Antico-operative binding of initiator transfer RNAMet to methionyl-transfer RNA synthetase from Escherichia coli: neutron scattering studies.

The interaction of methionyl-tRNA synthetase with initiator tRNA^Met has been investigated by neutron scattering. On the basis of parallel fluorescence measurements, two types of titrations have been performed. (1) In the presence of 10 mm-MgCl₂, a condition which insures antico-operative binding of two tRNA molecules to the enzyme dimer. (2) With saturating amounts of 5′-AMP and l-methioninol, in the presence of 50 mm-MgCl₂, conditions which allow two transfer RNA molecules to bind the dimer with very similar affinities.Varying the solvent density (²H₂O fraction) in the samples has allowed the identification by neutron scattering of changes in the radius of gyration and in the degree of dissociation of the enzyme dimer upon tRNA binding. In buffer containing 10 mm-MgCl₂, at each contrast studied, the binding process involves two steps. Firstly, one tRNA^met_f molecule binds easily to one dimeric enzyme molecule with an associated decrease of the radius of gyration of the enzyme moiety. The centre of mass of this tRNA lies very close to the centre of mass of the protomer with which it associates. Then, at higher tRNA concentration, a second tRNA molecule binds to the enzyme. However, the affinity of this second site is very much weaker. With the binding of the second tRNA, the radius of gyration of the enzyme moiety increases markedly. Concomitant limited dissociation of the dimer is suggested by the experimental data. These observations combined with the fact that, in 50 mm-MgCl₂ both the increased radius of gyration and the partial dissociation of the enzyme are accomplished in the absence of tRNA and remain unaffected upon binding one or two tRNA, confirm that the hindrance to binding a second tRNA in 10 mm-MgCl₂ arises from the constrained conformation of the one tRNA-enzyme complex.     

Sizes of two amino acyl-tRNA synthetase complexes from E. coli with their cognate tRNAs.

The complexes of valyl-tRNA synthetase with tRNA_I^Val and arginyl-tRNA synthetase with tRNA_II^Arg from $\text{E. coli}$ were studied by light scattering measurements and analytical ultracentrifugation of concentrations as low as 40 μg/ml. The molecular weights determined from these studies were 260,000 ± 2,000 for the valyl-tRNA synthetase·tRNA complex, and 310,000 ± 1,500 for the arginyl-tRNA synthetase·tRNA complex at pH 7.1. The stoichiometry for the complexes are apparently 2:1 for valyl-tRNA synthetase and tRNA and 4:1 in the case of the arginyl-tRNA synthetase and tRNA. From the angular dependence of the scattered intensity a radius of gyration of 54.5 Å for the complex between valyl-tRNA synthetase and tRNA was found, whereas for the other complex a value of 59.1 Å was found.     

Laser-light scattering investigation of the micellar properties of gangliosides

The micellar properties of gangliosides in water solutions were investigated by quasielastic light scattering measurements. G_M1 and G_D1a gangliosides were isolated from calf brain, purified to more than 99% and dissolved in 0.025 M Tris—HCI buffer (pH 6.8) at 37°C. The average intensity of scattered light and the intensity correlation function were measured by an apparatus including a 5145 Å argon laser and a real-time digital correlator. The scattered intensity data allowed the derivation of an upper limit to the critical micelle concentration (c₀) and the evaluation of the molecular weight (M) of the micelle. The intensity correlation function gave the diffusion coefficient D, and hence the hydrodynamic radius R_H, and also contained information on the polydispersity of the sample. We find c_o < 1 × 10^?6 M for both G_M1 and G_D1a, M = 532 000 ± 50 000 and $\text{R}{}_{\mathrm{<mtext>H</mtext>}}\text{= 63.9 ± 2}\text{A}\text{?}$ for G_M1, and M = 417 000 ± 40 000 and $\text{R}{}_{\mathrm{<mtext>H</mtext>}}\text{= 59.5 ± 2}\text{A}\text{?}$ for G_D1a. The mixture 3:1 of the two gangliosides gave intermediate values for all examined parameters. The presence of cations, within the physiological concentration range. and, in particular of Ca²⁺, did not influence significantly the values of c_o and the main features of the micelle.     

