Axial Rings Research Articles

THE LOWER CAMBRIAN EODISCOID TRILOBITECALODISCUS LOBATUSFROM SWEDEN: MORPHOLOGY, ONTOGENY AND DISTRIBUTION

Abstract: Several thousand disarticulated remains together with a few complete enrolled specimens of the lower Cambrian eodiscoid trilobiteCalodiscus lobatus(Hall, 1847) have been collected at two outcrop areas in Sweden. The material reveals new details of morphology and morphogenesis during ontogenetic development. Size‐frequency analyses show that the material from the Fånån rivulet in Jämtland, central Sweden, represents a natural population dominated by juveniles, whereas the material from Gislövshammar in Scania, southern Sweden, has been sorted during postmortem transport. Three stages of protaspid development can be traced and defined as well as all subsequent ontogenetic stages for the cephalon, hypostome and pygidium. The early meraspid pygidium has a pronounced larval notch, which persists, though becoming progressively less distinct in later meraspides. The number of axial rings in the transitory pygidium increases throughout meraspid development until a third and final thoracic segment is liberated. During ontogeny the articulating half‐rings are strongly developed, and both meraspides and holaspides were capable of full sphaeroidal enrollment and outstretched postures. The hypostome undergoes some dramatic modifications; in M0 the anterior margin is axe‐shaped, by M1 the area of attachment greatly decreases and the hypostome becomes more elongated and pear‐shaped, before attaining its adult form, which has an overall resemblance to that of polymerid trilobites. During ontogeny, the hypostome changes from a conterminant attachment to a natant condition, thereby mirroring hypostomal evolution within trilobites generally. The morphology, ontogeny, enrollment, hypostomal development and the presence of calcified protaspides suggest polymerid rather than agnostoid affinities of the eodiscoids.

Cambrian trilobite Ovatoryctocara granulata Tchernysheva, 1962 and its biostratigraphic significance

The genus Ovatoryctocara Tchernysheva, 1962, and its key species Ovatoryctocara granulata Tchernysheva, 1962, are revised. Ovatoryctocara granulata occurs near the base of the Ovatoryctocara Zone and ranges up into the lower portion of the Kounamkites Zone in the Siberian Platform. O. granulata also appears in southeastern Guizhou, South China, but O. granulata in northern Greenland may represent an indefinite species. Specimens of Ovatoryctocara from Newfoundland cannot be identified to species level. Specimens including two cranidia and three pygidia from the lower part of the Aoxi Formation at Yaxi Village, Shizhu Town, eastern Tongren, northeastern Guizhou, were previously assigned to O. granulata, which is now reassigned as a new species O. yaxiensis sp. nov. It bears the following main features: glabella club-shaped, slightly expanded medially, with four pairs of lateral furrows, of which S1–S3 are triangular pits, S4 is shallow, connecting with axial furrow; shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, longer posterolateral area (exsag.); semielliptical pygidium consisting of seven axial rings with a terminal piece and with eight pairs of marginal tips giving a sawtooth-like shape of the lateral margins in dorsal view. Although O. granulata is a widely distributed species, the FAD of O. granulata, for a global stage boundary, still has some disadvantages. First, its distribution is not as wide as that of Oryctocephalus indicus (Reed, 1910). Second, specimens of O. granulata are common only in Siberia. Third, the stratigraphic range of the species has not been studied in detail in all continents. Fourth, O. granulata is relatively small and easily affected by post-burial distortion. Last but not least, there exists obviously a facies change between the Ovatoryctocara Zone (lower Amgan Stage; deeper water facies) and the underlying Anabaraspis splendens Zone (Toyonian Stage; shallow water facies) in Siberian Platform. Nevertheless, the stratigraphic correlation utility of O. granulata in South China and Siberia is discussed.

