PubMed Health⌕ Search

Biomedical subjects

A H White

Publications and source records attributed to A H White.

At least 19 recordsLinked to original sources
PubMed2001

Carboxymethylated cage amines: coordination and lactamization.

Depending upon the position and degree of substitution, carboxymethyl derivatives of cage amines of the "sarcophagine" (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) type vary considerably in the stability of their lactamized forms. For 1,8-diamino-3-carboxymethylsarcophagine, L(1), only indirect evidence for some involvement of a lactamized form of its Ni(II) complex has been obtained. Crystal structure determinations for [Cu(H(2)L(1))](NO(3))(3.5)Cl(0.5) x 2.5H(2)O and [Ni(HL(1))]Cl(3) x 3H(2)O show distorted octahedral coordination of all six endocyclic N-donor atoms in both cases. For related diaminosarcophagine derivatives with either two (1,8; L(2)) or three (1,1,8; L(3)) carboxymethyl substituents on the exocyclic N atoms, crystallographic studies have shown a dilactam form for the ligands in their Ni(II) and Cu(II) complexes which is of almost identical conformation to that of the diprotonated "free" ligand in [H(2)L(3)][ZnCl(4)] x 6H(2)O. The lactamized ligands appear to strongly favor square planar four-coordination, and the Co(II) complex of L(2) shows a remarkable lack of reactivity toward oxygen. Kinetic studies indicate that the hydrolytic stability of the lactam rings is comparable to that of uncoordinated analogues.

Journal Article↗
PubMed2001

Oxidative addition of the imidazolium cation to zerovalent Ni, Pd, and Pt: a combined density functional and experimental study.

Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental studies. The theoretical analysis shows that addition of imidazoliums to Pt(0) and Ni(0) is more exothermic than to Pd(0), and Ni(0) is predicted to react with a much lower barrier than either Pt(0) or Pd(0). Strongly basic supporting ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of 2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is possible, and a number of Pt- and Pd-carbene complexes were prepared via oxidative addition of imidazolium salts to M(0) precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur, and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh(3))(2) and Pt(PCy(3))(2) affords [PtH(dmiy)(PPh(3))(2)]BF(4) (10) and [PtH(dmiy)(PCy(3))(2)]BF(4) (11), while reaction between 3,4-dimethylthiazolium tetrafluoroborate and Pt(PCy(3))(2) yields [PtH(dmty)(PCy(3))(2)]BF(4) (12) (dmiy = 1,3-dimethylimidazolin-2-ylidene, dmty = 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate to Pt(PPh(3))(4) or Pd(dcype)(dba) yields [PtI(tmiy)(PPh(3))(2)]BF(4) (9) and [PdI(tmiy)(dcype)]BF(4) (14), respectively (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype = 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-carbene-based complexes.

Journal Article↗
PubMed2001

Isolation and detailed characterization of the trans-[(H2O)2FeCl4](-) anion: stabilization of novel iron(III) species by large organic cations.

1,2-Diaminoethane (en) and FeCl3 give (enH2) [FeCl5(H2O)] (1) in concentrated HCl, extending the aquapentachloroferrate(III) series. For 1: C2H12N2Cl5OFe, orthorhombic, P2(1)2(1)2(1), a = 14.531(6) A, b = 10.772(4) A, c = 6.888(2) A, Z = 4. Diazabicyclo[2.2.2]octane dihydrochloride (DABCO-2HCl) and FeCl3 in concentrated HCl form a tetrachloroferrate(III) derivative whose subsequent ethanol treatment (restricted water access) results in the formation of a compound of composition (DABCOH2)2 [FeCl4(H2O)2]Cl3 (2). This contains the trans-[FeCl4(H2O)2](-) anion, in which the trans-Fe-O distances are 2.049(4) A. For 2: C12H32N4Cl7O2Fe, orthorhombic, Pnma, a = 16.378(3) A, b = 7.3323(6) A, c = 19.431(3) A, Z = 4. A combination of 57Fe Mössbauer spectroscopy and ac susceptibility data confirm uncanted 3D antiferromagnetic ground states with T(Néel) approximately 3.4 K for (enH2)[FeCl5(H2O)] and approximately 2.0 K for [DABCOH2]2[FeCl4(H2O)2]Cl3.