Physical mapping of the transfer RNA genes on lambda-h80dglytsu+36

The three Escherichia coli transfer RNA genes of the DNA of the transducing phage λ80cI857S^?t68dglyTsu⁺₃₆tyrTthrT (abbreviated λh80T), which specify the structures of tRNA^Gly₂(su⁺₃₆), tRNA^Tyr₂ and tRNA^Thr₃, have been mapped by hybridizing ferritin-labeled E. coli tRNA to heteroduplexes of λh80T DNA with the DNA of the parental phage (λh80cI857S^?t68) and examining the product in the electron microscope. The DNA of λh80T contains a piece of bacterial DNA of length 0·43 λ unit³ that replaces a piece of phage DNA of length 0·46 λ unit, proceeding left from B · P′ (the junction of bacterial DNA and phage DNA) (i.e. att⁸⁰). A cluster of three ferritin binding sites, and thus of tRNA genes, is seen at a position of 0·24 λ unit (1·1 × 10⁴ nucleotides) to the left of B· P′. The three tRNA genes of the cluster are separated by the unequal spacings of 260 (±30) and 140 (± 30) nucleotides, proceeding left from B·P′. The specific map positions have been identified by hybridization competition between ferritin-labeled whole E. coli tRNA with unlabeled purified tRNA^Tyr₂ and with unlabeled partially purified tRNA^Gly₂. The central gene of the cluster is tRNA^Tyr₂. The tRNA^Gly₂gene is probably the one furthest from B·P′. Thus, the gene order and spacings, proceeding left from B·P′, are: tRNA^Thr₃, 260 nucleotides, tRNA^Try₂, 140 nucleotides, tRNA^Gly₂.     

Adenosine A1 Agonists and the Ca2+ Channel Agonist Bay K 8644 Produce a Synergistic Stimulation of the GTPase Activity of Go in Rat Frontal Cortical Membranes

Abstract: The identity and role of G proteins in coupling adenosine receptors to effectors have been studied to a limited degree. We have identified the G proteins whose GTPase activity is stimulated by adenosine receptor agonists in neuronal membranes. (R)-Phenylisopropyladenosine, 2-chloroadenosine, and N-ethylcarboxamideadenosine produced a concentration-dependent stimulation of GTPase. At 10^?5M, the increase above basal GTPase in frontal cortex was 25 ± 4, 20 ± 3, and 8 ± 1%, respectively, and in the cerebellum 55 ± 2, 41 ± 4, and 22 ± 2%, respectively. The effects of (R)-phenylisopropyladenosine and 2-chloroadenosine were inhibited by (1) A₁ antagonists (76–96% reduction), (2) pretreatment with pertussis toxin (90–100% reduction), and (3) antibodies raised against the α-subunit of G_i and G_o (55–57% reduction by each), suggesting that A₁ receptors interact equally with G_i and G_o. (R)-Phenylisopropyladenosine increased the binding of a nonhydrolyzable analogue of GTP to membranes in a pertussis toxin-sensitive manner, indicative of activation of G_i or G_o. Previously, (±)-Bay K 8644 enhanced GTP hydrolysis by G_o but not G_i. Now we report a profound synergistic stimulation of GTPase in the presence of (R)-phenylisopropyladenosine and (±)-Bay K 8644 (10^?7 to 10^?5M). (±)-Bay K 8644 had no effect on nucleotide exchange and, thus, cannot activate G_o. It appears that a positive cooperative stimulation of G_o occurs when it is first activated by A₁ receptors and subsequently interacts with the L-type Ca²⁺ channel.     

Purification and characterization of a new ribopolynucleotide synthesizing enzyme from Escherichia coli.

