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At least 505 records · Page 28Linked to original sources
PubMed2001

Pars plana vitreous tap for phacoemulsification in the crowded eye.

This technique facilitates phacoemulsification and foldable intraocular lens (IOL) implantation in eyes with extremely crowded anterior segments. An automated pars plana vitreous tap is used to expand the anterior segment when extremely shallow anterior chambers do not deepen sufficiently with viscoelastic injection alone. This technique permits successful completion of pupilloplasty, capsulorhexis, phacoemulsification, and foldable IOL implantation in these high-risk eyes.

Aged↗
PubMed1995

Treatment of a crowded Class II malocclusion with significant maxillary incisor protrusion.

A case report of a 10-year-old girl with a skeletal and dental Class II, Division 1 malocclusion is presented. There was marked arch length deficiency in the mandibular arch and significant maxillary incisor protrusion. The patient was treated with maxillary premolar extractions and early maxillary treatment for incisor retraction. The treatment was completed nonsurgically with full treatment later and a single mandibular incisor extraction. The Class II malocclusion and crowding were resolved with Class II molar and Class I canine relationships and a positive profile change. [This case was presented to the American Board of Orthodontics in partial fulfillment of the requirements for the certification process conducted by the Board.]

Bicuspid↗
PubMed2004

Long-term outcome in a patient with Class I malocclusion with severe crowding treated without extractions.

The treatment of a patient with a Class I malocclusion with significant skeletal transverse maxillary discrepancy and severe crowding is presented. The maxilla was narrow compared with the mandible, and the posterior teeth were compensated, with the maxillary teeth inclined buccally and the mandibular teeth inclined lingually. Treatment included rapid palatal expansion to correct the transverse maxillary deficiency and to gain arch length. Long-term follow-up (20 years) with a stable result is reported.

Cephalometry↗
PubMed2001

Household crowding in early adulthood and schizophrenia are unrelated in Denmark: a nested case-control study.

A number of studies have found that being born in an urban area is a risk factor for developing schizophrenia. It has been hypothesized that increased exposure to infectious agents through household crowding might account for this association. Using Danish longitudinal registers, we have established a population-based sample of 191 cases of schizophrenia where the first admission occurred between 1981 and 1993. These cases were compared with 17413 individually matched controls of the same gender and age. Information regarding parents' and siblings' psychiatric history, urbanization, season and place of birth, and square meter per dweller were included in a conditional logistic regression model. We found square meter per dweller to be insignificant and without any trend when included as a risk factor for schizophrenia, whereas previous findings of schizophrenia associated with being born in an urban area and with schizophrenia in parents and siblings were replicated.

Adult↗
PubMed2001

High resolution 1H NMR spectroscopic studies of the composition of the haemolymph of crowd- and solitary-reared nymphs of the desert locust, Schistocerca gregaria.

High resolution 1H NMR spectroscopy provided a rapid and non-invasive means of establishing a multicomponent profile of the low molecular weight organic metabolites present in the haemolymph of crowd-reared and solitary-reared desert locusts. Assignments of the 1H NMR resonances detected in the one-dimensional (1D) spectra were supported by the comparison of chemical shifts with those of commonly found metabolites, the application of 2D NMR spectroscopy, and spiking of authentic standards into the sample. The haemolymph metabolite profile was found to comprise trehalose, a series of amino acids, organic acids, lipids, ethanol and the polyamine putrescine. In total, it was possible to detect simultaneously and potentially quantify over 20 endogenous compounds. An interesting difference between the phases was the higher putrescine concentrations present in solitary-reared nymphs, which may indicate a role of this compound in phase determination. In addition, differences in the concentrations of trehalose, lipids, acetate and ethanol were also observed between solitary-reared and gregarious-reared nymphs enabling the ready differentiation of the two phases based on the NMR spectra of the haemolymph.

Amino Acids↗
PubMed1995

Leeway space and the resolution of crowding in the mixed dentition.

The leeway space provides adequate space to resolve crowding that is present in the mixed dentition in the majority of individuals. This space can be maintained by preserving arch length with a lingual arch as the primary teeth begin to exfoliate, unless conditions such as the premature loss of a primary canine require earlier intervention. A lip bumper can also be inserted after the eruption of the first premolars to preserve arch length.

