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Biomedical subjects

R Chandrasekaran

Publications and source records attributed to R Chandrasekaran.

14 recordsLinked to original sources
PubMed1992

The "pseudo double-helical" structure of the gel-forming capsular polysaccharide from Rhizobium trifolii.

X-ray diffraction analysis of oriented fibers of the gel-forming, neutral, doubly branched, galactoserich capsular polysaccharide from Rhizobium trifolii has been used to determine and refine the molecular structure to a final R value of 0.28. The polysaccharide forms a 2-fold single helix of pitch 20.2 A, is stabilized by a series of hydrogen bonds that involve the side chains, and has the appearance of a double helix. Packing calculations reveal that the short-range ordering of these "pseudo double helices" is brought about by intermolecular hydrogen bonds that involve the side chains. Complete or partial removal of side chains will weaken the molecular association and is detrimental to the gelation process.

Carbohydrate Conformation↗
PubMed1992

Structure of poly d(A).poly d(T).

On the basis of the x-ray data from polycrystalline and well oriented fibers of the sodium salt of poly d(A).poly d(T) (Arnott et al, Nucl. Acids Res. 11, 4141-4155 (1983), a revised B'-DNA model incorporating B-like adenine and thymine strands is shown to give a much better x-ray agreement (R = 0.25) than the previously assigned model consisting of mixed sugar conformations in the two strands. The narrowing of the minor and the widening of the major grooves are promiscuous features of B'-DNA, which are common to all poly d(purine).poly d(pyrimidine) duplexes with two hydrogen bonded base-pairs and are in marked contrast with classical B-DNA. Due to modest propeller (-15 degrees), the cross strand diagonal hydrogen bonds (0.37 nm) in this duplex are not as strong as those in A,T-rich oligonucleotide crystal structures.

Base Composition↗
PubMed1977

Conformational analysis of cyclo (L-cystine).

Conformational analysis of cyclo(L-cystine) shows that the diketopiperazine ring has to exist only in the boat form. With this geometry, the molecule can adopt two distinct forms differing mainly in the chirality of the disulphide bridge. In both the P- and M-models, corresponding to dihedral angles of nearly +90 degrees and -90 degrees respectively about the S-S- bond, the molecule displays an approximate two-fold symmetry. According to our semi-empirical energy calculations, the minimum energy of the M-model is --9.2 kcal/mol, only 0.3 kcal/mol lower than that of the P-model. Because the difference between the two minima is so small, neither form is clearly superior to the other. However, the number of low energy conformations of the M-model in the allowed conformational space is significantly larger than that of the P-model by a ratio of 3 to 1, and therefore the former is likely to be thermodynamically favoured.

Cystine↗
PubMed1977

Conformation of polypeptide chains containing both L- and D-residues. II. Double-helical structures of poly-LD-peptides.

Polypeptides with alternating L- and D-amino acid residues can take up stereo-chemically satisfactory coaxial double-helical structures, both antiparallel and parallel, which are stabilized by systematic interchain NH...O hydrogen bonds. Semiempirical energy calculations over allowed regions of conformational space have yielded the characteristics of these double-helices. There are four possible types of antiparallel double-helices - A3, A4, A5 and A6, with n, the number of LD peptide units per turn, around 2.8, 3.6, 4.5 and 5.5 respectively, while for the parallel double-helices there are two types, P3 and P4, having similar helical parameters as in A3 and A4. The hydrogen-bonding scheme restricts the pitch in all the models to the narrow range of 10.0 to 11.5 A. All these helices have large central cores whose radii increase proportionately with n. In this respect, A3 and A4 are suitable models for the structure of gramicidin A. In terms of their relative energies, antiparallel double-helices are marginally more stable than those with parallel strands. Our results indicate that the energy differences amongst the members in the antiparallel family are not significant and thus provide an explanation for the polymorphism reported for poly (gamma-benzyl-LD-glutamate).

Amino Acids↗
PubMed1976

Combined lactose-D-xylose tolerance test in infancy.

A modified combined oral lactose-D-xylose tolerance test done on ten normal infants showed peak levels of blood glucose and D-xylose levels comparable to the peaks obtained when these tests were done separately in the same subjects. The combined test is recommended to evaluate digestive absorptive function in preference to two separate tolerance tests.

Blood Glucose↗
PubMed1975

Carbohydrate intolerance in infants with acute diarrhoea and its complications.

Two hundred and seventy-one infants with acute diarrhoea were studied for the presence of carbohydrate malabsorption and 110 infants (40.6%) were found to have carbohydrate intolerance. Malnutrition and severe diarrhoea were found to increase the predisposition to carbohydrate intolerance. The incidence of major complications, protracted diarrhoea and mortality were significantly higher in the carbohydrate intolerant infants as compared to those with carbohydrate tolerance.

Acid-Base Equilibrium↗