Physical Sciences and Mathematics Commons^™

Inhibition Of The Aminopeptidase From Aeromonas Proteolytica By L-Leucinephosphonic Acid. Spectroscopic And Crystallographic Characterization Of The Transition State Of Peptide Hydrolysis, Carin Stamper, Brian Bennett, Tanya Edwards, Richard Holz, Dagmar Ringe, Gregory Petsko

Richard C. Holz

The nature of the interaction of the transition-state analogue inhibitor l-leucinephosphonic acid (LPA) with the leucine aminopeptidase from Aeromonas proteolytica (AAP) was investigated. LPA was shown to be a competitive inhibitor at pH 8.0 with a K_i of 6.6 μM. Electronic absorption spectra, recorded at pH 7.5 of [CoCo(AAP)], [CoZn(AAP)], and [ZnCo(AAP)] upon addition of LPA suggest that LPA interacts with both metal ions in the dinuclear active site. EPR studies on the Co(II)-substituted forms of AAP revealed that the environments of the Co(II) ions in both [CoZn(AAP)] and [ZnCo(AAP)] become highly asymmetric and constrained upon the addition of …

Analyzing The Catalytic Mechanism Of The Fe-Type Nitrile Hydratase From Comamonas Testosteroni Ni1, Saroja Rao, Richard Holz

Richard C. Holz

In order to gain insight into the catalytic mechanism of Fe-type nitrile hydratases (NHase), the pH and temperature dependence of the kinetic parameters k_cat, K_m, and k_cat/K_m along with the solvent isotope effect were examined for the Fe-type NHase from Comamonas testosteroni Ni1 (CtNHase). CtNHase was found to exhibit a bell-shaped curve for plots of relative activity vs pH over pH values 4−10 for the hydration of acrylonitrile and was found to display maximal activity at pH ∼7.2. Fits of these data provided a pK_ES1 value of …

Function Of The Signal Peptide And N- And C-Terminal Propeptides In The Leucine Aminopeptidase From Aeromonas Proteolytica, Krzysztof Bzymek, Ventris D'Souza, Guanjing Chen, Heidi Campbell, Alice Mitchell, Richard Holz

Richard C. Holz

The leucine aminopeptidase from Aeromonas proteolytica (also known as Vibrio proteolyticus) (AAP) is a metalloenzyme with broad substrate specificity. The open reading frame (ORF) for AAP encodes a 54 kDa enzyme, however, the extracellular enzyme has a molecular weight of 43 kDa. This form of AAP is further processed to a mature, thermostable 32 kDa form but the exact nature of this process is unknown. Over-expression of different forms of AAP in Escherichia coli (with AAP's native leader sequence, with and without the N- and/or C-terminal propeptides, and as fusion protein) has allowed a model for the processing of wild-type …

Divalent Metal Binding Properties Of The Methionyl Aminopeptidase From Escherichia Coli, Ventris D'Souza, Brian Bennett, Alicja Copik, Richard Holz

Richard C. Holz

The metal-binding properties of the methionyl aminopeptidase from Escherichia coli (MetAP) were investigated. Measurements of catalytic activity as a function of added Co(II) and Fe(II) revealed that maximal enzymatic activity is observed after the addition of only 1 equiv of divalent metal ion. Based on these studies, metal binding constants for the first metal binding event were found to be 0.3 ± 0.2 μM and 0.2 ± 0.2 μM for Co(II)- and Fe(II)-substituted MetAP, respectively. Binding of excess metal ions (>50 equiv) resulted in the loss of ∼50% of the catalytic activity. Electronic absorption spectral titration of a 1 …

The Proteomics Of N-Terminal Methionine Cleavage, Frédéric Frottin, Aude Martinez, Philippe Peynot, Sanghamitra Mitra, Richard Holz, Carmela Giglione, Thierry Meinnel

Richard C. Holz

Methionine aminopeptidase (MAP) is a ubiquitous, essential enzyme involved in protein N-terminal methionine excision. According to the generally accepted cleavage rules for MAP, this enzyme cleaves all proteins with small side chains on the residue in the second position (P1′), but many exceptions are known. The substrate specificity of Escherichia coli MAP1 was studied in vitro with a large (>120) coherent array of peptides mimicking the natural substrates and kinetically analyzed in detail. Peptides with Val or Thr at P1′ were much less efficiently cleaved than those with Ala, Cys, Gly, Pro, or Ser in this position. Certain residues …