A new type of ribopolynucleotide-synthesizing enzyme was found both on cytoplasmic membranes and in protein-DNA complexes isolated from Escherichia coli. The enzyme was purified by exploiting a specific, reversible enzyme aggregation with ATP and spermidine. The purified enzyme (more than 90% pure) was free from other enzymatic activities such as ATPase and polynucleotide phosphorylase. The enzyme (molecular weight 270,000 ± 15%) contains two kinds of polypeptide chain (molecular weights 91,000 ± 10%, and 45,000 ± 10%) and these polypeptides are not common with those of DNA-dependent RNA polymerase. The enzyme catalyses the synthesis of ribopolynucleotides from nucleoside triphosphates in the presence of 1 mm-MgCl₂. Rifampicin and streptolydigin do not affect the enzyme reaction.     

Activation parameters for directly determined first order rate constants for outer sphere electron transfer reactions,and crystal structure of a pertinent pentaammine of CoIII

The outer sphere reductions of Co(NH₃)₅B³⁺ by Fe(CN)₅A³⁻ have been studied. The observed pseudo first order rate constants (Co complex in excess) obey the dependence k_obs=K_osk_et[Co]/(1 +K_os[Co]), as expected for outer sphere electron transfer reactions. Values of the fundamental electron transfer rate constants k_et have been determined, along with the equilibrium constant K_os for a range of reactions in which A and B are pyridyl ligands of different sizes. The first order electron transfer rate constants vary in a manner that is consistcnt with adiabatic electron transfer. The outer sphere ion pairing equilibrium constants K_os have been calculated: K_os=8.6 ± 0.1 × 10² M⁻¹ when A and B=pyridine; K_os=1.07 ± 0.09 × 10³ M⁻¹ where A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; K_os=1.86 ± 0.11 × 10³ M⁻¹ when A=4,4′-bipyridine, B=pyridine; K_os=1.27 ± 0.08 × 10³ M⁻¹ when A=4,4′-bipyridine, B=4-phenylpyridine. Distances of closest approach between the metal centers in the reactive ion pairs are compared, and it is concluded that there is a common mechanism, in which the ammonia side of the cobalt complex approaches the cyano side of the iron complex in each reactive ion pair.The distance of closest approach between the two metal centers (a) was calculated from the experimental values for the ion pairing equilibrium constant K_os at 25 °C: 5.2 Å when A=4,4′-bipyridine, B=pyridine; 5.4 Å when A=4,4′-bipyridine, B=4-phenylpyridine; 5.5 Å when A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; 5.7 Å when A=B=pyridine. These relatively short metal-metal distances, when compared to the X-ray structure of the compound [Co(NH₃)₅(4-phenylpyridine)]₂[S₂O₆]₃· 4H₂O, do not support an ion pair orientation in which the two substituted pyridine ligands A and B are oriented toward each other. [P2₁/c,a=7.399(3), b=22.355(10), c=13.776(4) Å, β=92.02(3)°, R=0.070.] The crystallographic results show that if the two pseudo-octahedral coordination spheres are oriented in the reactive ion pair so that an ammonia face of the cobalt complex is at hydrogen bonding distance from a cyano face on the iron complex, the metal-metal distance is 5.3 Å, a distance which is in agreement with the kinetic results.     

