Symmetry-related Positions Research Articles

Engineering of Pyroelectric Crystals Decoupled from Piezoelectricity as Illustrated by Doped α-Glycine.

Design of pyroelectric crystals decoupled from piezoelectricity is not only a topic of scientific curiosity but also demonstrates effects in principle that have the potential to be technologically advantageous. Here we report a new method for the design of such materials. Thus, the co-doping of centrosymmetric crystals with tailor-made guest molecules, as illustrated by the doping of α-glycine with different amino acids (Threonine, Alanine and Serine). The polarization of those crystals displays two distinct contributions, one arising from the difference in dipole moments between guest and host and the other from the displacement of host molecules from their symmetry-related positions. These contributions exhibit different temperature dependences and response to mechanical deformation. Thus, providing a proof of concept for the ability to design pyroelectric materials with reduced piezoelectric coefficient (d22 ) to a minimal value, below the resolution limit of the method (<0.005 pm/V).

Rapid discovery of stable materials by coordinate-free coarse graining.

A fundamental challenge in materials science pertains to elucidating the relationship between stoichiometry, stability, structure, and property. Recent advances have shown that machine learning can be used to learn such relationships, allowing the stability and functional properties of materials to be accurately predicted. However, most of these approaches use atomic coordinates as input and are thus bottlenecked by crystal structure identification when investigating previously unidentified materials. Our approach solves this bottleneck by coarse-graining the infinite search space of atomic coordinates into a combinatorially enumerable search space. The key idea is to use Wyckoff representations, coordinate-free sets of symmetry-related positions in a crystal, as the input to a machine learning model. Our model demonstrates exceptionally high precision in finding unknown theoretically stable materials, identifying 1569 materials that lie below the known convex hull of previously calculated materials from just 5675 ab initio calculations. Our approach opens up fundamental advances in computational materials discovery.

Tancaite-(Ce), ideally FeCe(MoO&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;)&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; ● 3H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O: description and average crystal structure

Abstract. Tancaite-(Ce), ideally FeCe(MoO4)3⚫3H2O, is a new mineral occurring within cavities in the quartz veins which cut the granite at Su Seinargiu, Sarroch (CA), Sardinia, Italy. It is a secondary mineral formed in the oxidation zone of a sulfide ore vein. Associated minerals are quartz, muscovite, molybdenite, pyrite, and a mendozavilite-like phase. Tancaite-(Ce) is red or pale brown in colour, with a vitreous to adamantine lustre. Electron microprobe analyses give (wt %) SiO2 0.34, CaO 0.09, Fe2O3 11.29, SrO 0.02, La2O3 5.04, Ce2O3 10.35, Pr2O3 1.07, Nd2O3 3.66, Sm2O3 0.19, ThO2 2.58, UO2 0.17, MoO3 58.62, and H2O (calculated) 7.43, with a sum of 100.85, from which the empirical formula is calculated. The empirical formula Fe1.033+(Ce0.46La0.23Nd0.16Pr0.05Sm0.01U0.01Th0.07)Σ=0.99(Mo2.96Si0.04)Σ=3.00O12⚫3H2O can be simplified as Fe3+(REE)(MoO4)3⚫3H2O and idealized as FeCe(MoO4)3⚫3H2O. The presence of H2O was confirmed by micro-Raman spectrometry (stretching and bending vibrations of O–H). The calculated density is 3.834 g cm−3. The X-ray diffraction pattern of tancaite-(Ce) is characterized by a set of strong reflections, which point to a cubic subcell with a=6.870(1) Å and space group Pm3¯m, plus a set of superstructure reflections. Tancaite-(Ce) displays a new structure type not previously reported in natural and synthetic molybdates. By considering only the strong reflections, it was possible to solve and refine its average structure (R1=0.038 for 192 unique reflections with I&gt;2σ(I)). The crystal structure consists of FeO6 octahedra centred at the origin of the cubic subcell and linked together through MoO4 tetrahedra by corner sharing. The Mo-centred tetrahedra are statistically distributed in four symmetry-related positions, with one-fourth occupancy. In the centre of the cubic unit cell the REE cations exhibit a 6+3 coordination, bonding six oxygen atoms and three H2O molecules, each of them being disorderly distributed in four symmetry-related positions. One of the possible supercells, with a 48-fold volume with respect to the primitive cubic small subcell, corresponded to a rhombohedral lattice, with a≈19.43 and c≈47.60 Å in the hexagonal setting. Several unsuccessful trials were performed to solve the real crystal structure of tancaite, by indexing the additional superstructure reflections and using their intensities to refine an ordered structural model. The new mineral has been approved by the IMA CNMNC (no. 2009-097). The name comes from Giuseppe Tanca, an Italian amateur mineralogist, who discovered the mineral and gave it to us for studying.