Unraveling The Catalytic Mechanism Of Nitrile Hydratases, Sanghamitra Mitra, Richard Holz

Richard C. Holz

To elucidate a detailed catalytic mechanism for nitrile hydratases (NHases), the pH and temperature dependence of the kinetic constants kcat and Km for the cobalt-type NHase from Pseudonocardia thermophila JCM 3095 (PtNHase) were examined. PtNHase was found to exhibit a bell-shaped curve for plots of relative activity versus pH at pH 3.2–11 and was found to display maximal activity between pH 7.2 and 7.8. Fits of these data provided pKES1 and pKES2 values of 5.9 ± 0.1 and 9.2 ± 0.1 (kcat′ = 130 ± 1 s-1), respectively, and pKE1 and pKE2 values of 5.8 ± 0.1 and 9.1 ± …

Proton Nmr Investigation Of The [4fe-4s]1+ Cluster Environment Of Nitrogenase Iron Protein From Azotobacter Vinelandii: Defining Nucleotide-Induced Conformational Changes, William Lanzilotta, Richard Holz, Lance Seefeldt

Richard C. Holz

This work presents the complete assignment of the isotropically shifted 1H NMR resonances of Azotobacter vinelandii nitrogenase iron protein (Fe protein) to β-CH2 and α-CH protons of the [4Fe- 4S]1+ cluster cysteinyl ligands. Four resonances were observed for the reduced Fe protein with chemical shifts of 49, 23, 17, and 13 ppm. T1 measurements and analysis of relative peak areas coupled with one-dimensional nuclear Overhauser effect (NOE) difference spectra were used to assign the two most downfield-shifted resonances (49 and 23 ppm) to cysteinyl ligand β-CH2 protons and the 17 and 14 ppm resonances to cysteinyl ligand α-CH protons. Temperature …

Hydrolysis Of Thionopeptides By The Aminopeptidase From Aeromonas Proteolytica:  Insight Into Substrate Binding, David Bienvenue, Danuta Gilner, Richard Holz

Richard C. Holz

A series of l-leucine aniline analogues were synthesized that contained either a carbonyl or thiocarbonyl as a part of the amide bond. Additionally, the para-position on the phenyl ring of several substrates was altered with various electron-withdrawing or donating groups. The kinetic constants Km and kcat were determined for the hydrolysis of each of these compounds in the presence of the aminopeptidase from Aeromonas proteolytica (AAP) containing either Zn(II) or Cd(II). The dizinc(II) form of AAP ([ZnZn(AAP)]) was able to cleave both carbonyl and thiocarbonyl containing peptide substrates with similar efficiency. However, the dicadmium(II) form of AAP ([CdCd(AAP)]) was unable …

Electrochemical Attachment Of Motile Bacterial Cells To Gold, Sergey Rozhok, Richard Holz

Richard C. Holz

Selective attachment of Escherichia coli K-12 bacterial cells to charged gold surfaces was demonstrated. Electrostatic binding of E. coli K-12 bacterial cells to positively charged surfaces was observed starting at +750 mV. The binding of E. coli K-12 cells to positively charged gold surfaces is proposed to occur due to long-range electrostatic interactions between the negatively charged O-chain of lipopolysaccharide (LPS) molecules protruding the bacterial cell body and the electrode surface. Removing LPS alters the cellular surface charge and results in cellular attachment to negatively charged surfaces. Thus, applying an electrical potential allows for the direct, real time detection of …

Synthesis, Molecular Structure, And Reactivity Of Dinuclear Copper(Ii) Complexes With Carboxylate-Rich Coordination Environments, Richard Holz, John Bradshaw, Brian Bennett

Richard C. Holz

The dinucleating ligand N,N‘-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-(carboxymethyl)glycine) (CH₃HXTA) has been used to synthesize the dinuclear Cu(II) bis(pyridine) complex Na[Cu₂(CH₃HXTA)(Py)₂]·1.5(1,4-dioxane) (Na(1)):  triclinic space group P1̄ (a = 12.550(3) Å, b = 13.413(3) Å, c = 13.540(4) Å, α = 117.12(2)°, β = 104.70 (2)°, and γ = 92.13(2)°). The structure shows two distinct distorted square pyramidal Cu(II) centers with each Cu(II) ion bound by two carboxylate oxygen atoms, one amine nitrogen atom, a phenolate oxygen atom, and one pyridine nitrogen atom. The Cu--Cu separation is 3.531 Å, …