Affinity labelling of tryptophanyl-transfer RNA synthetase

A double affinity-labelling approach has been developed in order to convert an oligomeric enzyme with multiple active centres into a single-site enzyme.Tryptophanyl-transfer RNA synthetase (EC 6.1.1.2) from beef pancreas is a symmetric dimer, α₂ An ATP analogue, γ-(p-azidoanilide)-ATP does not serve as a substrate for enzymatic aminoacylation of tRNA^Trp but acts as an effective competitive inhibitor in the absence of photochemical reaction, with K₁ = 1 × 10^?3m (K_mfor ATP = 2 × 10^?4m). The covalent photoaddition of azido-ATP³ results in complete loss of enzymatic activity in both the ATP-[³²P]pyrophosphate exchange reaction and tRNA aminoacylation. ATP completely protects the enzyme against inactivation. However, covalent binding of azido-ATP is also observed outside the active centres. The difference between covalent binding of the azido-ATP in the absence and presence of ATP corresponds to 2 moles of the ATP analogue per mole of the enzyme.Two binding sites for tRNA^Trp have been found from complex formation at pH 5.8 in the presence of Mg²⁺. The two tRNA molecules bind, with K_dis = 3.6 × 10^?8m and K_dis = 0.9 × 10^?6m, respectively, pointing to a strong negative co-operativity between the binding sites for tRNA.N-chlorambucilyl-tryptophanyl-tRNA^Trp and TRSase form a complex with K_dis = 5.5 × 10^?8m at pH 5.8 in the presence of 10 mm-Mg²⁺. This value is similar to the value of K_dis for tryptophanyl-tRNA of 4.8 × 10^?8m. Under the same conditions a 1:1 complex (in mol) is formed between the enzyme and Trp-tRNA or N-chlorambucilyl-Trp-tRNA. On incubation, a covalent bond is formed between N-chlorambucilyl-Trp-tRNA and TRSase; 1 mole of affinity reagent alkylates 1 mole of enzyme independently of the concentration of the modifier. The alkylation reaction is completely inhibited by the presence of tRNA^Trp whereas the tRNA devoid of tRNA^Trp does not affect the rate of alkylation. In the presence of either ATP or tryptophan, or a mixture of the two, the alkylation reaction is inhibited even though these ligands have no effect on the complex formation between TRSase and the tRNA analogue. Photoaddition of the azido-ATP completely prevents the reaction of the enzyme with the tRNA analogue, although the non-covalent complex formation is not affected.Exhaustive alkylation of TRSase partially inhibits the reaction of ATP [³²P]pyrophosphate exchange and completely blocks the aminoacylation of tRNA^Trp. Cleavage of the tRNA which is covalently bound to TRSase restores both the ATP-[³²P]pyrophosphate exchange and aminoacylation activity.The TRSase which is covalently-bound to R-Trp-tRNA is able to incorporate only one ATP molecule per dimeric enzyme into the active centre. This doubly modified enzyme is completely enzymatically inactive. Removal of the tRNA residue from the doubly modified enzyme results in the formation of the derivative with one blocked ATP site. Therefore, a “single-site” TRSase may be generated either by alkylation of the enzyme with Cl-R-Trp-tRNA or after the removal of covalently bound tRNA from the doubly labelled protein.Tryptophanyl-tRNA synthetase containing blocked ATP and/or tRNA binding site(s) seems to bo a useful tool for investigation of negative co-operativity and may help in the elucidation of the structure function relationships between the active centres.     

Interactions of yeast valyl-tRNA synthetase with RNAs and conformational changes of the enzyme.

Different conformations have been identified for the enzyme valyl-tRNA synthetase from yeast inside its complex with one tRNA molecule by neutron scattering. One form is identical to that of the free enzyme in solution; the other form is more contracted, having a radius of gyration which is smaller by 10% and a specific volume which is smaller by 1%. The contracted conformation has been found for the complexes with tRNA^Val and tRNA^Asp in phosphate buffer (pH 6.3) provided the ionic strength is lower than about 150 mm. In higher ionic strength (up to about 500 mm) the enzyme still forms a complex with tRNA^Val but its conformation remains that of the free protein in solution. In the complex with tRNA₃^Leu, the enzyme conformation is that of the free state even at the lowest ionic strength examined (that of the phosphate buffer, 60 mm). The free enzyme is an elongated molecule of radius of gyration 40 Å (a compact protein of the same molecular weight would have a radius of gyration of 30 Å).The positioning within the complex of tRNA^Val, on the one hand, and tRNA₃^Leu, on the other, is very different. The first tRNA is intimately associated with the enzyme, lying predominantly closer to the centre of mass of the complex than the protein. In the complex with tRNA₃^Leu, the tRNA lies further away from the centre of mass of the complex than the protein.Small concentrations of tRNA^Val, tRNA^Asp, tRNA₃^Leu or Escherichia coli 5 S ribosomal RNA cause the enzyme to aggregate into dimers, trimers and higher aggregates provided the ionic strength of the buffer is below 150 mm. In higher ionic strength or for [RNA]: [enzyme] > 1 the aggregates are dissociated to yield the one-to-one RNA-enzyme complex.     