Ringlike cores of cylindrically confined nematic point defects

Nematic liquid crystals confined in a cylindrical capillary and subjected to strong homeotropic anchoring conditions is a long-studied fundamental problem that uniquely incorporates nonlinearity, topological stability, defects, and texture physics. The observed and predicted textures that continue to be investigated include escape radial, radial with a line defect, planar polar with two line defects, and periodic array of point defects. This paper presents theory and multiscale simulations of global and fine scale textures of nematic point defects, based on the Landau-de Gennes tensor order parameter equations. The aim of this paper is to further investigate the ringlike nature of point defect cores and its importance on texture transformation mechanisms and stability. The paper shows that the ringlike cores can be oriented either along the cylinder axis or along the radial direction. Axial rings can partially expand but are constrained by the capillary sidewalls. Radial rings can deform into elliptical structures whose major axis is along the capillary axis. The transformation between several families of textures under capillary confinement as well as their stability is discussed in terms of defect ring distortions. A unified view of nematic textures found in the cylindrical cavities is provided.

A restudy of Oryctocephalus indicus (Reed, 1910)

Abstract Restudy on morphology of Oryctocephalus indicus (Reed, 1910), based on suecimens from Guizhou, China and Nevada USA, suggests that the subspecies regarded previously as Oryctocephalus indicus indicus (Reed, 1910), O. indicus latus Zhoa et Yuan 2002, and O. indicus kobayashi Saito, 1934, are synonymous, and Oryctocephalus americanus Sundberg et Mc Collum, 2003 is a similar form of O. indicus. Oryctocephalus indicus is rediagnosed as having a glabella that is subconical in outline and tapering forward slightly; a thorax that comprises 12 segments, and a small pygidium that bears 2–3 axial rings with a terminal piece and a postaxial ridge. Oryctocephalus indicus is widely distributed in eastern Guizhou and ranges through a great interval of Kaili Formation. Its first appearance is almost identical with these events of trilobite extinction-recovery occurring at the end of Early Cambrian, the alternation of acritach assemblages, the change of trace elements (RFE) and stable isotopes (carbon) at the Wu...

Analytic solutions to Maxwell–London equations and levitation force for a general magnetic source in the presence of a long type-II superconducting cylinder

The interaction between a general magnetic source and a long type-II superconducting cylinder in the Meissner or mixed state is studied within the London theory. We first study the Meissner state and solve the Maxwell–London equations when the source is a magnetic monopole located at an arbitrary position. Then the field and supercurrent for a more complicated magnetic charge distribution can be obtained by superposition. A magnetic point dipole with arbitrary direction is studied in detail. It turns out that the levitation force on the dipole contains in general an angular as well as a radial component. By integration we obtain the field and supercurrent when the source is a two-dimensional monopole (a magnetically charged long thread along the axial direction), from which the results for a two-dimensional point dipole easily follow. In the latter case the levitation force points in the radial direction regardless of the orientation of the dipole. The case for a current carrying long straight wire parallel to the cylindrical axis is solved separately. The limit of ideal Meissner state is discussed in most cases. The case of mixed state is discussed briefly. It turns out that vortex lines along the axial direction and vortex rings concentric with the cylinder have no effect outside the cylinder and the levitation forces remain the same as in the case of the Meissner state.

Characterization of Heme-Coordinating Histidyl Residues of Cytochrome b5 Based on the Reactivity with Diethylpyrocarbonate: A Mechanism for the Opening of Axial Imidazole Rings