The role of Cinnamaldehyde and Phenyl ethyl alcohol as two types of precipitants affecting protein hydration levels

Release of water is the main force which drives proteins towards crystallisation (giving rise to protein crystals for crystallography) and aggregation (main cause of neurodegenerative diseases), and as such it is possible to make changes in the crystallisation/aggregation process by using compounds which are able to reduce the amount of water molecules around proteins. Cinnamaldehyde and Phenyl ethyl alcohol are the active constituents of cinnamon and rose flower, respectively. Traditional Iranian Medicine (TIM) suggests the use of cinnamon and rose flower for the reduction of excess coldness and wetness from the brain of patients suffering from Dementia. Using crystallisation as a model system and X-ray crystallography, we tested whether Cinnamaldehyde or Phenyl ethyl alcohol can mimic the role of precipitants resulting in the formation of crystals of HEWL (as a model protein) by releasing water from the surrounding protein environment. Results have revealed that both Cinnamaldehyde and Phenyl ethyl alcohol, in particular, were capable to adequately act as ‘precipitants’ but in the presence of NaCl (as a salt), resulting in better crystals of HEWL by changing the amount of charge and/or making water molecules unavailable in the symmetry related position, in line with the role suggested for these compounds by TIM.

Local and Global Electric Field Asymmetry in Photosynthetic Reaction Centers.

The origin of unidirectional electron transfer in photosynthetic reaction centers (RCs) has been widely discussed. Despite the high level of structural similarity between the two branches of pigments that participate in the initial electron transfer steps of photosynthesis, electron transfer only occurs along one branch. One possible explanation for this functional asymmetry is the differences in the electrostatic environment between the active and the inactive branches arising from the charges and dipoles of the organized protein structure. We present an analysis of electric fields in the RC of the purple bacterium Rhodobacter sphaeroides using the intrinsic carbonyl groups of the pigments as vibrational reporters whose vibrational frequency shifts can be converted into electric fields based on the vibrational Stark effect and also provide Stark effect data for plant pigments that can be used in future studies. The carbonyl stretches of the isolated pigments show pronounced Stark effects. We use these data, solvatochromism, molecular dynamics simulations, and data in the literature from IR and Raman spectra to evaluate differences in fields at symmetry-related positions, in particular at the 9-keto and 2-acetyl positions of the pigments involved in primary charge separation.

Crystal structure of (diethyl ether-κO)[5,10,15,20-tetra-kis-(2-iso-thio-cyanato-phen-yl)porphyrinato-κ4 N]zinc diethyl ether solvate.

The crystal structure of the title compound, [Zn(C48H24N8S4)(C4H10O)]·C4H10O, consists of discrete porphyrin complexes that are located on a twofold rotation axis. The ZnII cation is fivefold coordinated by four N atoms of the porphyrin moiety and one O atom of a diethyl ether mol-ecule in a slightly distorted square-pyramidal environment with the diethyl ether mol-ecule in the apical position. The porphyrin backbone is nearly planar with the metal cation slightly shifted out of the plane towards the coordinating diethyl ether mol-ecule. All four iso-thio-cyanato groups of the phenyl substituents at the meso-positions face the same side of the porphyrin, as is characteristic for picket fence porphyrins. In the crystal structure, the discrete porphyrin complexes are arranged in such a way that cavities are formed in which additional diethyl ether solvate mol-ecules are located around a twofold rotation axis. The O atom of the solvent mol-ecule is not positioned exactly on the twofold rotation axis, thus making the whole mol-ecule equally disordered over two symmetry-related positions.