Child↗
PubMed2003

Patterns of the tapeworm Raillietina trapezoides infection in the fat sand rat Psammomys obesus in Tunisia: season, climatic conditions, host age and crowding effects.

The tapeworm Raillietina trapezoides was studied in a Tunisian population of the fat sand rat Psammomys obesus. Seasonal changes in the abundance of parasite and host were monitored in a longitudinal field survey lasting 20 months. In total 582 intestinal samples were collected during 10 trapping sessions between May 1995 and January 1997 and examined. The impact of abiotic and biotic factors on the prevalence and parasite burden were explored, using generalized linear models. R. trapezoides showed a seasonal pattern with most transmission occurring in summer and autumn. In August-September when the rodents were at low density and most adult (90-100%) were infected. In winter, infections with R. trapezoides were highly prevalent in the cohort of adults only. The worm burden by number was also high in late summer and autumn. Prevalence and parasite burden by number or by biomass were highly age dependent. Season and age act in synergy so that the rodents were highly infected in late summer and autumn. Unusually dry conditions in winter seemed to increase prevalence and to reduce the number of worms. There was no difference between males and females in prevalence or parasite burden (by number or by biomass). Prevalence was not correlated with the relative density of the hosts whereas parasite burden depended upon their densities. The more numerous the worms were, the smaller they were individually, reflecting a crowding effect. The possibility of an immune response regulating P. obesus is discussed, in regard to the very high natural prevalence and the variation in worm burden, and compared with other parasite-host systems.

Aging↗
PubMed1994

Effects of molecular crowding on protein self-association: a potential source of error in sedimentation coefficients obtained by zonal ultracentrifugation in a sucrose gradient.

Theoretical and experimental studies have illustrated a potential source of error in sedimentation coefficients obtained by sucrose density gradient centrifugation of proteins undergoing reversible self-association. The error stems from the excluded volume (molecular crowding) effect of the sucrose on the activity coefficients of monomeric and polymeric states. The consequent displacement of the equilibrium position in favor of polymeric state(s) is a function of sucrose concentration, and can therefore result in failure to detect the equilibrium coexistence of monomer if 5% sucrose suffices to displace the equilibrium completely toward dimer. In less extreme situations, it may result in the evaluation of an average sedimentation coefficient whose magnitude is a function of sucrose concentration and hence of the distance migrated into the sucrose gradient. These features are illustrated by the results of computer-simulated sedimentation of reversibly dimerizing systems in a sucrose gradient, and by conventional sedimentation velocity experiments on yeast enolase.

Centrifugation, Zonal↗
PubMed2001

Interpretation of the reversible inhibition of adenosine deaminase by small cosolutes in terms of molecular crowding.

Published results on the inhibitory effects of small cosolutes on adenosine deamination by adenosine deaminase [Kurz, L. C., Weitkamp, E., and Frieden, C. (1987) Biochemistry 26, 3027-3032; Dzingeleski, G., and Wolfenden, R. (1993) Biochemistry 32, 9143-9147] have been reexamined. Results for sucrose, dioxane, methanol, and ethanol are shown to be qualitatively consistent with thermodynamic interpretation in terms of molecular crowding effects arising from the occurrence of a minor increase in enzyme volume and/or asymmetry during the kinetic reaction--a conformational transition that could be either preexisting or ligand induced. Direct evidence for the existence of the putative isomeric transition is provided by active enzyme gel chromatography on Sephadex G-100, which demonstrates a negative dependence of enzyme elution volume upon substrate concentration and is therefore consistent with substrate-mediated conformational changes that favor a larger (or more asymmetric) isomeric state of the enzyme. There are thus experimental grounds for adopting the present description of the inhibitory effects of unrelated cosolutes on the kinetics of adenosine deamination by adenosine deaminase in terms of thermodynamic nonideality.

Adenosine↗
PubMed2002

Crowding by trisaccharides and the 2:1 cytochrome c-cytochrome c peroxidase complex.