Journal Article↗
PubMed2001

Systematic structural coordination chemistry of p-tert-butyltetrathiacalix[4]arene: 1. Group 1 elements and congeners.

Determinations of the crystal structures of complexes of the alkali metal ions with, in the case of Li, the dianion and, in the cases Na-Cs, the monoanion of p-tert-butyltetrathiacalix[4]arene have shown that both the sulfur atoms which form part of the macrocyclic ring, as well as the pendent phenolic/phenoxide oxygen donor atoms, are involved in coordination to these metals. Although the Li and Na complex structures are similar to those of the corresponding complexes of p-tert-butylcalix[4]arene, there is no similarity in the structures of the Cs complexes, with the present structure showing no evidence of polyhapto Cs(+)-pi interactions. Instead, the complex crystallizes as a ligand-bridged (S-, O-donor) aggregate of three Cs ions, solvent molecules, and four calixarenes, somewhat like the Rb complex, though here four Rb ions are present, and higher in aggregation than the K+ complex, where two K+ ions are sandwiched between two calixarene moieties. The triethylammonium complex of the thiacalixarene monoanion, though formally analogous in that it involves a monocation, has a simpler structure than any of the alkali metal derivatives, based formally on proton coordination (H-bonding). However, interestingly, it can be isolated in both solvated (dmf, dmso) and unsolvated forms, as indeed can the "free", p-tert-butyltetrathiacalix[4]arene ligand itself.

Journal Article↗
PubMed2001

Simple syntheses, structural diversity, and Tishchenko reaction catalysis of neutral homoleptic rare earth(II or III) 3,5-di-tert-butylpyrazolates--the structures of.

The homoleptic rare-earth pyrazolate complexes [Sc(tBu2pz)3], [Ln2(tBu2pz)6] (Ln = La, Nd, Sm, Lu), [Eu4(tBu2pz)8] and the mixed oxidation state species [Yb2(tBu2pz)5] (tBu2pz = 3,5-di-tert-butylpyrazolate) have been prepared by a simple reaction between the corresponding rare-earth metal and 3,5-di-tert-butylpyrazole, in the presence of mercury, at elevated temperatures. In addition, [Yb2(tBu2pz)6] was prepared by redox transmetallation/ligand exchange between ytterbium, diphenylmercury(II) and tBu2pzH in toluene, whilst the same reactants in toluene under different conditions or in diethyl ether gave [Yb2(tBu2pz)5]. The complexes of the trivalent lanthanoids display dimeric structures [Ln2(tBu2pz)6] (Ln = La, Nd, Yb, Lu) with chelating eta2-terminal and eta2:eta2-bridging pyrazolate coordination. The considerably smaller Sc3+ ion forms monomeric [Sc(tBu2pz)3] of putative D3h molecular symmetry, with pyrazolate ligands solely eta2-bonded. [Eu4(tBu2pz)8] is a structurally remarkable tetranuclear EuII complex with two types of europium centres in a linear array. The outer two are bonded to one terminal and two bridging pyrazolates, and the inner two are coordinated by four bridging ligands. Unprecedented mu-eta5:eta2 pyrazolate ligation is observed, with each outer Eu2+ sandwiched between two eta5-bonded pyrazolate groups, which are also eta2-linked to an inner Eu2+. The two inner Eu2+ ions are linked together by two equally occupied components of each of two symmetry related, disordered pyrazolate groups with one component eta4:eta2 bridging and one eta3:eta2 bridging. [La2(tBu2pz)6] has also been shown to be a Tishchenko reaction catalyst with several organic substrates.

Journal Article↗
PubMed2000

Carboxymethylation of cage amines: control of alkylation by metal ion coordination.

