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Y H Sheng

Publications and source records attributed to Y H Sheng.

7 recordsLinked to original sources
PubMed2001

Theoretical studies on alkyne addition to molybdenum alkylidenes: the preference of alpha- and beta-additions.

The addition of alkynes HC=CR to Mo(NH)(CH(2))(OR')(2) (R = H, Me, Ph; R' = CH(3), CF(3)) has been studied with both ab initio molecular orbital and density functional calculations. Geometry optimizations were carried out with the HF/3-21G, HF/HW3, and B3LYP/HW3 methods. The transition structures for these addition reactions are in distorted trigonal bipyramidal geometries, similar to those of alkene additions. The calculated activation enthalpy for HC=CH addition to Mo(NH)(CH(2))(OR')(2) is about 10.3 kcal/mol for R' = CH(3) and about 2.3 kcal/mol for R' = CF(3), indicating a significant preference for acetylene addition to Mo(NH)(CH(2))(OCF(3))(2) over Mo(NH)(CH(2))(OCH(3))(2). These barriers are higher than those of the corresponding ethylene addition by about 2-4 kcal/mol, even though the reaction of acetylene is much more exothermic. The alpha-addition of HC=CR (R = Me, Ph) is found to be considerably more favorable than the beta-addition to Mo(NH)(CH(2))(OR')(2). Interestingly, the alpha-addition has a lower activation energy, while the beta-addition has a higher activation energy, compared to that of the parent acetylene addition. Thus, alpha-addition is intrinsically favored over beta-addition by over 4 kcal/mol. This preference is reduced by solvent effect. All these can be explained by a destabilizing interaction between the nonreacting pi-orbital of alkyne and one of the lone pairs on the imido nitrogen. The steric effect of the bulky ligands in the real catalysts is also investigated qualitatively by the PM3 method. These studies give results in good accord with the experimentally observed regioselectivity.

Journal Article↗
PubMed2000

Epitope studies with anti-beta 2-glycoprotein I antibodies from autoantibody and immunized sources.

This paper examines the methodology of anti-beta(2)-glycoprotein I (beta(2)-GPI) epitope determination and provides further epitope studies using human sera containing anti-beta(2)-GPI autoantibodies. Studies in this field may be misleading as the antigen coating density using mutant forms of beta(2)-GPI may be below the threshold required for monogamous divalent binding by low affinity anti-beta(2)-GPI autoantibodies, while being easily detected by high affinity anti-beta(2)-GPI from immunized animals. The antigen density threshold effect is found in anti-beta(2)-GPI autoantibodies from humans and from monoclonal anti-beta(2)-GPI derived from mice with models of autoimmune disease. Anti-beta(2)-GPI from an autoimmune mouse and from 18/21 human sera did not bind above background levels to a domain-I-deleted mutant. In addition, single point mutations in domain I result in dramatic changes in the binding of many human sera containing anti-beta(2)-GPI. These findings support a conclusion that domain I of beta(2)-GPI contains significant epitopes for the anti-beta(2)-GPI antibodies found in the antiphospholipid syndrome.

Animals↗
PubMed1998

Human beta 2-glycoprotein I binds to endothelial cells through a cluster of lysine residues that are critical for anionic phospholipid binding and offers epitopes for anti-beta 2-glycoprotein I antibodies.

Beta 2-Glycoprotein I (beta 2GPI) is a phospholipid-binding protein recognized by serum autoantibodies from the anti-phospholipid syndrome both in cardiolipin- and beta 2GPI-coated plates. We found that: 1) recombinant wild-type beta 2GPI bound to HUVEC and was recognized by both human monoclonal IgM and affinity-purified polyclonal IgG anti-beta 2GPI anti-phospholipid syndrome Abs; and 2) a single amino acid change from Lys286 to Glu significantly reduced endothelial adhesion. Double and triple mutants (from Lys284,287 to Glu284,287, from Lys286,287 to Glu286,287, and from Lys284,286,287 to Glu284,286,287) completely abolished endothelial binding. A synthetic peptide (P1) spanning the sequence Glu274-Cys288 of the beta 2GPI fifth domain still displayed endothelial adhesion. Another peptide (P8), identical with P1 except that Cys281 and Cys288 were substituted with serine residues, did not bind to HUVEC. Anti-beta 2GPI Abs, once bound to P1 adhered to HUVEC, induced E-selectin expression and up-regulated IL-6 secretion. Control experiments conducted with irrelevant Abs as well as with the P8 peptide did not show any endothelial Ab binding nor E-selectin and IL-6 modulation. Our results suggest that: 1) beta 2GPI binds to endothelial cells through its fifth domain; 2) the major phospholipid-binding site that mediates the binding to anionic phospholipids is also involved in endothelial binding; 3) HUVEC provide a suitable surface for beta 2GPI binding comparable to that displayed by anionic phospholipids dried on microtiter wells; and 4) the formation of the complex between beta 2GPI and the specific Abs leads to endothelial activation in vitro.

Amino Acid Sequence↗
PubMed1996

The role of beta 2-glycoprotein I in the antiphospholipid syndrome.

Antiphospholipid antibodies were originally thought to bind negatively-charged (anionic) phospholipids. Current evidence suggest that the target antigen is considerably more complex and includes beta 2-glycoprotein I, a phospholipid-binding plasma protein. Our understanding of the pathophysiology of the antiphospholipid syndrome has increased exponentially with a number of studies into the interactions of antiphospholipid antibodies and beta 2-glycoprotein I.

Amino Acid Sequence↗
PubMed1996

beta 2-Glycoprotein I: target antigen for autoantibodies in the 'antiphospholipid syndrome'.

Antiphospholipid' (aPL) antibodies are of important clinical significance because of their association with thrombosis both arterial and venous, recurrent foetal loss, specific neurological sequelae like seizures and chorea, cardiac valvular abnormalities and thrombocytopenia. Traditionally these autoantibodies have been assayed using phospholipid (PL) dependent tests and are classified as lupus anticoagulants (LA) and anticardiolipin (aCL) antibodies based on the method of detection. The antibodies thus, had been thought to bind PLs but it has now become clear that the true antigens are PL-binding proteins. The major protein consistently found as the target antigen for these autoantibodies is beta 2-glycoprotein I (beta 2-GPI). Other candidate PL-binding proteins have also been investigated including prothrombin, protein C and protein S but thus far appear to play less important roles in the binding of these antibodies.

Antiphospholipid Syndrome↗
PubMed1993

[Vitreous prolapse during cataract surgery].

Clinical data of 427 cases (458 eyes) of extracapsular cataract extraction and IOL implantation gave the impression that vitreous prolapse due to disruption of the zonula and the posterior capsule was a malpractical complication. Forcible capsulotomy too large or deep and uneven irrigating pressure were the common causes of zonular disruption. Obstructed passage and insufficient separation of the nucleus from the posterior cortical bed leading to difficult nuclear delivery, aspiration of cortex in the posterior chamber, poorly filled anterior chamber, insufficient room for the IOL, improper insertion of the posterior haptics and traumatic dialing were risk factors for posterior capsular rupture and vitreous prolapse.

Cataract Extraction↗