a mixture of two diastereomers) with 1 equiv. of AgOTf. These observations indicated that the ligand exchange reaction of 8 or 9 with AgOTf contained the following three steps: (i) abstraction of one of the two prochiral iododes by AgOTf, (ii) recoordination of the oxazoline ring, and (iii) exchange of the remaining iodide for the triflate by AgOTf. The stereochemistry around the metal center was determined at the second step. All complexes have been characterized by usual spectroscopic methods as well as elemental analyses, and 4 and 13 have been characterized by X-ray analyses.     

Structural characterization of zirconium isopropoxide precursors modified by di- and trichloroacetic acids

Reactions of Zr(OⁱPr)₄(PrⁱOH) with di- and trichloroacetic acid in 1:1 molar ratio in toluene gave the products Zr₂(μ-OⁱPr)₂(μ-OOCCHCl₂)(OⁱPr)₄(OOCCHCl₂)(HOⁱPr) (1) and Zr₂(μ-OⁱPr)₂(μOOCCCl₃)(OⁱPr)₄(OOCCCl₃)(HOⁱPr) (2), respectively, in quantitative yields. The molecular geometry of both (1) and (2) is constituted by a slightly distorted edge-shared bioctahedron and both have almost similar bond dimensions. Addition of dichloroacetic acid in 1:2 molar ratio to Zr(OⁱPr)₄(HOⁱPr) in toluene although yielded the bis-substituted crude product Zr(OⁱPr)₂(OOCCHCl₂)₂(HOⁱPr) (3) but its solution in toluene left for crystallization formed a tri-nuclear oxo product Zr₃(μ₃-O)(μ-OⁱPr)₂(μ-OOCCHCl₂)₃(η-OOCCHCl₂)₂(OⁱPr)₃ (3a). The three zirconium atoms in the structure of (3) are forming an isosceles triangle with a triply bridged oxo moiety at its center.     

Activation of the Si-Si σ-bond of 1,2-divinyldisilane catalyzed by Ru(II) complexes

In the presence of a catalytic amount of RuHCl(CO)(PR₃)_n (R=ⁱPr, n=2; R=Ph, n=3), 1,1,2,2-tetramethyl-1,2-divinyldisilane (1) undergoes unexpected and clean isomerization via the Si-Si bond cleavage to yield a mixture of 6- and 5-membered cyclic compounds, 1,1,4,4-tetramethyl-1,4-disilacyclohex-2-ene and 1,1,2,3,3,-pentamethyl-1,3-disilacyclopent-4-ene, the former being the major product.     

Mapping Intercellular CO2 Mole Fraction (Ci) in Rosa rubiginosa Leaves Fed with Abscisic Acid by Using Chlorophyll Fluorescence Imaging : Significance of Ci Estimated from Leaf Gas Exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO₂ assimilation (A_n) and the stomatal water vapor conductance (g_s). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O₂) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO₂ concentration (7400 μmol mol⁻¹) in the leaf atmosphere. It was concluded that ABA primarily affected A_n by decreasing the CO₂ supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the A_n versus intercellular mole fraction (C_i) relationship was assumed not to be affected by ABA, and images of C_i and g_s were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of g_s remained unimodal following ABA treatment. A comparison of calculations of C_i from images and gas exchange in ABA-treated leaves showed that the overestimation of C_i estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO₂, when leaf conductance to water is low.     

Intracellular pH Increase Driven by an Na+/H+ Exchanger upon Activation of Surf Clam Oocytes

Intracellular pH (pH_i) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pH_iof 0.3 pH unit after activation by excess K⁺. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pH_iwas increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K⁺induced GVBD but the pH_iof K⁺-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pH_iincreases in K⁺-activated oocytes were otherwise proportional to the external Na⁺concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10–50 μM) efficiently inhibited the K⁺-induced increase of pH_ibut did not block GVBD. These two derivatives were able, however, to retard K⁺-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K⁺-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pH_iincrease, driven by a typical Na⁺/H⁺exchanger, follows activation of surf clam oocytes but that this pH_iincrease is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.     