Reactions between chloroacetate and both "free" and coordinated forms of the cage amine diaminosarcophagine (1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) result in the introduction of between one and four pendent carboxymethyl substituents on the nitrogen atoms of the cage. While at least the first two steps in the reaction of the free ligand have been found to involve only the secondary nitrogen centers, in both the Cu(II) and Co(III) complexes alkylation occurs only at the primary (1,8) centers, the greater ease of achieving a higher degree of alkylation of the Cu(II) complex being attributed to the lower charge of this species causing a lesser reduction of the nucleophilicity of the uncoordinated primary nitrogen atoms. All the new ligands have been characterized by X-ray structure determinations of their Cu(II) or Co(III) complexes. In some cases, this has shown that the methods used to isolate the crystalline complexes result in lactamization of the ligand.

Journal Article↗
PubMed2000

Anterior lumbar fusion improves discogenic pain at levels of prior posterolateral fusion.

STUDY DESIGN: A descriptive case review. OBJECTIVES: To assess the outcomes of anterior lumbar interbody fusion for painful discs within a solid posterolateral spinal fusion. SUMMARY OF BACKGROUND DATA: Some patients continue to have pain after posterolateral spinal fusion despite apparently solid arthrodesis. One potential etiology is pain that arises from a disc within the fused levels. METHOD: Retrospective review of 176 patients with anterior interbody fusion, which located 20 who had anterior interbody fusion levels of prior posterolateral spinal fusion. All had low back pain, solid posterolateral spinal fusion, and painful disc(s) at the posterolateral spinal fusion level(s) but not elsewhere. Pain was measured by the Numerical Rating Scale, function by Oswestry Disability Questionnaire, and patient satisfaction by the North American Spine Society Outcome Questionnaire. RESULTS: Follow-up data were available for 18 patients (90%). Mean follow-up was 58 months (25 to 102). There were 10 men and 8 women. Mean age was 45 years (26 to 72). Diagnoses were degenerative discs, herniated nucleus pulposus, spondylolisthesis, and spinal stenosis. Eight patients had injuries after the previous posterolateral spinal fusion that precipitated new symptoms. Two patients had one level fusion, 14 had two levels, and 1 each had three and four levels. Four patients had one prior surgery, 5 had two, and 9 had three or more. All patients had solid anterior interbody fusion by radiograph. Mean Numerical Rating Scale improved from 7.9 before surgery to 4.7 after (P< 0.001). Mean Oswestry Disability Questionnaire improved from 56.3 before surgery to 47.9 after (P = 0.04). Of 15 patients unable to work before anterior interbody fusion, 5 returned to work. Sixteen patients (89%) were satisfied with their results. CONCLUSION: Low back pain that continues or recurs after apparently solid posterolateral spinal fusion may be caused by painful disc(s) at motion segment(s) within the fusion. A solid posterolateral spinal fusion may not protect the residual disc(s) from injury. Anterior interbody fusion can provide significant improvements in pain and function and a high degree of patient satisfaction in this clinical setting.

Disability Evaluation↗
PubMed2000

Coordination chemistry of a novel tetramacrocyclic ligand derived from 1,4,7-triazacyclononane: synthesis, structure, and properties of nickel(II) and copper(II) complexes.