We investigated the reactivity of heme-coordinating imidazole with diethylpyrocarbonate using a soluble domain of cytochrome b(5). Analyses with various spectroscopic methods including MALDI-TOF-MS indicated that two axial His residues (His44 and His68) of cytochrome b(5) were protected from the modification by several factors, i.e., limited steric exposure of the axial imidazole to the solvent, the Fe-N(epsilon2) coordination bond, and protonation of the N(delta1) position by forming a hydrogen bond with its immediate surroundings. However, once N-carbethoxylation at the N(epsilon2) position of the axial His residues occurred with a higher concentration of diethylpyrocarbonate, displacement of heme prosthetic group from the protein moiety continued. Simultaneously, it facilitated the second N-carbethoxylation to take place at the N(epsilon1) position of the same imidazole ring, leading to a bis-N-carbethoxylated derivative and further to a ring-opened derivative. A similar mechanism seemed in operation for one non-axial His residue (His85), in which the N(delta1) atom works as a hydrogen acceptor in a strong hydrogen-bond and the other N(epsilon2) atom is in a protonated form, resulting in a formation of the ring-opened derivative upon treatment with a higher concentration of diethylpyrocarbonate. These results suggested that the use of diethylpyrocarbonate for MALDI-TOF-MS analysis might provide a unique method to characterize the protonation state of His residues and the strength of their hydrogen-bondings at the active site of enzymes.

Vibrational shifts of OCS in mixed clusters of parahydrogen and helium

We present a detailed theoretical study of the solvation structure and solvent induced vibrational shifts for an OCS molecule embedded in pure parahydrogen clusters and in mixed parahydrogen/helium clusters. The use of two recent OCS-(parahydrogen) and OCS-helium ab initio potential energy surfaces having explicit dependence on the asymmetric stretch of the OCS molecule allows calculation of the frequency shift of the OCS nu(3) vibration as a function of the cluster size and composition. We present results for clusters containing up to a full first solvation shell of parahydrogen (N=17 molecules), and up to M=128-N helium atoms. Due to the greater interaction strength of parahydrogen than helium with OCS, in the mixed clusters the parahydrogen molecules always displace He atoms in the first solvation shell around OCS and form multiple axial rings as in the pure parahydrogen clusters. In the pure clusters, the chemical potential of parahydrogen shows several magic numbers (N=8,11,14) that reflect an enhanced stability of axial rings containing one less molecule than required for complete filling at N=17. Only the N=14 magic number survives in the mixed clusters, as a result of different filling orders of the rings and greater delocalization of both components. The OCS vibration shows a redshift in both pure and mixed clusters, with N-dependent values that are in good agreement with the available experimental data. The dependence of the frequency shift on the cluster size and its composition is analyzed in terms of the parahydrogen and helium density distributions around the OCS molecule as a function of N and M. The frequency shift is found to be strongly dependent on the detailed distribution of the parahydrogen molecules in the pure parahydrogen clusters, and to be larger but show a smoother dependence on N in the presence of additional helium, consistent with the more delocalized nature of the mixed clusters.

( P-Bis(pentafluorophenyl) substituted) PCP-pincer Ru(II) complexes: A theoretical study of the molecular structure and electronic properties

The differences between the molecular structures of the PCP-pincer complex [RuCl{C 6H 3(CH 2P(C 6H 5) 2) 2-2,6}(PPh 3)] ([RuCl(PCP H)(PPh 3)], 1) and its tetrakis-pentafluorophenyl substituted analogue [RuCl{C 6H 3(CH 2P(C 6F 5) 2) 2-2,6}(PPh 3)] ([RuCl(PCP F20)(PPh 3)], 2) have been rationalised by performing calculations on the cations [Ru(PCP H)(PPh 3)] + ( 1cat) and [Ru(PCP F20)(PPh 3)] + ( 2cat). The molecular interactions between the chloride ligand and the axial rings, as found in 1 and 2, respectively, have been studied computationally in the model systems [(C 6X 5PH 2) 2Cl −] (X = H, F). The calculations on 2cat show that in 2 it is most likely the attractive electrostatic interaction between the chloride ligand and the fluorinated phenyl rings that forces the C ipso atom to occupy an axial position rather than an equatorial one in the observed (X-ray of 2) square pyramidal arrangement. In 1, however, repulsive steric hindrance forces the PPh 3 ligand to take the apical position. The applicability of the TD-DFT method for the calculation of the electronic spectra of the PCP-pincer compounds 1 and 2 has been tested. The results indicate that the excitation energies calculated for both complexes are in a reasonable agreement with the experimental absorption maxima. However, for 1, all the calculated transition energies are underestimated.

