scholarly journals Synthesis of rigidified flavin–guanidinium ion conjugates and investigation of their photocatalytic properties

2009 ◽  
Vol 5 ◽  
Author(s):  
Harald Schmaderer ◽  
Mouchumi Bhuyan ◽  
Burkhard König
Keyword(s):  
Binding Site ◽  
Redox Reactions ◽  
Substrate Binding ◽  
Alder Reaction ◽  
Benzyl Ester ◽  
Diels Alder ◽  
Substrate Binding Site ◽  
X Ray ◽  
Diels Alder Reaction ◽  
Acid Structure

Flavin chromophores can mediate redox reactions upon irradiation by blue light. In an attempt to increase their catalytic efficacy, flavin derivatives bearing a guanidinium ion as oxoanion binding site were prepared. Chromophore and substrate binding site are linked by a rigid Kemp’s acid structure. The molecular structure of the new flavins was confirmed by an X-ray structure analysis and their photocatalytic activity was investigated in benzyl ester cleavage, nitroarene reduction and a Diels–Alder reaction. The modified flavins photocatalyze the reactions, but the introduced substrate binding site does not enhance their performance.

scholarly journals Molecular recognition of sugar binding in a melibiose transporter MelB by X-ray crystallography

2020 ◽  
Author(s):  
Lan Guan ◽  
Parameswaran Hariharan
Keyword(s):  
Molecular Recognition ◽  
Binding Site ◽  
Crystal Structures ◽  
Mammalian Cells ◽  
Substrate Binding ◽  
Structural Basis ◽  
Cooperative Binding ◽  
Recognition Mechanism ◽  
Substrate Binding Site ◽  
X Ray

AbstractThe symporter melibiose permease MelB is the best-studied representative from MFS_2 family and the only protein in this large family with crystal structure determined. Previous thermodynamic studies show that MelB utilizes a cooperative binding as the core mechanism for its obligatory symport. Here we present two sugar-bound X-ray crystal structures of a Salmonella typhimurium MelB D59C uniport mutant that binds and catalyzes melibiose transport uncoupled to either cation, as determined by biochemical and biophysical characterizations. The two structures with bound nitrophenyl-α-D-galactoside or dodecyl-β-D-melibioside, which were refined to a resolution of 3.05 or 3.15 Å, respectively, are virtually identical at an outward-facing conformation; each one contains a α-galactoside molecule in the middle of protein. In the substrate-binding site, the galactosyl moiety on both ligands are at an essentially same configuration, so a galactoside specificity determinant pocket can be recognized, and hence the molecular recognition mechanism for the binding of sugar in MelB is deciphered. The data also allow to assign the conserved cation-binding pocket, which is directly connected to the sugar specificity determinant pocket. The intimate connection between the two selection sites lays the structural basis for the cooperative binding and coupled transport. This key structural finding answered the long-standing question on the substrate binding for the Na+-coupled MFS family of transporters.SignificanceMajor facilitator superfamily_2 transporters contain >10,000 members that are widely expressed from bacteria to mammalian cells, and catalyze uptake of varied nutrients from sugars to phospholipids. While several crystal structures with bound sugar for other MFS permeases have been determined, they are either uniporters or symporters coupled solely to H+. MelB catalyzes melibiose symport with either Na+, Li+, or H+, a prototype for Na+-coupled MFS transporters, but its sugar recognition has been a long-unsolved puzzle. Two high-resolution crystal structures presented here clearly reveal the molecular recognition mechanism for the binding of sugar in MelB. The substrate-binding site is characterized with a small specificity groove adjoining a large nonspecific cavity, which could offer a potential for future exploration of active transporters for drug delivery.

scholarly journals Two-Carbon Ring Enlargement of Five-, Six-, and Seven-Membered 1-Aza-2-vinylcycloalk-2-enes with Dimethyl Acetylenedicarboxylate and Subsequent Thermal Isomerization Reactions