A new polynucleating ligand, 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene (Ldur), has been prepared and characterized as its dodecahydrobromide salt. Addition of base to an aqueous solution of this salt and 4 molar equivalents (m.e.) of a Ni(II) salt produces a mixture of bi- and trinuclear complexes, which can be separated by cation-exchange chromatography (CEC) and crystallized as [Ni2Ldur](ClO4)(4).2H2O (1) and [Ni3Ldur(H2O)6](ClO4)(6).9H2O (2). The "full capacity" tetranuclear complex, [Ni4Ldur(H2O)12](ClO4)(8).8H2O (3), is obtained by slow addition of Ldur to a refluxing aqueous solution of excess Ni2+ ions, followed by CEC purification. Treatment of Ldur with 4 m.e. of a copper(II) salt produces exclusively the tetranuclear complex, [Cu4Ldur(H2O)8](ClO4)(8).9H2O (4), while reaction with only 2 m.e. of Cu2+ ions yields the binuclear complex, [Cu2Ldur](ClO4)(4).4H2O (5). The X-ray structures of complexes 1,2,4, and [Cu2Ldur](ClO4)(4).3H2O (5') have been determined; all are monoclinic, P2(1)/c: for 1, a = 9.497(3) A, b = 13.665(5) A, c = 19.355(6) A, beta = 100.57(2) degrees, V = 2469(1) A3, and Z = 2; for 2, a = 22.883(7) A, b = 15.131(6) A, c = 20.298(8) A, beta = 97.20(3) degrees, V = 6973(4) A3, and Z = 4; for 4, a = 16.713(7) A, b = 16.714(6) A, c = 14.775(11) A, beta = 108.24(5) degrees, V = 3920(4) A3, and Z = 2; and for 5', a = 9.5705(1) A, b = 13.0646(1) A, c = 20.1298(2) A, beta = 103.1618(8) degrees, V = 2450.81(4) A3, and Z = 2. The metal centers in 1 and 5' lie in distorted octahedral environments, each facially coordinated by two of the triamine rings of Ldur, the cation in each case being centrosymmetric. In 2, one of the nickel(II) centers is similarly sandwiched by two triamine rings, while the other two nickel(II) centers are each coordinated by a single triamine ring from the ligand, with their distorted octahedral coordination spheres each being completed by three water molecules. In 4, the four triamine rings of Ldur bind to separate copper(II) centers, with two water molecules occupying the remaining two sites of the distorted square pyramidal (SP) coordination spheres, the cation again being centrosymmetric.

Journal Article↗
PubMed2000

Patient satisfaction after circumferential lumbar fusion.

STUDY DESIGN: A review of 141 consecutive patients who underwent instrumented circumferential lumbar fusions. Outcome was assessed by an independent third party after a minimum follow-up of 2 years. OBJECTIVES: To evaluate clinical outcomes and assess patient satisfaction with circumferential lumbar fusions. SUMMARY OF BACKGROUND DATA: Circumferential lumbar fusion has been used as a salvage procedure for revision spine surgery. METHODS: Data were collected from patient interviews, patient satisfaction questionnaire, and review of records and radiographs of 141 consecutive patients who underwent circumferential lumbar fusion under a single anesthetic. RESULTS: There were follow-up data in 133 of the 141 patients (92%). Mean duration of follow-up was 37.2 months. There were 41 (31%) primary and 92 (69%) revision surgeries. There were 73 (55%) workers' compensation and 60 (45%) non-workers' compensation patients. There were radiographs available at 12-month follow-up in 86 patients (65%). There was solid radiographic fusion in 85 (99%) of the 86 patients. The overall complication rate was 20%. Clinical outcome was determined by the patient satisfaction questionnaire. There were 14 (11%) who chose the statement, "surgery met my expectations"; 68 (51%) who chose, "surgery improved my condition enough that I would go through it again for the same outcome"; 26 (20%) who chose, "surgery helped me but I would not go through it again for the same outcome"; and 25 (19%) who chose, "I am the same or worse compared with before surgery." There was no statistical difference in patient satisfaction between primary and revision surgeries or between workers' compensation versus non-workers' compensation groups. CONCLUSIONS: Circumferential lumbar fusion is a useful procedure for a patient with difficult reconstructive disease. There is a very high fusion rate. Overall, 62% of patients are satisfied with the result, especially those with a diagnosis of pseudarthrosis or spondylolisthesis with stenosis. Patients who are working before surgery and patients who are not injured workers also tended to progress well.

Adult↗
PubMed2000

3,5-Dimethoxy-3-(1,1-dimethylprop-2-enyl)-3,4-dihydropsoralen-4-one: an unusual furanocoumarin derivative from Halfordia kendack.