Structurally Distinct Active Sites In The Copper(Ii)-Substituted Aminopeptidases From Aeromonas Proteolytica And Escherichia Coli, Brian Bennett, William E. Antholine, Ventris M. D'Souza, Guanjing Chen, Leila Ustinyuk, Richard C. Holz

Richard C. Holz

The aminopeptidase from Aeromonas proteolytica (AAP) was titrated with copper, which bound sequentially at two distinct sites. Both the mono- and disubstituted forms of AAP exhibited catalytic hyperactivity relative to the native dizinc enzyme. Monosubstituted AAP exhibited an axial Cu(II) EPR spectrum with slight pH dependence:  at pH 6.0 g|| = 2.249, g⊥ = 2.055, and A||(63/65Cu) = 1.77 × 10-2 cm-1, whereas at pH 9.65 g|| = 2.245, g⊥ = 2.056, and A||(63/65Cu) = 1.77 × 10-2 cm-1. These data indicate oxygen and nitrogen ligation of Cu. AAP further substituted with copper exhibited a complex signal with features around …

Proton Nmr Spectroscopy As A Probe Of Dinuclear Copper(Ii) Centers, Richard Holz, Julie Brink

Richard C. Holz

No abstract provided.

Dinuclear Copper(Ii) Complexes With Carboxylate-Rich Coordination Environments. Models For Substituted Copper(Ii) Aminopeptidases, Richard Holz, Julie Brink, Feben Gobena, Charles O'Connor

Richard C. Holz

The dinucleating ligand N,N'-(2-hydroxy-5-methyl1-, 3-xylylene)bis(N-carboxymethylglycine) (CH₃HXTA) has been used to synthesize the dinuclear Cu(II) diaqua complex [Cu₂(CH₃HXTA) (H₂O)₂]H•4H₂O (1): monoclinic space group P2₁In (a = 15.092(3) Å, b = 10.842(3) Å, c = 15.219(4) Å, and β = 104.70(2)°). The structure shows two distinct square pyramidal Cu(II) centers with each Cu(II) ion bound to two carboxylate oxygen atoms, one amine nitrogen atom, the phenolate oxygen atom, and one water oxygen atom. The Cu- -Cu separation is 3.726 Å and Cu1-O1-Cu2 angle …

Structures And Properties Of Dibridged (Μ-Oxo)Diiron(Iii) Complexes. Effects Of The Fe-O-Fe Angle, Richard Norman, Richard Holz, Stephane Menage, Charles O'Connor, Jian Zhang, Lawrence Que

Richard C. Holz

A series of (μ-oxo)diiron(lll) complexes of tris(2-pyridylmethy1)amine (TPA), [Fe2(TPA)2O(L)] (CIO4)n, were synthesized and characterized where L represents the bridging ligands carbonate, hydrogen maleate, diphenyl phosphate, diphenylphosphinate, maleate, and phthalate. Together with the linear dichloride complex, this series of compounds provides a unique opportunity to systematically study the effects of the Fe-0-Fe angle (125-180°) on the electronic spectral and magnetic properties of the (μ-oxo)diiron(lll) core. [Fe2(TPA)2O(CO3)](CIO4)2.2CH3OH (1) crystallizes in the monoclinic space group P21/c with a = 11.282 (7) Å, b = 18.253 (9) Å, c = 20.390 (7) Å, and β= 95.02 (4)°. The structure was determined at -50°C from …

Attachment Of Motile Bacterial Cells To Prealigned Holed Microarrays, Sergey Rozhok, Zhifang Fan, Dorjderem Nyamjav, Chang Liu, Chad Mirkin, Richard Holz