Proteins are designed to function under crowded conditions where the solute concentration can reach 400 g/L, but they are almost always studied in dilute solutions. To address this discrepancy, we have undertaken a series of studies to determine the effects of high solute concentrations on the thermodynamics of protein equilibria. Recently, we used isothermal titration calorimetry (ITC) to show that high concentrations of mono-, di-, and tetrasaccharides have a small stabilizing effect on the crystallographically defined cytochrome c binding site on yeast ferricytochrome c peroxidase [Morar, A. S., Wang, X., and Pielak, G. J. (2001) Biochemistry 40, 281-285]. Here, we use this technique to show that trisaccharides increase the apparent thermodynamic binding constants for both cytochrome c binding sites on the peroxidase. Mutagenesis studies confirm that the second site includes Asp 148 on the peroxidase. Binding of both cytochrome c molecules is exothermic. The data are interpreted by assuming either the presence or absence of intersite interactions.

Aspartic Acid↗
PubMed2002

Calorimetric demonstration of the potential of molecular crowding to emulate the effect of an allosteric activator on pyruvate kinase kinetics.

A method based on isothermal calorimetry is described for the direct kinetic assay of pyruvate kinase. In agreement with earlier findings based on the standard coupled assay system for this enzyme in the presence of a fixed ADP concentration, the essentially rectangular hyperbolic dependence of initial velocity upon phosphoenolpyruvate concentration is rendered sigmoidal by the allosteric inhibitor phenylalanine. This effect of phenylalanine can be countered by including a high concentration of a space-filling osmolyte such as proline in the reaction mixtures. This investigation thus affords a dramatic example that illustrates the need to consider potential consequences of thermodynamic nonideality on the kinetics of enzyme reactions in crowded molecular environments such as the cell cytoplasm.

Allosteric Regulation↗
PubMed2003

Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins.

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.

Crystallography, X-Ray↗
PubMed2005

Cd(II), Zn(II), and Pd(II) complexes of an isoindoline pincer ligand: consequences of steric crowding.

The sterically crowded isoindoline pincer ligand, 6'-MeLH, prepared by condensation of 4-methyl-2-aminopyridine and phthalonitrile, exhibits very different reaction chemistry with Cd2+, Zn2+, and Pd2+. Three different ligand coordination modes are reported, each dependent upon choice of metal ion. This isoindoline binds to Cd2+ as a charge-neutral, zwitterionic, bidentate ligand using imine and pyridine nitrogen atoms to form the eight-coordinate fluxional complex, Cd(6'-MeLH)2(NO3)2. In the presence of Zn2+, however, loss of a pyridine arm occurs through solvolysis and tetrahedrally coordinated complexes are formed with coordination of pyrrole and pyridine nitrogen atoms. Reaction with Pd2+ produces the highly distorted, square planar complex Pd(6'-MeL)Cl in which a deprotonated isoindoline anion coordinates as a tridentate pyridinium NNC pincer ligand.

Cadmium↗
PubMed2001

Synthesis of an extremely crowded naphthalene via a stable norbornadienone.

Computational studies at the HF/3-21G and B3LYP/6-31G(d) levels suggest that 5,6,8-tri(tert-butyl)-1,2,3,4-tetraphenylnaphthalene (2) is perhaps the most crowded naphthalene derivative that will show normal stability; more highly congested naphthalenes will prefer to exist as the corresponding Dewar isomers. Initial attempts to prepare 2 by reacting 3,4,5,6-tetraphenylbenzyne with 2,3,5-tri(tert-butyl)cyclopentadienone at 83 degrees C gave instead a stable norbornadienone, 1,2,4-tri(tert-butyl)-5,6,7,8-tetraphenyl-9-oxo-1,4-dihydro-1,4-methanonaphthalene (3), which was characterized by X-ray crystallography. The experimental and calculated (HF/3-21G) activation energies (E(a)) for the decarbonylation of 3 were quite high: 39 and 46 kcal/mol, respectively, a manifestation of the substantially increased strain in the transition state as the tert-butyl groups are forced together. The naphthalene 2 was obtained in good yield by heating 3 in refluxing toluene, and its X-ray structure showed exceptional distortions from a normal naphthalene geometry. However, 2 is not completely stable, and it decomposes upon prolonged heating in xylenes.

Journal Article↗
PubMed2002

Synthesis, structure, and redox properties of the extremely crowded triarylpnictogens: tris(2,4,6-triisopropylphenyl)phosphine, arsine, stibine, and bismuthine.