NMR Crystallography, Hydrogen Bonding and Optical Properties of the Novel 2D Hybrid Oxyfluorotitanate [H2taz]2·(Ti5O5F12)

The new 2D hybrid oxyfluorotitanate [H2taz]2·(Ti5O5F12), where [H2taz]+ represents 1,4-diH-1,2,4-triazolium cation ([C2N3H4]+), has been prepared, and its structure has been refined from powder X-ray diffraction data. It is built up from ∞(Ti5O5F12)2– inorganic layers separated by [H2taz]+ cations and adopts the same space group (Cmm2) and, considering the inorganic part, the same structure as [H2gua]2·(Ti5O5F12), where [H2gua]+ represents guanidinium cation ([C(NH2)3]+). The substitution of [H2gua]+ by [H2taz]+ was aimed at reduction of the refractive index, while a high optical band gap was maintained. The substitution effect is small, but [H2taz]2·(Ti5O5F12) could also allow a high UV protection with a good aesthetic effect. In [H2taz]2·(Ti5O5F12), a 50% site occupancy is attributed to one H atom site and two mirror symmetry related positions have a mixed C and N composition, preventing DFT calculations from being performed for the structural cell and preventing the cation configuration from being dete...

Dynamics of local symmetry correlators for interacting many-particle systems.

Recently [P. A. Kalozoumis et al. Phys. Rev. Lett. 113, 050403 (2014)] the concept of local symmetries in one-dimensional stationary wave propagation has been shown to lead to a class of invariant two-point currents that allow to generalize the parity and Bloch theorem. In the present work, we establish the theoretical framework of local symmetries for higher-dimensional interacting many-body systems. Based on the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, we derive the equations of motion of local symmetry correlators which are off-diagonal elements of the reduced one-body density matrix at symmetry related positions. The natural orbital representation yields equations of motion for the convex sum of the local symmetry correlators of the natural orbitals as well as for the local symmetry correlators of the individual orbitals themselves. An alternative integral representation with a unique interpretation is provided. We discuss special cases, such as the bosonic and fermionic mean field theory, and show in particular that the invariance of two-point currents is recovered in the case of the non-interacting one-dimensional stationary wave propagation. Finally we derive the equations of motion for anomalous local symmetry correlators which indicate the breaking of a global into a local symmetry in the stationary non-interacting case.

C-Terminal Domain of Integrase Binds between the Two Active Sites.

HIV integrase (HIV-IN), one of three HIV enzymes, is a target for the treatment of AIDS, but the full biological assembly has been difficult to characterize, hampering inhibitor design. The recent crystallographic structures of integrase from prototype foamy virus (PFV-IN) with bound DNA were a breakthrough, revealing how viral DNA organizes two integrase dimers into a tetramer that has the two active sites appropriately spaced for insertion of the viral DNA into host DNA. The organization of domains within each PFV-IN protein chain, however, varies significantly from that found in HIV-IN structures. With the goal of identifying shared structural characteristics, the interactions among components of the PFV-IN and HIV-IN assemblies were investigated with the macromolecular docking program DOT. DOT performs an exhaustive, rigid-body search between two macromolecules. Computational docking reproduced the crystallographic interactions of the PFV-IN catalytic and N-terminal domains with viral DNA and found similar viral DNA interactions for HIV-IN. Computational docking did not reproduce the crystallographic interactions of the PFV-IN C-terminal domain (CTD). Instead, two symmetry-related positions were found for the PFV-IN CTD that indicate formation of a CTD dimer between the two active sites. Our predicted CTD dimer is consistent with cross-linking studies showing interactions of the CTD with viral DNA that appear to be blocked in the PFV-IN structures. The CTD dimer can insert two arginine-rich loops between the two bound vDNA molecules and the host DNA, a region that is unoccupied in the PFV-IN crystallographic structures. The positive potential from these two loops would alleviate the large negative potential created by the close proximity of two viral vDNA ends, helping to bring together the two active sites and assisting host DNA binding. This study demonstrates the ability of computational docking to evaluate complex crystallographic assemblies, identify interactions that are influenced by the crystal environment, and provide plausible alternatives.

Different effects of identical symmetry-related mutations near the bacteriochlorophyll dimer in the photosynthetic reaction center of Rhodobacter sphaeroides.

In the bacterial photosynthetic reaction center (RC), asymmetric protein environment of the bacteriochlorophyll (BChl) dimer largely determines the photophysical and photochemical properties of the primary electron donor. Previously, we noticed significant differences in properties of Rhodobacter sphaeroides RCs with identical mutations in symmetry-related positions- I(M206)H and I(L177)H. The substitution I(L177)H resulted in covalent binding of BChl PA with the L-subunit, as well as in 6-coordination of BChl BB, whereas in RC I(M206)H no such changes of pigment-protein interactions were found. In addition, the yield of RC I(M206)H after its isolation from membranes was significantly lower than the yield of RC I(L177)H. This study shows that replacement of amino acid residues in the M203-M206 positions near BChls PB and BA by symmetry-related residues from the L-subunit near BChls PA and BB leads to further decrease in RC amount in the membranes associated obviously with poor assembly of the complex. Introduction of a new hydrogen bond between BChl PB and its protein environment by means of the F(M197)H mutation stabilized the mutant RC but did not affect its low yield. We suggest that the mutation I(M206)H and substitution of amino acid residues in M203-M205 positions could disturb glycolipid binding on the RC surface near BChl BA that is important for stable assembly of the complex in the membrane.