The human tRNA(m(2)(2)G(26))dimethyltransferase: functional expression and characterization of a cloned hTRM1 gene

This paper presents the first example of a complete gene sequence coding for and expressing a biologically functional human tRNA methyltransferase: the hTRM1 gene product tRNA(m²₂G)dimethyltransferase. We isolated a human cDNA (1980 bp) made from placental mRNA coding for the full-length (659 amino acids) human TRM1 polypeptide. The sequence was fairly similar to Saccharomyces cerevisiae Trm1p, to Caenorhabditis elegans TRM1p and to open reading frames (ORFs) found in mouse and a plant (Arabidopsis thaliana) DNA. The human TRM1 gene was expressed at low temperature in Escherichia coli as a functional recombinant protein, able to catalyze the formation of dimethylguanosine in E.coli tRNA in vivo. It targeted solely position G₂₆ in T7 transcribed spliced and unspliced human tRNA^Tyr in vitro and in yeast trm1 mutant tRNA. Thus, the human TRM1 protein is a tRNA(m²₂G₂₆)dimethyltransferase. Compared with yeast Trm1p, hTRM1p has a C-terminal protrusion of ~90 amino acids which shows similarities to a mouse protein related to RNA splicing. A deletion of these 90 C-terminal amino acids left the modification activity in vitro intact. Among point mutations in the hTRM1 gene, only those located in conserved regions of hTRM1p completely eliminated modification activity.     

UAG readthrough during TMV RNA translation: isolation and sequence of two tRNAs with suppressor activity from tobacco plants

The hypothetical replicase or replicase subunit cistron in the 5'-proximal part of tobacco mosaic virus (TMV) RNA yields a major 126-K protein and a minor 183-K `readthrough' protein in vivo and in vitro. Two natural suppressor tRNAs were purified from uninfected tobacco plants on the basis of their ability to promote readthrough over the corresponding UAG termination codon in vitro. In a reticulocyte lysate the yield of 183-K readthrough protein increases from ˜10% in the absence of added tobacco plant tRNA up to ˜35% in the case of pure tRNA^Tyr added. Their amino acid acceptance and anticodon sequence (GψA) identifies the two natural suppressor tRNAs as the two normal major cytoplasmic tyrosine-specific tRNAs. tRNA^Tyr₁ has an A:U pair at the base of the TψC stem and an unmodified G₁₀, whereas tRNA^Tyr₂ contains a G:C pair in the corresponding location and m²G in position 10. This is the first case that, in a higher eukaryote, the complete structure is known of both the natural suppressor tRNAs and the corresponding viral RNA on which they exert their function. The corresponding codon-anticodon interaction, which is not in accordance with the wobble hypothesis, and the possible biological significance of the readthrough phenomenon is discussed.     

Antico-operative binding of bacterial and mammalian initiator tRNAMet to methionyl-tRNA synthetase from Escherichia coli