The brachymetopid trilobite Radnoria in the Silurian (Wenlock) of New York State and Arctic Canada

Radnoria bretti n. sp., from the Wenlock (Sheinwoodian) Rochester Formation of western New York State, is the best preserved and most completely known member of its genus. It provides the first definitive information on Radnoria's hourglass-shaped rostral plate, the first known hypostome of a Silurian member of the genus, demonstrates that Radnoria engaged in sphaeroidal enrollment, and reveals that early holaspid individuals had tubercles on the posterior thoracic axes and pygidial axial rings that were effaced with maturity. Three new species from the Wenlock of the Cape Phillips Formation of Nunavut are known from sparse material and are reported in open nomenclature. Together, the species greatly increase knowledge of Laurentian Silurian brachymetopids, which have until now been known from a single cranidium from the Wenlock of Arkansas.

Hydrogen bonding in diruthenium(II,III) tetraacetate complexes with biologically relevant axial ligands

Three diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru 2(μ-O 2CCH 3) 4L 2](PF 6), where L are the biologically relevant ligands imidazole, 1, 7-azaindole, 2, and caffeine, 3, were synthesized and characterized by elemental analysis, IR and UV–Vis spectroscopy and X-ray crystallography. In order to further elucidate the potential interactions of these dimers with DNA, the nature of the ligand coordination and the secondary inter- and intramolecular hydrogen-bonding interactions in all three complexes were assessed. Complex 1 · CH 2Cl 2 shows, exclusively, intermolecular interactions with the PF 6 - counterion whereas complexes 2 · ClCH 2CH 2Cl and 3 · OC(CH 3) 2 · H 2O, in addition to extensive intermolecular interactions, show intramolecular hydrogen bonding from the axial ligand to the bridging acetate oxygens, locking the ligand mean planes in place between the bridging acetate mean planes. In addition, all three complexes display π–π stacking of axial ligand rings on adjacent diadduct units.

Crystal and Molecular Structure of P(C6H5)5 · 0.5 THF

Abstract Pentaphenylphosphorus crystallizes from tetrahydrofuran (THF) as P(C6H5)5・0.5 THF (triclinic space group: P1̅, a = 10.095(4), b = 10.252(3), c = 12.725(3) A° , α = 71.21(1), β = 76.98(3), γ = 87.12(1)°, Z = 2). Its molecular structure is an almost perfect trigonal bipyramid with significantly longer axial than equatorial P-CPh bonds (P-Cax 1.982(2)/1.979(2), P-Ceq 1.853(2)/1.845(2)/1.847(2) Å). It differs from the well established structure of solvent-free P(C6H5)5 (P. J. Wheatley, J. Chem. Soc. 2206 (1964)) in the relative orientation of the phenyl rings with respect to each other (axial rings) and with respect to the equatorial PC3 plane (equatorial rings) but not in the trigonal-bipyramidal (tbp) geometry at phosphorus. Differences in the geometry around the central atom had been found previously for Sb(C6H5)5 (square pyramid) and Sb(C6H5)5 · 0.5C6H12 (tbp) but not in As(C6H5)5 and As(C6H5)5 · 0.5 C6H12 (both tbp).

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

We determine the structure and energetics of complexes of the linear OCS molecule with small numbers of para-hydrogen molecules, N=1–8, using zero temperature quantum Monte Carlo methods. Ground state calculations are carried out with importance-sampled rigid body diffusion Monte Carlo and excited state calculations with the projection operator imaginary time spectral evolution methodology. The ground states are found to be highly structured, with a gradual build up of two axial rings as N increases to 8. Analysis of the azimuthal density correlations around the OCS molecule shows that these rings are quite delocalized for small N values, but become strongly localized for N⩾5. Excited state calculations are made for a range of total cluster angular momentum values and the rotational energy levels fitted to obtain effective rotational and distortion constants of the complexed OCS molecule as a function of cluster size N. Detailed analysis of these spectroscopic constants indicates that the complexes of OCS with para-hydrogen have an unusually rich variation in dynamical behavior, with sizes N=1–2 showing near rigid behavior, sizes N=3–4 showing extremely floppy behavior, and the larger sizes N=5–8 showing more rigid behavior again. The large values of the distortion constant D obtained for N=3–4 are rationalized in terms of the coupling between the OCS rotations and the “breathing” mode of the first, partially filled ring of para-hydrogen molecules.