Klebelsbergite, Sb4O4SO4(OH)2: Stability relationships, formation in Nature, and refinement of its structure

The extent to which secondary Sb minerals control Sb dispersion in the supergene environment is yet to be fully understood. Stability studies of klebelsbergite have been undertaken to better under- stand its role in controlling Sb mobility and relationships with other secondary Sb minerals. Solubility in aqueous 0.1084 M HNO3 was determined at 298.15 K and the data obtained used to derive ∆Gfθ(klebelsbergite, s, 298.15 K) = -2056.4 ± 5.0 kJ/mol. Solubility data have been used to deduce the conditions under which the mineral can form as a thermodynamically stable phase. The single-crystal X-ray structure of synthetic klebelsbergite has been determined 293 K and is essentially the same as that reported earlier for atoms with Z ≥ 8. Crystal data: orthorhombic, space group Pca21, a = 5.7563(4), b = 11.2538(7), c = 14.8627(9) Å, V = 962.81(11) Å3, Z = 4. Refinement converged to R1 = 0.0154 for 2206 unique reflections with I > 2σ(I). The present study has located the hydroxyl H atoms on both O5 and O9. The H-bond arrangements are somewhat different to those proposed earlier with the quasi-linear O9-H···O3 interaction having <(DHA) = 171(6)°. The O5 hydroxyl H atom lies in a different position to that proposed earlier and is involved in a bifurcated H-bond arrangement with O2 and with itself in a symmetry-related position, with <(DHA) = 133(5) and 125(5)°, respectively.

Ab-initio phasing using nanocrystal shape transforms with incomplete unit cells.

X-ray free electron lasers are used in measuring diffraction patterns from nanocrystals in the 'diffract-before-destroy' mode by outrunning radiation damage. The finite-sized nanocrystals provide an opportunity to recover intensity between Bragg spots by removing the modulating function that depends on crystal shape, i.e. the transform of the crystal shape. This shape-transform dividing-out scheme for solving the phase problem has been tested using simulated examples with cubic crystals. It provides a phasing method which does not require atomic resolution data, chemical modification to the sample, or modelling based on the protein databases. It is common to find multiple structural units (e.g. molecules, in symmetry-related positions) within a single unit cell, therefore incomplete unit cells (e.g. one additional molecule) can be observed at surface layers of crystals. In this work, the effects of such incomplete unit cells on the 'dividing-out' phasing algorithm are investigated using 2D crystals within the projection approximation. It is found that the incomplete unit cells do not hinder the recovery of the scattering pattern from a single unit cell (after dividing out the shape transforms from data merged from many nanocrystals of different sizes), assuming that certain unit-cell types are preferred. The results also suggest that the dynamic range of the data is a critical issue to be resolved in order to apply the shape transform method practically.

Dipole relaxation and proton transport in polycrystalline γ-cyclodextrin hydrate: A dielectric spectroscopy study

The polycrystalline γ-cyclodextrin hydrate (γ-CD·12.2H 2O) has been investigated via dielectric spectroscopy over a frequency range of 0–100 kHz and the temperature ranges of 108.0–298.5 K (cooling) and 109.0–433.0 K (heating). At T < 250.0 K, the electrical properties of the sample accept a great contribution from the flip-flop proton orientational disorder and a much lesser one from the positional fluctuations of the water molecules. Moreover, a strong synergy is observed between the stability of the γ-CD molecules and the dynamic disorder of the infinite flip-flop chains. This type of disorder disappears upon cooling ( T trans = 186.7 K) and reappears upon heating ( T trans = 194.5 K). At T > 250.0 K, the dielectric permittivity ε′ and loss ε″ increase abruptly due to the proton dc-conductance of γ-CD·12.2H 2O which has been interpreted in terms of a theoretical model (Pnevmatikos, 1988) being consistent with the generation of ionic defects and their combination with the dipole reorientations in a collective motion of soliton-type. The influence of the simultaneous dehydration process on this charge transport mechanism relies on the very sensitive balance between the diffusive motion of water molecules (exchange between symmetry related positions) and their removal from the crystal lattice. The Arrhenius semiconductive behavior of the ac-conductivity in the ranges of 257.1–313.2 K ( E a = 0.42 eV) and 331.2–385.1 K ( E a = 0.39 eV) implies the dominance of water diffusion which conserves the structural integrity of the endless hydrogen-bonded chains and the proton transfer along them. The limited decrease of the ac-conductivity from 313.2 to 331.2 K along with its rapid decrease above 385.1 K, indicates that the removal of the water molecules rules out their diffusive motion. The Cole–Cole diagrams ( ε″ vs. ε′) make clear that during the heating process the grain boundary polarization gradually becomes more significant than the grain interior one. In the range of 348.0–385.1 K, the constrictive grain boundary resistances are totally eliminated allowing the extensive proton transport through the grains of the polycrystalline specimen.