The reaction scheme of methionyl-tRNA synthetase from Escherichia coli with the initiator tRNA_s^Met from E. coli and rabbit liver, respectively, has been resolved. The statistical rate constants for the formation, k_R, and for the dissociation, k_D, of the 1:1 complex of these tRNAs with the dimeric enzyme have been calculated. Identical k_R values of 250 μm^?1 s^?1 reflect similar behaviour for antico-operative binding of both tRNA_s^Met to native methionyl-tRNA synthetase. Advantage was taken of the difference in extent of tryptophan fluorescence-quenching induced by the bacterial and mammalian initiator tRNA_s^Met to measure the mode of exchange of these tRNAs antico-operatively bound to the enzyme. Analysis of the results reveals that antico-operativity does not arise from structural asymmetric assembly of the enzyme subunits. Indeed, both subunits can potentially bind a tRNA molecule. Exchange between tRNA molecules can occur via a transient complex in which both sites are occupied. Either strong and weak sites reciprocate between subunits on the transient complex or occupation of the weak site induces symmetry of this complex. While in the present case, these two alternatives are kinetically indistinguishable, they do account for the observation that, upon increasing the concentration of the competing mammalian tRNA, the rate of exchange of the E. coli initiator tRNA^Met is enhanced, due to its faster rate of dissociation from the transient complex. Finally, it has been verified that in the case of the trypsin-modified methionyl-tRNA synthetase which cannot provide more than one binding site for tRNA, exchange of enzymebound bacterial tRNA by mammalian tRNA does proceed to a limiting rate independent of the mammalian tRNA concentration present in the solution.     

Synthesis and crystal structure of bis(tetramethylammonium) aquotetra-fluorodioxouranate(VI) dihydrate, [(CI3)4 N]2[UO2 F4(H2 O)]·2H2O

The title compound has been synthesized and subjected to crystal structure analysis. M_r = 548.50, m.p. 108.1 °C (decom.), orthorhombic, Im2m,a = 7.006(2), b = 8.938(2), c = 13.619(2) Å V = 852.8(3) ų, Z = 2, D_x = 2.136, D_m, (flotation in CCl₄/CH₂I₂) = 2.128 g cm^?3, λ(Mo-Kα) = 0.71069 Å, μ = 90.79 cm^?1, F(000) = 519.89, T = 295 K, final R_F = 0.036 and R_G = 0.044 for 566 observed reflections. The discrete [UO₂F₄(H₂0)]^2? anion has site symmetry m2m, its virtually linear uranyl moiety being surrounded by fluoro and aquo ligands occupying the vertices of a pentagon in the equatorial plane. Watet molecules serve to link the complex anions by hydrogen bonds into layers, between which the organic cations are accommodated.     

New preparations and molecular structures of cis-MCl2(Ph2PCH2PPh2)2, M = Ru,Os and trans-RuCl2(Ph2PCH2PPh2)2

The title compounds were made by reacting bis(diphenylphosphino)methane (dppm) with reduced solutions of OsCl₆^4? and Ru₂OCl₁₀^4?. The crystal and molecular structures of these compounds have been determined form three-dimensional X-ray study. The cis-isomers crystallize with one CHCl₃ per molecule of the complex. All three compounds crystallize in the monoclinic space group P2₁/n with unit cell dimensions as follows: Cis-OsCl₂(dppm)₂·CHCl₃: a = 13.415(4) Å, b = 22.859(4) Å, c = 16.693(3) Å, β = 105.77(3)°, V = 4926(3) ų, Z = 4. cis-RuCl₂(dppm)₂·CHCl₃: a = 13.442(3) Å, b = 22.833(7) Å, c = 16.750(4) Å, β = 105.53(2)°, V = 4953(3) ų, Z = 4. trans-RuCl₂(dppm)₂: a = 11.368(7) Å, b = 10.656(6) Å, c = 18.832(12) Å; β = 103.90(6)°, V = 2213(7) ų; Z = 2. The structures were refined to R = 0.044 (R_w = 0.055) for cis-OsCl₂(dppm)₂·CHCl₃; R = 0.065 (R_w = 0.079) for cis-RuCl₂(dppm)₂·CHCl₃ and R = 0.028 (R_w = 0.038) for trans-RuCl₂(dppm)₂. The complexes are six coordinate with stable four-membered chelate rings. The PMP angle in the chelate rings is ca. 71° in each case.     