Design of Mechanically Robust High-Tg Polymers: Synthesis and Dynamic Mechanical Relaxation Behavior of Glassy Poly(ester carbonate)s with Cyclohexylene Rings in the Backbone

High-Tg yet mechanically robust polymers are desirable but difficult to realize. To explore whether the insertion of a motionally active molecular entity in the backbone could produce polymers with both high Tg and yet are not brittle, three series of high-Tg glassy polycarbonates/polyesters/poly(ester carbonate)s were designed and synthesized. The secondary relaxation behavior of these polymers was studied by dynamical mechanical analysis (DMA). Incorporation of cyclohexylene groups (C-rings) in the main chain of polymers made from two rigid and bulky monomers, spirobiindane bisphenol (SBI) and trimethylcyclohexylbisphenol (Tmc), respectively, changes the γ relaxation process dramatically in peak shape and position in these polymers. No such change is observed in polycarbonate polymers made from cyclohexylbisphenol (BPAZ), or ZPC for short. Through monitoring the γ relaxation with gradual structural change, C-rings in the main chain backbone are proposed to undergo active ring inversion. The γ relaxation of ZPC is proposed to be mainly due to side-chain C-ring inversion. The narrowing on the high-temperature side of the γ relaxation peak of the polycarbonate of Tmc is proposed to be due to the restricted motion of its axial phenyl rings. A linear correlation between relaxation strength and total C-ring concentration was found for all the polymers.

Crystal structure of cis-polar, trans(Cl,O 5)-Na 2[Rh(1,3-pddadp)Cl] · 7H 2O and structural correlations between octahedral pentadentate metal(III) complexes with diaminopolycarboxylato-type ligands

The pentadentate complex cis-polar, trans(Cl,O 5)-Na 2[Rh(1,3-pddadp)Cl] · 7H 2O (where 1,3-pddadp is the 1,3-propanediamine- N, N ′-diacetate- N, N ′-di-3-propionate ion) was prepared and characterized. The structure of the complex was determined by X-ray analysis. The rhodium(III) ion is surrounded octahedrally by the two N and three O atoms of the 1,3-pddadp ligand and one chloride anion. The chloride anion occupies the trans position with respect to the oxygen atom of the axial glycinate ring and the cis-polar orientation with respect to the tertiary nitrogen atom bonded to the uncoordinated 3-propionate group. The investigated pentadentate complex derives directly from the trans(O 5O 6) isomer of the corresponding hexadentate [Rh(1,3-pddadp)] − complex. The conformations of the chelate rings were found to be envelope and flat for in-plane (G ′′) and axial (R ′) glycinate rings, and sofa and chair for in-plane β-alaninate (G ′) and 1,3-propanediamine (T) rings, respectively. The electronic absorption spectrum of cis-polar, trans(Cl,O 5)-[Rh(1,3-pddadp)Cl] 2 − is also given and compared with those of hexadentate trans(O 5O 6)-[Rh(1,3-pddadp)] − and trans(O 5)-[Rh(1,3-pddadp)] − complexes.

Nonplanar distortions of bis-base low-spin iron(II)-porphyrinates: absorption and resonance Raman investigations of cross-trans-linked iron(II)-basket-handle porphyrin complexes.