A series of the extremely crowded triarylpnitogens, tris(2,4,6-triisopropylphenyl)phosphine (1), arsine (2), stibine (3), and bismuthine (4), were synthesized by the reaction of 2,4,6-triisopropylphenylcopper(I) with the corresponding pnictogen trichlorides. Introduction [corrected] of the three bulky aryl groups resulted in the unusual structures and redox properties, which were studied by X-ray crystallography and cyclic voltammetry. The triarylpnictogens 1, 2, and 3 had extremely large bond angles around pnictogen atoms (1: 111.5 degrees , 2: 109.2 degrees , 3: 106.7 degrees ), and not only 1 and 2, but also stibine 3 displayed a reversible redox wave in the cyclic voltammograms at very low potentials (1: 0.16 V, 2: 0.50 V, 3: 0.57 V vs Ag/Ag+), which suggests considerable stability of the corresponding cation radicals.

Journal Article↗
PubMed2006

Synthesis of a sterically crowded atropisomeric 1,8-diacridylnaphthalene for dual-mode enantioselective fluorosensing.

An efficient synthetic route to a sterically crowded 1,8-diheteroarylnaphthalene-derived enantioselective fluorosensor that operates in two different detection modes utilizing fluorescence lifetime and intensity has been developed. Screening of palladium-catalyzed Negishi, Kumada, Suzuki, Hiyama, and Stille coupling methods showed that the latter affords highly congested 1,8-diarylnaphthalenes in superior yields. Despite severe steric hindrance, axially chiral 1,8-bis(3-(3',5'-dimethylphenyl)-9-acridyl)naphthalene, 1, was obtained in 68% yield from 1,8-dibromonaphthalene, 14, and 3-(3',5'-dimethylphenyl)-9-tributylstannylacridine, 13, via two consecutive Stille cross-coupling steps using tetrakis(triphenylphosphine)palladium(0) as catalyst in the presence of copper(II) oxide. The preparation of 1 involved formation of 4-(3',5'-dimethylphenyl)-2-chlorobenzoic acid, 7, through microwave-assisted Suzuki coupling of 4-bromo-2-chlorobenzoic acid, 10, and 3,5-dimethylphenylboronic acid, 11, followed by regioselective amination with aniline and acridine ring construction in phosphorus oxybromide. Lithiation, subsequent treatment with trimethylstannyl chloride, and Stille cross-coupling then completed the five-reaction sequence providing 1 in 57% overall yield. The enantiomers of 1 were separated by semipreparative HPLC on a (R,R)-Whelk-O 1 column and successfully employed in enantioselective fluorosensing of N-t-Boc-protected serine, 20, glutamine, 22, proline, 23, and 2-hydoxy-2-methylsuccinic acid, 21. Fluorescence titration experiments with 23 revealed that both static and dynamic quenching occur with distinctive enantioselectivity. Addition of (R)-23 to a solution of (+)-1 in acetonitrile resulted in stronger fluorescence quenching than titration with the (S)-enantiomer of 23. The fluorescence lifetime, tau, of 1 was determined as 18.8 ns and steadily decreased to 7.5 and 6.8 ns in the presence of 0.1 M of (S)-23 and (R)-23, respectively.

Journal Article↗
PubMed2006

Remote asymmetric induction about a crowded aromatic core.

Described are among the first highly diastereoselective, one-pot organometallic addition and hydride reduction reactions (>95% de) involving three symmetry-equivalent carbonyl centers, each that bears a 1,5-relationship to its nearest neighbor. Three-fold methyllithium addition to 2,4,6-trimethoxybenzene-1,3,5-tricarbaldehyde gives the anti,syn triol exclusively (by 1H NMR); addition of HMPA to the reaction or replacement of the substrate's methoxy groups with ethyl groups affords a statistical 3:1 (anti,syn:syn,syn) diastereomeric product ratio. Analogous asymmetric induction is found upon hydride reduction (using LiAlH4 or NaBH4) of the complementary triketone, 2,4,6-trimethoxybenzene-1,3,5-triethanone. Chelation and steric (gearing) effects about the crowded aromatic core contribute to the observed stereoselectivity.

Journal Article↗
PubMed1996

Phospholes with Reduced Pyramidal Character from Steric Crowding. 2. Photoelectron Spectral Evidence for Some Electron Delocalization in 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole.

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively).

Journal Article↗