Inhibition of plant phytases by phloroglucinol

Plant phytases (myoinositol hexaphosphate phosphohydrolase, EC 3.1.3.8) were inhibited by phloroglucinol (1,3,5-benzenetriol) in vitro. The inhibition of the Cucurbita maxima phytase was found to be non-competitive and pH dependent with an apparent inhibition constant (K_i) value of 2.3 × 10^?1 M at optimum pH (4.8) and temperature (50°). The apparent number of inhibitor molecules (n) bound per enzyme molecule was found to be 2.2 suggesting that the positive cooperativity phenomenon may be present.     

Triangular matrix representation of dimensionless helical hydrophobic moment ratios

If the hydrophobicity of the ith residue in an α-helix is denoted by H_i, one can (following Eisenberg) define a corresponding vector, h_i = a_iH_i. Here a_i is a unit vector directed radially from the helix axis through C_i^α. The vector sum of the h_i for the N residues in an α-helix is denoted by h, and h² is the square of the magnitude of h. In the absence of any correlation in the h_i we obtain h² = ∑ H_i², where the sum extends over all N residues in the helix. The dimensionless ratio h²/∑ H_i² has the value 1 when the h_i are uncorrected. However, if hydrophobic residues occur on the opposite surface of a helix from the hydrophilic residues, h²/∑ H_i² > 1. The h²/∑ H_i² for the n(n − 1)/2 helices that might occur in a chain of n residues are conveniently displayed as the elements in a symmetric n × n matrix. The jkth and kjth elements are h²/∑ H_i² for the helix that initiates at residue j and terminates at residue k. All of the h²/∑ H_i² are retained in either of the triangular matrices that together together constitute the symmetric matrix. The largest element in the triangular matrix for glucagon is the entry for a helix that is nearly identical with the major helix found by Wüthrich when the peptide is bound to a zwitterionic micelle. Three related hormonal peptides have their maximum h²/∑ H_i² in the same region of their respectively triangular matrices. The triangular matrix for α-tropomyosin shows that the h²/∑ H_i² are larger in the portion of the chain that forms the more stable helix in the in-register dimer.     

d-Xylose,an anomer-specific inhibitor of α-galactosidase

d-xylose is a highly anomer-specific, powerful and competitive inhibitor of plant α-galactosidases. The apparent inhibition constant (K_i) for the interaction of d-xylose with chick pea (Cicer arietinum) α-galactosidase and the apparent number of inhibitor molecules (n) bound per enzyme molecule, using p-nitrophenyl-α-d-galactopyranoside as substrate, were found to be 0.4 × 10^?2M and 0.8, respectively.     

Synthesis and physico-chemical studies of bimetallic alkoxy derivatives of chromium(III) with niobium(V) and tantalum(V)

Reactions of CrCl₃·3thf with KM(OPrⁱ)₆ in 1:3 molar ratios in benzene yield soluble complexes of the type Cr[M(OPrⁱ)₆]₃ (M=Nb or Ta). On heating under vacuum, these complexes tend to disproportionate into Cr(OPrⁱ)₃ and M(OPrⁱ)₅ (M=Nb or Ta). A number of bimetallic alkoxides have also been synthesized by the alcoholysis of Cr[M(OPrⁱ)₆]₃ with alcohols (methanol, ethanol and t-amyl alcohol). The IR, visible, electron spin resonance and magnetic properties of these newly synthesized complexes throw light on the structural features.     

Purification and properties of esterase from Bacillus stearothermophilus

An enzyme, which hydrolyzes p-nitrophenyl and m-carboxyphenyl esters of n-fatty acids, is purified from Bacillus stearothermophilus. The enzyme reaction obeys the Michaelis-Menten theory. The Michaelis constant (K_m) decreases with increasing the length of carbon number of the acids, but the maximum velocity (V) is maximum for n-caproate. The enzyme is inhibited by diisopropyl fluorophosphate (DFP),² and 1 mole of DFP reacts with 1 mole of the enzyme of the molecular weight of 42,000–47,000. The enzyme is considered to be carboxylic ester hydrolase (EC 3.1.1.1). The effects of temperature on K_m or V for p-nitrophenyl n-caproate and on the inhibitor constant (K_i) for n-laurate suggest a thermal transition in the conformation of the enzyme protein at 55 °C. The enzyme is strongly inhibited by sulfhydryl reagents such as p-chloromercuribenzoate and 5,5′-dithiobis (2-nitrobenzoic acid) at 65 °C, but less at 30 °C. The relationship between the inhibition of the activity by p-chloromercuribenzoate and temperature may suggest that a thermal transition of the enzyme protein accompanies some structural change around sulfhydryl group.     

