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Biomedical subjects

S K Arora

Publications and source records attributed to S K Arora.

At least 91 records · Page 5Linked to original sources
PubMed1985

Ectopic atrial rhythm with progressively increasing exit block.

A case of ectopic atrial rhythm with progressively increasing exit block is reported in a case of ischemic heart disease. Such a rhythm suggests significantly diseased atria which are unable to sustain a regular rhythm for a long period. This arrhythmia should warn the clinician of impending atrial arrest.

Aged↗
PubMed1985

Correlation of structure and activity in ansamycins: structure, conformation, and interactions of antibiotic rifamycin S.

The crystal and molecular structure of the DNA-dependent RNA polymerase inhibiting antibiotic rifamycin S (C37H45O12N) as a dihydrate has been determined, and the conformation necessary for activity has been correlated with those of other active rifamycins. The orthorhombic unit cell, space group P212121 with dimensions of a = 13.010 (2), b = 14.236 (2), c = 20.571 (4) A, contains 4 molecules. The structure was solved by a combination of vector search and direct methods and refined anisotropically to an R factor of 0.048 for 2855 reflections. The conformation of the ansa chain differs from those of rifampicin and rifamycin B but resembles that of rifamycin SV at the joining points, C(2) and C(12), of the ansa chain to the naphthoquinone chromophore. The middle part of the ansa chain, which is essential for its activity against the enzyme, has the same conformation as other active rifamycins. The effect of the 3-substitution on the ansa chain conformation is that the carboxyl (C(15) = O) group wings around the N-C(16) direction, depending upon the electronegativity of the 3-substituent. The hydrogen bonding involves O(1), O(2), O(8), O(9), O(10), and the water molecules. A possible four-stage model for the interaction of the rifamycins with the enzyme DNA-dependent RNA polymerase has been speculated.

DNA-Directed RNA Polymerases↗
PubMed1985

Molecular structure, stereochemistry and interactions of steffimycin B, and DNA binding anthracycline antibiotic.

The crystal and molecular structure of anthracycline antibiotic steffimycin B(C29H32O13) has been determined by X-ray diffraction and the stereochemistry revealed. The orthorhombic crystals belong to space group P2(1)2(1)2(1), with the dimensions; a = 8.253 (2), b = 8.198 (2), c = 40.850 (8) A and Z = 4. Intensity data were collected for 2518 independent reflections. The structure was solved by direct methods and refined to an R value of 0.066 for 1410 reflections. The configuration in ring A is 7R,8S,9S. Ring A adopts half chair conformation, while the sugar ring has the regular chair conformation. The molecule most probably binds to double helical DNA through intercalation and hydrogen bonding.

Anthracyclines↗
PubMed1983

Correlation of structure and activity in ansamycins. Molecular structure of sodium rifamycin SV.

The crystal and molecular structure of the sodium salt of rifamycin SV (clinically known as rifacin) as the monohydrate ethanol solvate has been determined to study the conformation of the ansa chain in unsubstituted rifamycins and also to clarify the metal complexation with rifamycins. The crystals belong to the space group P2(1)2(1)2(1) with cell dimensions (estimated standard deviations in parentheses) of a = 12.061 (2), b = 13.936 (2), and c = 24.731 (4) A. The structure was solved by direct methods and refined to an R factor of 0.069. The conformation of the ansa chain differs from that of other active rifamycins, e.g., rifampcin and rifamycin B at the joining point of the ansa chain to the naphthohydroquinone chromophore. The conformation of the middle part of the ansa chain, which is essential for activity against DNA-dependent RNA polymerase, remains the same. The sodium ion is penta-coordinated and has a trigonal bipyramidal geometry. The intermolecular hydrogen bonding involves O(9), O(10), O(5), and O(6) through water and ethanol molecules. A two-step mode of action of rifamycins has been postulated, and the conformations of antibiotics suitable for penetration of the membrane barrier and that for antibiotic-enzyme complex formation have been suggested.

Anti-Bacterial Agents↗