2006 ◽  
Vol 61 (4) ◽  
pp. 385-395 ◽  
Author(s):  
Gerhard Maas ◽  
Robert Reinhard ◽  
Hans-Georg Herz
Keyword(s):  
Elevated Temperatures ◽  
Alder Reaction ◽  
Thermal Isomerization ◽  
Diels Alder ◽  
Dimethyl Acetylenedicarboxylate ◽  
X Ray ◽  
Ring Enlargement ◽  
Diels Alder Reaction ◽  
Carbon Ring ◽  
Electrocyclic Reaction

2-Aminodienes, in which the enamine function is incorporated in a five-, six-, or seven-membered ring, react with dimethyl acetylenedicarboxylate in a sequence of [2+2] cycloaddition and electrocyclic ring-opening to form the two-carbon ring expanded unsaturated heterocycles, i.e., 3,4- dicarboxylate substituted 6,7-dihydro-1H-azepines 3, 8 and 21, 1,6,7,8-tetrahydroazocines 22, and 6,7,8,9-tetrahydro-1H-azonines 13. Similarly, 2-[(2-thienyl)ethynyl]-4,5,6,7-tetrahydro-1H-azepine 9 is converted into 2-[(2-thienyl)ethynyl]-6,7,8,9-1H-azonine-3,4-dicarboxylate 10 which was characterized by X-ray structure determination. The eight- and nine-membered azaheterocycles 22 and 13, which have not been isolated, undergo thermal isomerization at elevated temperatures. Thus, ring contraction by a 6π-electrocyclic reaction takes place for N-methyl substituted azonine 13, while the N-allyl moiety of azocines 22 engages in an intramolecular Diels-Alder reaction or a 1,7- electrocyclization reaction

ChemInform Abstract: Abnormal Diels-Alder Reaction of 5-Alkoxyoxazoles with Tetracyanoethylene and X-Ray Crystal Structure of an Adduct.

ChemInform ◽  
2010 ◽  
Vol 23 (44) ◽  
pp. no-no
Author(s):  
T. IBATA ◽  
Y. ISOGAMI ◽  
H. NAKAWA ◽  
H. TAMURA ◽  
H. SUGA ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Alder Reaction ◽  
Diels Alder ◽  
X Ray ◽  
Diels Alder Reaction

Molecular Topology of Linked Anthracenes: X-Ray Structure Analyses of Alkyl Di-9-anthryl-glycolates, 9-Anthryl 9-Anthrylmethyl Ethers and Their Photoisomers

1989 ◽  
Vol 42 (11) ◽  
pp. 1869 ◽  
Author(s):  
HD Becker ◽  
BW Skelton ◽  
AH White
Keyword(s):  
Single Crystal ◽  
Alder Reaction ◽  
Diels Alder ◽  
Molecular Structures ◽  
Molecular Topology ◽  
X Ray Diffraction ◽  
X Ray ◽  
Diels Alder Reaction ◽  
Photochemical Isomerization ◽  
Spatial Demand

The molecular structures of ethyl di-9-anthrylglycolate (1) and methyl di-9-anthrylglycolate (2), as well as those of three isomerization products of (2), have been established by single-crystal X-ray diffraction studies. In both (1) and (2), the anthracenemoieties are characterized by marked deviations from planarity. The stereospecific course of the thermal and photochemical isomerization of (2) by Diels -Alder reaction is suggested to be governed by the spatial demand of the alkoxycarbonyl group. Base-catalysed isomerization of (2) by migration of 9-anthryl from carbon to oxygen is associated with steric relief.

X-Ray Crystallographic Analysis of 2,5-Bis(diphenylmethylsilyl)thiophene Monooxide and the Diels-Alder Reaction of Thiophene Monooxide with Dienophiles

Heterocycles ◽  
1998 ◽  
Vol 47 (2) ◽  
pp. 793 ◽  
Author(s):  
Naomichi Furukawa ◽  
Shao-Zhong Zhang ◽  
Ernst Horn ◽  
Ohgi Takahashi ◽  
Soichi Sato ◽  
...  
Keyword(s):  
Alder Reaction ◽  
Crystallographic Analysis ◽  
Diels Alder ◽  
X Ray ◽  
Diels Alder Reaction
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