Monomeric molybdenum(V) complexes. 4. The structure of tetraphenylarsonium oxodichloro(N-2-oxophenylsalicylideniminato)molybdate(V), [Ph4As] [MoOCl2(SalphO)]

The structure of [Ph₄As] [MoOCl₂(SalphO)], where SalphO is N-2-oxophenylsalicylideniminate dianion, has been determined by X-ray crystallography. The complex crystallizes in the monoclinic space group P2₁/n with a = 11.829(2), b = 16.149(3), c = 17.410(3) Å, β = 97.485(15)° and Z = 4. The calculated and observed densities and 1.566 and 1.573(10) g cm^?3, respectively. Block-diagonal least-squares refinement of the structure using 4722 independent reflections with I ? 3σ(I) converged at R = 0.0345 and R_w = 0.0484. The crystal contains [Ph₄As]⁺ cations and [MoOCl₂(SalphO)]^? anions. The Mo atom in the anion is in a distorted octahedral coordination environment. A planar terdentate Schiff base ligand occupies meridional positions with the N atom trans to the terminal oxo group (O_t). Two Cl atoms are cis to the O_t atom. The Mo atom is displaced by 0.33 Å from the equatorial plane toward the O_t atom. The MoO_t distance is 1.673(3) Å. The MoN bond trans to the O_t atom is 2.298(4) Å. The two MoCl bond lengths are 2.371(1) and 2.408(1) Å. The difference of 0.037 Å is significant (30 σ). Preparations of the title complex and the related complexes are also described.     

Cyclopentadienyl iron xanthate complexes. Crystal and molecular structure and electrochemical reduction of η-C5R5Fe(CO)2(η1-SC(S)OEt) (RH or CH3)

Complexes η-C₅R₅Fe(CO)₂(η¹-SC(S)OEt) (RH, (1) and RCH₃ (2)) have been analysed by X-ray diffraction techniques. 1: P2₁2₁2₁ 11.3560(5), 10.8595(4), 10.1158(3) Å, Z=4; 1023 observed reflexions, R and R_w 0.069 and 0.073. 2: Pbca, 15.6907(11), 15.4566(13), 14.3083(11) Å, Z=8; 2271 observed reflexions, R and R_w being 0.071, 0.073. The coordination is quite similar for both compounds with the xanthate monodentate ligand almost perpendicular to the ring planes and a relative twist, one from each other of about 10°. The reductive electrochemistry of both complexes has been examined by cyclic voltammetry and coulometry. In a carbon electrode the first-one electron reduction step can be ascribed to the formation of corresponding carbonyl dimers. In a mercury electrode, the first reduction step of 1 leads to a bond rupture process with formation of a mercury compound [CpFe(CO)₂]₂Hg and further reduction to the anion CpFe(CO)₂⁻. However, the behaviour of the pentamethylcyclopentadienyl complex (2) is quite different, and it is reduced in a three step process.     

Hydrodynamic properties and structure of fd virus

A length of 8950 ± 200 Å and a diameter of 90 ± 10 Å have been obtained for fd virus from a simultaneous solution of the Broersma equations relating the length and diameter of a rod-like particle to its rotational, D_R, and translational, D_T, diffusion coefficients. Measurements of D_R were by transient electric birefringence, and of D_T by low-angle intensity fluctuation spectroscopy. A mass of (16.4 ± 0.6) × 10⁶ daltons was calculated from the Svedberg equation using our measured values of D_T, the sedimentation coefficient and the density increment. These results, together with the molecular weight of fd DNA, give a total number of major coat protein subunits of 2710 ± 110 and a ratio of nucleotides to protein subunits which is definitely non-integral, 2.30 ± 0.11. These measurements help delineate significant structural differences between fd and other filamentous viruses. Also included in this paper is an Appendix (by L. A. Day & S. A. Berkowitz) concerning the number of nucleotides, 6370 ± 140, and the density and refractive index increments of fd DNA.