Electronic absorption and Soret-excited resonance Raman (RR) spectra are reported for bis-N-alkylimidazole and bis-pyridine complexes of various cross-trans-linked iron(II)-"basket-handle" porphyrins (Fe(II)-BHP) in methylene chloride. These compounds enable us to characterize the spectroscopic properties of ruffled six-coordinated low-spin Fe(II)-porphyrin complexes. The visible absorption spectra show that the Q and B bands are progressively red-shifted when the handles are shortened and/or when the steric hindrance of the axial ligands is increased. This effect is accompanied by both a decrease in RR frequency of the nu(2) mode and an increase in frequency of the nu(8) and nu(s)(Fe-ligand(2)) modes. More precisely, an inverse linear correlation is found between the frequencies of the nu(2) and nu(8) modes. For each ligation state, the positions of the absorption bands are also linearly correlated with the frequency of the nu(2) or nu(8) mode. All of these spectroscopic data reveal that the degree of ruffling of the Fe(II)-BHP complexes is increased by the N-methylimidazole --> pyridine axial substitutions, presumably because the mutual steric strains between the axial ligand rings, the porphyrin macrocycle and the porphyrin handles are increased. The present study provides a first basis for discerning ruffled conformations from planar and other nonplanar structures in ferrous heme proteins.

Absorption and resonance Raman investigations of ligand rotation and nonplanar heme distortion in bis-base low-spin iron(II)-tetrakis(o-pivalamidophenyl)porphyrin complexes.

The absorption and resonance Raman (RR) spectra of the bis-N-methylimidazole, bis-1,5-dicyclohexylimidazole, and bis-pyridine complexes of the meso-alphaalphabetabeta and meso-alphabetaalphabeta atropisomers of Fe(II)-tetrakis(o-pivalamidophenyl)porphyrins (Fe(II)TpivPP) were obtained in methylene chloride. The different spatial arrangements of the o-pivalamide pickets in these two Fe(II)TpivPP compounds are expected to control the absolute and relative positions of the axial ligand rings with respect to the Fe-N(pyrrole) bonds. In particular, the spectroscopic data obtained for the bis-N-methylimidazole and bis-dicyclohexylimidazole complexes of the Fe(II)[alphabetaalphabeta-TpivPP] derivative showed the most important differences. Redshifts of the B and Q absorption bands (+ 4-5 nm) as well as an upshift of the low frequency nu(8) RR mode (+ 5 cm(-)(1)) were observed. No shift of the skeletal high frequency modes was detected. These spectral effects were associated with a change in relative position of the axial imidazole rings from nearly parallel in the bis-N-methylimidazole complex to nearly perpendicular in the bis-dicyclohexylimidazole complex. On the basis of stereochemical considerations as well as previous spectroscopic investigations, the data were interpreted in terms of change in porphyrin structure from planar to saddled. Complementing to a parallel study on bis-base Fe(II) "basket handle" porphyrin complexes, this spectroscopic investigation provides an additional means to distinguish planar, ruffled, and saddled conformations for ferrous hemes included in proteins.

The solution structure of the oxidized bovine microsomal cytochrome b5 mutant V61H

Using 1488 NOE constraints, 19 stereo-specific assignments, 13 pairs of H-bond constraints, and 140 pseudo-contact shift constraints, a family of 35 structures of bovine microsomal cytochrome b 5 mutant V61H has been obtained through the program PSEUDYANA. The family has been further refined by restrained energy minimization to give a family of final structures. The RMSD values of final structures with respect to the average structure are 0.45 ± 0.11 and 0.96 ± 0.10 Å for backbone and heavy atoms, respectively. The final Δ χ ax and Δ χ rh values are 2.34 × 10 −32 and −0.67 × 10 −32 m 3, respectively. The comparisons between the solution structures of mutant V61H and WT cytochrome b 5, and X-ray structure of the mutant V61H show that the global folding of the molecule in solution is unchanged and the side-chain of His61 deviates from the heme pocket and extends into the solvent like in its crystal structure. However, the helices around the heme pocket undergo outward global displacement while their local conformations are well maintained. Meanwhile, the heme ring shows a little off the heme pocket, which accounts for the lower stability of the mutant. Additionally, the axial ligand rings counterclockwise rotate around His39 N–Fe axis due to the mutation, which is confirmed by variation of the hyperfine shifts of the heme protons of V61H compared to those of WT cytochrome b 5.