Richard C. Holz

Construction of biomotors is an exciting area of scientific research that holds great promise for the development of new technologies with broad potential applications in areas such as the energy industry and medicine. Herein, we demonstrate the fabrication of prealigned microarrays of motile Escherichia coli bacterial cells on SiOx substrates. To prepare these arrays, holed surfaces with a gold layer on the bottom of the holes were utilized. The attachment of bacteria to the holes was achieved via nonspecific interactions using poly-l-lysine hydrobromide (PLL). Our data suggest that a single motile bacterial cell can be selectively attached to an individual …

Two-Dimensional 1H Nmr Studies On Octahedral Nickel(Ii) Complexes, Richard Holz, Evgenij Evdokimov, Feben Gobena

Richard C. Holz

The dinucleating ligand ethylene glycol−bis(β-aminoethyl ether) N,N,N‘,N‘-tetrakis[(2-(1-ethylbenzimidazoyl)] (EGTB-Et; 1) was used to synthesize the dinuclear Ni(II) tetraacetonitrile complex cation [Ni2(EGTB-Et)(CH3CN)4]2+ (2):  triclinic space group P1̄ (a = 12.273(5) Å, b = 12.358(7) Å, c = 12.561(6) Å, α = 90.43(4)°, β = 110.26(3)°, γ = 99.21 (4)°, and Z = 1). The structure shows two identical octahedral Ni(II) centers each bound to two benzimidazole ring nitrogen atoms, one amine nitrogen atom, an ether oxygen atom, and two acetonitrile nitrogen atoms. The Ni(II) ions are tethered together by a diethyl ether linkage with a crystallographic center of inversion between the methylene …

Arge-Encoded N-Acetyl-L-Ornithine Deacetylase From Escherichia Coli Contains A Dinuclear Metalloactive Site, Wade C. Mcgregor, Sabina I. Swierczek, Brian Bennett, Richard C. Holz

Richard C. Holz

The catalytic and structural properties of the argE-encoded N-acetyl-l-ornithine deacetylase (ArgE) from Escherichia coli were investigated. On the basis of kinetic and ITC (isothermal titration calorimetry) data, Zn(II) binds to ArgE with Kd values that differ by ∼20 times. Moreover, ArgE exhibits ∼90% of its full catalytic activity upon addition of one metal ion. Therefore, ArgE behaves similarly to the aminopeptidase from Aeromonas proteolytica (AAP) in that one metal ion is the catalytic metal ion while the second likely plays a structural role. The N-acetyl-l-ornithine (NAO) deacetylase activity of ArgE showed a linear temperature dependence from 20 to 45 °C, …

Proton Nuclear Magnetic Resonance Investigation Of The [2fe-2s]1--Containing “Rieske-Type” Protein From Xanthobacter Strain Py2, Richard Holz, Frederick Small, Scott Ensign

Richard C. Holz

Proton NMR spectra of the Rieske-type ferredoxin from Xanthobacter strain Py2 were recorded in both H2O and D2O buffered solutions at pH 7.2. Several well-resolved hyperfine-shifted 1H NMR signals were observed in the 90 to −20 ppm chemical shift range. Comparison of spectra recorded in H2O and D2O buffered solutions indicated that the signals at −11.4 (L) and −15.5 (M) ppm were solvent-exchangeable and thus were assigned to the two histidine Nε2H protons. The remaining observed signals were assigned based upon chemical shift, T1 values, and one-dimensional nuclear Overhauser effect (nOe) saturation transfer experiments to either CβH or CαH protons …

Spectroscopic Characterization Of Some Rare Earth Complexes Of Triethylenetetraaminehexaacetic Acid, Richard Holz, William Horrocks

Richard C. Holz

The Eu3+ complexes formed with triethylenetetraaminehexaacetic acid (TTHA) were characterized in solution by laser-induced Eu3+ luminescence spectroscopy. Both 2:1 and 1:1 metal:ligand complexes were detected at pH 6.0 depending on the metal- ligand stoichiometry. These two species exhibit distinct excited-state lifetimes of 260 and 1210 μs, respectively. The 2:1 complex coordinates three or four water molecules at each Eu3+ ion while the 1:1 complex has no coordinated water molecules. The 7F0→5D0 excitation spectra revealed multiple isomeric forms for both the 2:1 and 1:1 complexes. Spectra and excited-state lifetimes were taken as a function of pH for a 1:1 metal:ligand solution. …

Yttrium-89 Nmr Spectroscopy, A New Probe For Calcium-Binding Proteins, Richard Holz, William Horrocks

Richard C. Holz

No abstract provided.