Yeast inorganic pyrophosphatase substrate recognition

Monodentate Co(NH₃)₅PP_i was determined not to be a substrate for yeast inorganic pyrophosphatase while P¹,P²-bidentate Co(NH₃)₄PP_i was turned over by the enzyme at a rate of 7.5 min^?1. A kinetic analysis of the substrate activities of the P¹,P²-bidentate complexes, Co(en)₂PP_i, Cr(NH₃)₄PP_i, Cr(H₂O)(NH₃)₃PP_i, Cr(H₂O)₂(NH₃)₂PP_i, and Cr(H₂O)₄PP_i was carried out in order to access the potential role of the metal-water ligands in productive binding. While substitution of the H₂O ligands with NH₃ ligands had a minimal affect on the K_m for Mg²⁺, the binding affinity of the complexes decreased with an increasing $\text{NH}{}_3\text{H}{}_2\text{O}$ ligand ratio as did the turnover number of the corresponding central complexes. The Co(en)₂PP_i complex was hydrolyzed at a rate approximately 0.6% of that for the Co(NH₃)₄PP_i complex. The substrate activities of β,γ-bidentate Co(NH₃)₄PPP_i and α,β,γ-tridentate Co(NH₃)₃PPP with pyrophosphatase were also tested. While both complexes were shown to bind tightly to the Mg²⁺-activated enzyme neither was hydrolyzed. On the other hand, in the presence of the Zn²⁺-activated enzyme the tridentate complex was turned over at a rate of 0.17 min^?1 while the bidentate complex remained inert to hydrolysis.     

A linear potassium metalated vanadium(IV) polymeric alkoxide: Structural and spectroscopic studies

We have been interested in the preparation of alkoxometallate “bricks” of the M′{M(OR)_n} type, with M′ = alkaline metal, M = vanadium(IV) and R = isopropyl or tert-butyl, to be employed as starting materials in salt-elimination reactions with other transition metal complexes. Here, we report the synthesis, spectroscopic characterisation and molecular structure of [{K₂(VO)₂(OPrⁱ)₆(PrⁱOH)₂}_∞] (1), prepared by a combination of Lewis acid–base and micro-hydrolysis reactions. The linear polymeric chain contains planar four-membered {(VO)₂(μ-OPrⁱ)₂} rings with vanadyl groups in anti-coplanar configuration; the rings are connected by “bridging” K(HOPrⁱ)⁺ units. Powder and solution EPR spectra suggest a spin triplet ground state, with a very weak ferromagnetic interaction between the vanadium(IV) centres at room temperature.     

Fluctuations of an alpha-helix.

Fluctuations in backbone dihedral angles in the α-helical conformation of homopolypeptides are studied based on an assumption that the conformational energy function of a polypeptide consisting of n amino-acid residues can be approximated by a 2n-dimensional parabola around the minimum point in the range of fluctuations. A formula is derived that relates 〈Δθ_iΔθ_j〉, the mean value of the product of deviations of dihedral angles ?_i and ψ_i (collectively designated by θ_i) from their energy minimum values, with a matrix inverse to the second derivative matrix F ,_n of the conformational energy function at the minimum point. A method of calculating the inverse matrix F _n^?1 explicitly is given. The method is applied to calculating 〈Δθ_iΔθ_j〉 for the α-helices of poly(L -alanine) and polyglycine. The autocorrelations 〈(Δ?_i)²〉 and 〈(Δψ_i)²〉 at 300°K are found to be about 66 deg² and 49 deg², respectively, for poly(L -alanine), and 84 deg² and 116 deg², respectively, for polyglycine. The length of correlations of fluctuations along the chain is found for both polypeptides to be about eight residues long.