The aerial parts of Halfordia kendack (Rutaceae) have yielded a new furanocoumarin derivative which has been identified as 3,5-dimethoxy-3-(1,1-dimethylprop-2-enyl)-3,4-dihydropsoralen-4-one on the basis of spectroscopic and X-ray examination. This new compound, present in high concentration, is probably formed from a 4-O-prenylated precursor furanocoumarin via a Claisen rearrangement. No trace of the putative precursor was found in this sample.

Crystallography, X-Ray↗
PubMed1998

Controversy. The use of selective nerve root blocks: diagnostic, therapeutic, or placebo?

An increasing number of therapeutic spinal injections are performed each year despite little validation in randomized controlled trials. Additional injections are performed for diagnostic purposes of localizing symptomatic nerve roots, again without detailed evaluation of accuracy, specificity, or sensitivity. Drs. Slosar and White argue that selective nerve root blocks are extremely useful; Dr Wetzel believes that selective nerve root blocks have no role in selecting patients for surgery.

Humans↗
PubMed1997

Respiratory isolation in a teaching hospital with low-to-moderate rate of tuberculosis: compliance with Centers for Disease Control and Prevention guidelines for identifying patients who may have active tuberculosis.

BACKGROUND: Because of the limitation of isolation spaces suitable for tuberculosis, proper use of these spaces is prudent. We examined the current approach to respiratory isolation to determine the compliance with Centers for Disease Control and Prevention (CDC) guidelines for identifying active tuberculosis. METHODS: A retrospective review was performed of all patients placed in respiratory isolation and those with pulmonary tuberculosis. RESULTS: Seventy-seven instances of isolation and three admissions without isolation were encountered. Isolated patients met CDC guidelines for infectiousness in 59 of 77 instances (76.6%). The remaining patients were isolated with findings not characteristic of active tuberculosis (10 instances), normal chest radiograph (6 occasions), or without chest radiograph (2 instances). The time of implementing isolation was the first hospital day in 58 instances (75.3%) and 2 to 14 days in 19 instances (24.7%). Isolated human immunodeficiency virus-infected patients were more likely to meet CDC guidelines for infectiousness (21/22 [95.5%] vs 38/55 [69.1%] instances) and to be isolated on a timely basis (18/22 [81.8%] vs 40/55 [72.7%]). Tuberculosis was documented in 15 instances and isolation was delayed or never implemented in 5 and 3 instances, respectively. CONCLUSIONS: Compliance with CDC guidelines for respiratory isolation in patients not infected with human immunodeficiency virus is suboptimal. Many isolated patients do not meet these guidelines, and isolation is delayed or not implemented in patients who may have tuberculosis.

Adult↗
PubMed1996

Molecular structure of terrecyclodiol: a derivative of the antifungal metabolite terrecyclic acid A from Aspergillus terreus.

A strain of Aspergillus terreus, which was isolated from organic mulch and inhibited the growth of the plant pathogen Phytophthora cinnamomi, produces an antifungal metabolite when grown in liquid culture. This metabolite was isolated by bioassay-guided fractionation and identified as terrecyclic acid A (1). X-ray diffraction studies and spectroscopic details of the derived terrecyclodiol (2) are described.

Antifungal Agents↗
PubMed1996

Classroom assessment of the use of compact language drills: a technique borrowed from foreign language teaching.

Many deaf children have great difficulty learning how to use irregular English verbs. This research study seeks to exemplify how a single-case, ABA design can be used by classroom teachers in making a determination about the usefulness of classroom teaching strategies, and in the case of this study, to explore the use of compact language drills (CLDs) as a technique for helping a deaf child improve his ability to use 4 irregular verbs. The CLDs were conducted 5-10 minutes each day for five weeks at the close of school. The drills engaged the child in immediate recall of simple sentences loaded with semantic/syntactic clues which signalled the need for a past, present or future irregular verb within the target sentence. The results suggest that CLDs, when used systematically, can significantly improve a child's ability to use irregular verbs correctly, both immediately following treatment, and across time.

Child↗