THE UPPER CAMBRIAN TRILOBITES PLETHOPELTIS AND LEIOCORYPHE: MORPHOLOGY, PAEDOMORPHISM, CLASSIFICATION

Plethopeltis armatus, and species of Leiocorpyphe, are shown to have in common a median ventral cephalic suture and thoracic axial rings with a poorly-defined articulating furrow and a strong transverse ventral ridge. In other characters they differ greatly, and to group them in the Plethopeltidae is open to question; this family may be polyphyletic. The median ventral suture is regarded as a cardinal character of Asaphida, but it is uncertain whether this character had a single origin or arose more than once. Leiocoryphe gemma may be a paedomorphic species derived from some Upper Cambrian trilobite with an effaced cephalon.

The Upper Cambrian trilobitesPlethopeltisandLeiocoryphe: Morphology, paedomorphism, classification

Plethopeltis armatus, and species ofLeiocorpyphe, are shown to have in common a median ventral cephalic suture and thoracic axial rings with a poorly-defined articulating furrow and a strong transverse ventral ridge. In other characters they differ greatly, and to group them in the Plethopeltidae is open to question; this family may be polyphyletic. The median ventral suture is regarded as a cardinal character of Asaphida, but it is uncertain whether this character had a single origin or arose more than once.Leiocoryphe gemmamay be a paedomorphic species derived from some Upper Cambrian trilobite with an effaced cephalon.

MicroPET II: design, development and initial performance of an improved microPET scanner for small-animal imaging (This work was originally conducted at UCLA, Crump Institute for Molecular Imaging, and was continued and completed at UC Davis, Department of Biomedical Engineering.)

MicroPET II is a second-generation animal PET scanner designed for high-resolution imaging of small laboratory rodents. The system consists of 90 scintillation detector modules arranged in three contiguous axial rings with a ring diameter of 16.0 cm and an axial length of 4.9 cm. Each detector module consists of a 14 × 14 array of lutetium oxyorthosilicate (LSO) crystals coupled to a multi-channel photomultiplier tube (MC-PMT) through a coherent optical fibre bundle. Each LSO crystal element measures 0.975 mm × 0.975 mm in cross section by 12.5 mm in length. A barium sulphate reflector material was used between LSO elements leading to a detector pitch of 1.15 mm in both axial and transverse directions. Fused optical fibre bundles were made from 90 µm diameter glass fibres with a numerical aperture of 0.56. Interstitial extramural absorber was added between the fibres to reduce optical cross talk. A charge-division readout circuit was implemented on printed circuit boards to decode the 196 crystals in each array from the outputs of the 64 anode signals of the MC-PMT. Electronics from Concorde Microsystems Inc. (Knoxville, TN) were used for signal amplification, digitization, event qualification, coincidence processing and data capture. Coincidence data were passed to a host PC that recorded events in list mode. Following acquisition, data were sorted into sinograms and reconstructed using Fourier rebinning and filtered backprojection algorithms. Basic evaluation of the system has been completed. The absolute sensitivity of the microPET II scanner was 2.26% at the centre of the field of view (CFOV) for an energy window of 250–750 keV and a timing window of 10 ns. The intrinsic spatial resolution of the detectors in the system averaged 1.21 mm full width at half maximum (FWHM) when measured with a 22Na point source 0.5 mm in diameter. Reconstructed image resolution ranged from 0.83 mm FWHM at the CFOV to 1.47 mm FWHM in the radial direction, 1.17 mm FWHM in the tangential direction and 1.42 mm FWHM in the axial direction at 1 cm offset from the CFOV. These values represent highly significant improvements over our earlier microPET scanner (approximately fourfold sensitivity increase and 25–35% improvement in linear spatial resolution under equivalent operating conditions) and are expected to be further improved when the system is fully optimized.