Molecular Discrimination Of Type-I Over Type-Ii Methionyl Aminopeptidases, Krzysztof Swierczek, Alicja Copik, Sabina Swierczek, Richard Holz

Richard C. Holz

Two residues that are conserved in type-I methionyl aminopeptidases (MetAPs) but are absent in all type-II MetAPs are the cysteine residues (Escherichia coli MetAP-I:   C59 and C70) that reside at the back of the substrate recognition pocket. These Cys residues are 4.4 Å apart and do not form a disulfide bond. Since bacteria and fungi contain only type-I MetAPs while all human cells contain both type-I and type-II MetAPs, type-I MetAPs represent a novel antibiotic/antifungal target if type-I MetAPs can be specifically targeted over type-II. Based on reaction of the thiol-specific binding reagent 5,5‘-dithio-bis(2-nitrobenzoic acid) (DTNB) with the type-I MetAP …

Zinc Aminopeptidases, Aminopeptidase From Vibrio Proteolyticus (Aeromonas Proteolytica) As Prototypical Enzyme, Richard Holz, Anna Starus, Danuta Gillner

Richard C. Holz

No abstract provided.

Bayesian Decision Theoretic Approach To Directional Multiple Hypotheses Problems, Naveen K. Bansal, Klaus J. Miescke

Naveen Bansal

A multiple hypothesis problem with directional alternatives is considered in a decision theoretic framework. Skewness in the alternatives is considered, and it is shown that this skewness permits the Bayes rules to possess certain advantages when one direction of the alternatives is more important or more probable than the other direction. Bayes rules subject to constraints on certain directional false discovery rates are obtained, and their performances are compared with a traditional FDR rule through simulation. We also analyzed a gene expression data using our methodology, and compare the results to that of a FDR method.

Observation Of Antiferromagnetic Correlations In The Hubbard Model With Ultracold Atoms, Russell Hart, Pedro Duarte, Tsung-Lin Yang, Xinxing Liu, Thereza Paiva, Ehsan Khatami, Richard Scalettar, Nandini Trivedi, David Huse, Randall Hulet

Faculty Publications

Ultracold atoms in optical lattices have great potential to contribute to a better understanding of some of the most important issues in many-body physics, such as high-temperature superconductivity. The Hubbard model—a simplified representation of fermions moving on a periodic lattice—is thought to describe the essential details of copper oxide superconductivity. This model describes many of the features shared by the copper oxides, including an interaction-driven Mott insulating state and an antiferromagnetic (AFM) state. Optical lattices filled with a two-spin-component Fermi gas of ultracold atoms can faithfully realize the Hubbard model with readily tunable parameters, and thus provide a platform for …

Spitzer Space Telescope Measurements Of Dust Reverberation Lags In The Seyfert 1 Galaxy Ngc 6418, Billy Vazquez, Pasquale Galianni, Michael Richmond, Andrew Robinson, David J. Axon, Keith Horne, Triana Almeyda, Michael Fausnaugh, Bradley M. Peterson, Mark Bottorff, Jack Gallimore, Moshe Elitzur, Hagai Netzer, Thaisa Storchi-Bergmann, Alessandro Marconi, Alessandro Capetti, Dan Batcheldor, Catherine Buchanan, Giovanna Stirpe, Makoto Kishimoto, Christopher Packham, Enrique Perez, Clive Tadhunter, John Upton, Vincente Estrada-Carpenter

Physics and Astronomy Faculty Publications

We present results from a 15 month campaign of high-cadence (~3 days) mid-infrared Spitzer and optical (B and V) monitoring of the Seyfert 1 galaxy NGC 6418, with the objective of determining the characteristic size of the dusty torus in this active galactic nucleus (AGN). . . .