Functionalizable Collagen Model Peptides

The functionalizability and conformational properties of azidoproline (Azp)-containing collagen model peptides (CMPs) were studied. The results show that (4R)Azp has a similar stabilizing effect on the collagen triple helix as (4R)hydroxyproline and that functionalized CMPs are readily accessible by "click" chemistry. The resulting triazole-functionalized CMPs form stable triple helices, demonstrating that sterically demanding moieties in three symmetry-related positions in all strands are tolerated. The straightforward synthesis and facile functionalizability of the Azp-containing CMPs are intriguing for the development of functional collagen-based materials.

A mechanism of a phase transition based on a new symmetry and structure of bis(thiourea) thiazolium bromide

A redetermination of the crystal structure of bis(thiourea) thiazolium bromide (hereafter T 2ThBr) at 250 K (HT) and 100 K (LT) points to different space group symmetries than the ones reported in [A. Pajzderska, J. Wąsicki, H. Małuszyńska, P. Czarnecki, L. Toupet, E. Collet, J. Chem. Phys. 129 (2008) 104501]. Our structure determination of T 2ThBr shows that the two phases are centrosymmetric, Cmcm and Pmcn, and that the guest is disordered over at least four symmetry related positions in the HT form and over at least two positions in the LT form. The thiourea molecule is disordered over two positions independent from the temperature. A new model of the disordered structure is presented together with a new classification and a mechanism of the phase transition.

A polymorph of tetrakis(acetonitrile-κN)copper(I) tetrafluoridoborate

A P2(1)2(1)2(1) polymorph of the title compound, [Cu(CH(3)CN)(4)]BF(4), is reported. The crystal structure is very similar to the structure of the Pna2(1) polymorph reported by Jones & Crespo [Acta Cryst. (1998), C54, 18-20]. The anions and one of the three independent cations occupy similar positions in both polymorphs. Two of the four symmetry-related positions of the other two cations are also identical in the two polymorphs, and the other two positions are related by mirror symmetry. The crystal used for the structure determination contained a volume fraction of 0.088 (7) of the Pna2(1) polymorph.

EPR, ENDOR, and Special TRIPLE measurements of P•+ in wild type and modified reaction centers from Rb. sphaeroides

The influence of the protein environment on the primary electron donor, P, a bacteriochlorophyll a dimer, of reaction centers from Rhodobacter sphaeroides, has been investigated using electron paramagnetic resonance and electron nuclear double resonance spectroscopy. These techniques were used to probe the effects on P that are due to alteration of three amino acid residues, His L168, Asn L170, and Asn M199. The introduction of Glu at L168, Asp at L170, or Asp at M199 changes the oxidation/reduction midpoint potential of P in a pH-dependent manner (Williams et al. (2001) Biochemistry 40, 15403-15407). For the double mutant His L168 to Glu and Asn at L170 to Asp, excitation results in electron transfer along the A-side branch of cofactors at pH 7.2, but at pH 9.5, a long-lived state involving B-side cofactors is produced (Haffa et al. (2004) J Phys Chem B 108, 4-7). Using electron paramagnetic resonance spectroscopy, the mutants with alterations of each of the three individual residues and a double mutant, with changes at L168 and L170, were found to have increased linewidths of 10.1-11.0 G compared to the linewidth of 9.6 G for wild type. The Special TRIPLE spectra were pH dependent, and at pH 8, the introduction of aspartate at L170 increased the spin density ratio, rho (L)/rho (M), to 6.1 while an aspartate at the symmetry related position, M199, decreased the ratio to 0.7 compared to the value of 2.1 for wild type. These results indicate that the energy of the two halves of P changes by about 100 meV due to the mutations and are consistent with the interpretation that electrostatic interactions involving these amino acid residues contribute to the switch in pathway of electron transfer.

Diaqua(2,9-dimethyl-1,10-phenanthroline-κ2N,N′)(3-hydroxybenzoato-κ2O,O′)cobalt(II) nitrate

In the symmetric unit of the title compound, [Co(C7H5O3)(C14H12N2)(H2O)2]NO3, the Co2+ cation is coordinated by a bidentate 2,9-dimethyl-1,10-phenanthroline (dmphen) ligand, a bidentate 3-hydroxy­benzoate anion and two water mol­ecules in a distorted octa­hedral environment. The 3-hydroxy­benzoate ligand, Co atom and nitrate anion are situated on a crystallographic twofold rotation axis. The OH group of the benzoate anion is disordered over two symmetry-related positions with site-occupancy factors of 0.5. An extensive series of O—H⋯O hydrogen bonds, involving water mol­ecules and 3-hydroxy­benzoate and nitrate anions, and weak C—H⋯O hydrogen bonds lead to a supra­molecular network structure.

Redesigning symmetry‐related “mini‐core” regions of FGF‐1 to increase primary structure symmetry: Thermodynamic and functional consequences of structural symmetry

Previous reports detailing mutational effects within the hydrophobic core of human acidic fibroblast growth factor (FGF-1) have shown that a symmetric primary structure constraint is compatible with a stably folded protein. In the present report, we investigate symmetrically related pairs of buried hydrophobic residues in FGF-1 (termed "mini-cores") that are not part of the central core. The effect upon the stability and function of FGF-1 mutations designed to increase primary structure symmetry within these "mini-core" regions was evaluated. At symmetry-related positions 22, 64, and 108, the wild-type protein contains either Tyr or Phe side chains. The results show that either residue can be readily accommodated at these positions. At symmetry-related positions 42, 83, and 130, the wild-type protein contains either Cys or Ile side chains. While positions 42 and 130 can readily accommodate either Cys or Ile side chains, position 83 is substantially destabilized by substitution by Ile. Tertiary structure asymmetry in the vicinity of position 83 appears responsible for the inability to accommodate an Ile side chain at this position, and is known to contribute to functional half-life. A mutant form of FGF-1 with enforced primary structure symmetry at positions 22, 64, and 108 (all Tyr) and 42, 83, and 130 (all Cys) is shown to be more stable than the reference FGF-1 protein. The results support the hypothesis that a symmetric primary structure within a symmetric protein superfold represents a solution to achieving a foldable, stable polypeptide, and highlight the role that function may play in the evolution of asymmetry within symmetric superfolds.

Crystallization and preliminary analysis of a dsDNA bacteriophage capsid intermediate: Prohead II of HK97

HK97 Prohead II is an early intermediate in the maturation of HK97, a T = 7 dsDNA-tailed bacteriophage related to bacteriophage lambda. Previously, selected capsid-protein genes of HK97 were expressed in Escherichia coli and spontaneously assembled to form an icosahedral capsid that followed a maturation pathway closely similar to the authentic virion. The crystal structure of the mature HK97 capsid (Head II) made in this way was reported at 3.5 A resolution. Additional high-resolution structures of intermediates are needed to understand the maturation mechanism. The crystal structure of expressed Prohead II will elucidate the early steps of HK97 assembly. Crystals of the Prohead II mutant W336F were grown in 0.1 M HEPES pH 7.5, 0.2 M CaCl(2) and 2-3% PEG 4000 at a Prohead II concentration of 16.5 mg ml(-1). It was not possible to grow high-quality crystals of wild-type Prohead II. Diffraction was observed to 5 A resolution from these crystals on beamline 14BM-C at the Advanced Photon Source and data were collected to 5.5 A with a completeness of 77%. The space group was P2(1)3, with unit-cell parameter a = 707.0 A and four particles in the unit cell. The particles are on the body diagonals of the cubic cell, with icosahedral threefold axes coincident with crystallographic threefold axes. Self-rotation function and locked-rotation function analysis determined the particle orientation and a one-dimensional R-factor search along the body diagonal indicated that the particle centers were close to (1/4, 1/4, 1/4) and symmetry-related positions. Molecular-replacement averaging and phase extension are under way.