For the remainder of the abstract, please visit:

http://dx.doi.org/10.1088/0004-637X/801/2/127

Dual-Sensor Fluorescent Probes Of Surfactant-Induced Unfolding Of Human Serum Albumin, Christopher J. Abelt

Arts & Sciences Articles

Two extrinsic fluorescent probes, 3-(dimethylamino)-8,9,10,11-tetrahydro-7H-cyclohepta[a]naphthalen-7-one (1) and 7-(dimethylamino)-2,3-dihydrophenanthren-4(1H)-one (2), are used to probe the unfolding of human serum albumin by sodium dodecyl sulfate (SDS). These probes respond separately to the polarity and H-bond-donating ability of their surroundings. Competitive binding experiments show that fluorophore 1 binds to site I (domain IIA) and 2 binds to site II (domain IIIA). The local acidity of 1 in site I is out of the sensing range of 1, whereas the local acidity of 2 in site II is calculated to be nearly zero on Catalans solvent acidity index. Both probes show that the first …

Historical Common Names Of Great Plains Plants, With Scientific Names Index. Volume Ii: Scientific Names Index, Elaine Nowick

Zea E-Books Collection

Containing thousands of entries of both vernacular and scientific names of Great Plains plants, the literature that informs this exhaustive listing spans nearly 300 years. Author Elaine Nowick has drawn from sources as diverse as Linnaeus, Lewis and Clark, and local university extension publications to compile the gamut of practical, and often fanciful, common plant names used over the years. Each common name is accompanied by a definitive scientific name with references and authority information. Interspersed with scientifically-correct botanical line drawings, the entries are written in standard ICBN format, making this a useful volume for scholars as well as lay …

Historical Common Names Of Great Plains Plants, With Scientific Names Index. Volume I: Common Names, Elaine Nowick

Zea E-Books Collection

Containing thousands of entries of both vernacular and scientific names of Great Plains plants, the literature that informs this exhaustive listing spans nearly 300 years. Author Elaine Nowick has drawn from sources as diverse as Linnaeus, Lewis and Clark, and local university extension publications to compile the gamut of practical, and often fanciful, common plant names used over the years. Each common name is accompanied by a definitive scientific name with references and authority information. Interspersed with scientifically-correct botanical line drawings, the entries are written in standard ICBN format, making this a useful volume for scholars as well as lay …

Spectroscopic Confirmation Of An Ultramassive And Compact Galaxy At Z = 3.35: A Detailed Look At An Early Progenitor Of Local Giant Ellipticals, Z. Cemile Marsan, Danilo Marchesini, Gabriel B. Brammer, Mauro Stefanon, Adam Muzzin, Alberto Fernández-Soto, Stefan Geier, Kevin N. Hainline

Dartmouth Scholarship

We present the first spectroscopic confirmation of an ultra-massive galaxy at redshift z>3 using data from Keck-NIRSPEC, VLT-Xshooter, and GTC-Osiris. We detect strong [OIII] and Lyα emission, and weak [OII], CIV, and HeII, placing C1-23152 at a spectroscopic redshift of zspec=3.351. The modeling of the emission-line corrected spectral energy distribution results in a best-fit stellar mass of M∗=3.1+0.6−0.7×1011M⊙, a star-formation rate of <7 M⊙yr−1, and negligible dust extinction. The stars appear to have formed in a short intense burst ~300-500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z~4.1. From the analysis of the line ratios and widths, and the observed flux at 24μm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of ~1046ergs−1. Potential contamination to the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of 2×1011M⊙. HST/WFC3 H160 and ACS I814 images are modeled, resulting in an effective radius of re~1 kpc in the H160 band and a Sersic index n~4.4. This object may be a prototype of the progenitors of local most massive elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of its stars at z>4 in a highly dissipative, intense, and short burst of star formation. C1-23152 is completing its transition to a post-starburst phase while hosting a powerful AGN, potentially responsible for the quenching of the star formation activity.

Spectroscopic Orbital Periods For 29 Cataclysmic Variables From The Sloan Digital Sky Survey, John R. Thorstensen, Cynthia J. Taylor, Christopher S. Peters, Julie N. Skinner

Dartmouth Scholarship

We report follow-up spectroscopy of 29 cataclysmic variables from the Sloan Digital Sky Survey (SDSS), 22 of which were discovered by SDSS and seven of which are previously known systems that were recovered in SDSS. The periods for 16 of these objects were included in the tabulation by Gänsicke et al. While most of the systems have periods less than 2 hr, only one has a period in the 80–86 minutes "spike" found by Gänsicke et al., and 11 have periods longer than 3 hr, indicating that the present sample is skewed toward longer-period, higher-luminosity objects. Seven of the objects …