Search PubMed⌕ Search

Biomedical subjects

G Zundel

Publications and source records attributed to G Zundel.

33 records · Page 2Linked to original sources
PubMed1978

The H2(18O) enrichment of the leaf water of tropic trees: comparison of species from the tropical rain forest and the semi-arid region in Brazil.

The H2(18O) enrichment, delta, in the water of leaves from four Brazilian trees, was studied. In all trees the leaf water showed a periodic variation in delta, with a maximum in the early afternoon and a minimum around 6 a.m. In general delta was found to be either higher or lower than the stationary enrichment which is supposed to depend only on the relative atmospheric humidity. This effect is due to the slow response of the system to variations of the humidity. For a special case, where steady-state conditions could be anticipated, the kinetic enrichment was obtained to 20 +/- 3%, which agrees with theoretical predictions.

Brazil↗
PubMed1976

Influence of temperature and magnesium ions on the secondary and tertiary structures of tRNAPhe and 23 S RNA - infrared investigations.

Band splitting and/or bands shifting in opposite directions due to coupling of vibrations of neighboring groups observed in the infrared spectra of tRNAPhe and 23 S RNA give information on the secondary structure. The base pairing, dependent on temperature, is investigated, discussing coupling effects with the base residues' vibrations in the region 1700-1500 cm-1. The secondary structure of the backbone is studied, discussing coupling effects with vibrations in the region 1300-1000 cm-1. The 2'OH groups are cross-linked with the O atoms of the neighboring ribose residues via hydrogen bonds. Probably the greater than PO-2 groups are turned inward at the backbone, i.e. towards the base residues. The base pairs as well as the secondary structure of the backbone melt with increasing temperature and with dialysis against distilled water. The comparison of the Mg2+ and the K+ salts of the tRNAPhe shows that the changes of base pairing due to Mg2+ are small. At the backbone, however, Mg2+ favor somewhat more the discussed secondary structure than K+ does. All Mg2+ effects on secondary structure are, however, too small to explain the considerable increase in melting temperature due to Mg2+. Thus it is supposed that the rise in the melting temperature due to Mg2+ is not caused by a change in secondary but in the tertiary structure of tRNAPhe. Furthermore, the influence of Mg2+ on the secondary structure of 23 S RNA is studied. The following results are obtained: (1) The double helical regions become more compact and probably increase due to the influence of Mg2+. (2) At the backbone, Mg2+ induces strong hydrogen bonding between the 2'OH groups and the ether O atoms of neighboring ribose residues. Probably they turn the greater than PO-2 groups toward the base residues, i.e., inward at the backbone. Schulte, Morrison and Garrett found that a critical level of Mg2+ is required for binding certain proteins to rRNA (Biochemistry (1974) 13, 1032). Thus the observed conformation is probably necessary for binding these proteins.

Binding Sites↗
PubMed1975

The significance of the 2' OH group and the influence of cations on the secondary structure of the RNA backbone.

In the IR spectra, the coupling of vibrations leads to band splitting and/or bands shifting in opposite directions which provides information on the mutual orientation of groupings. From such band shifts in the range 1800 to 1500 cm-1 one can draw conclusions on the double helix formation of polynucleotides. These band shifts are caused either by vibrational coupling of stretching vibrations within pairs of base residues or by coupling of stretching vibrations with the bending (scissor) vibration of the -NH2 groups; the latter is indicated by band shifts after deuterium substitution within the amino groups. Couplings of phosphate and 1 ibose vibrations in the range 1300 to 1000 cm-1 provide information on the secondary structure of the backbone. In order to obtain information of the structure of the RNA backbone, the IR spectra of poly(ribonucleotides) were studied in neutral media in which they were single-stranded. The shift due to coupling of the band of the 2'OD bending vibration and that of the antisymmetric stretching vibration of the ether group of the ribose residue proves that ribose residues of the backbone are cross-linked via hydrogen bonds. These are formed between the 2'OD or 2'OH groups, respectively, and the O atoms of the ether group of the neighboring ribose residues. This is the reason for the difference between DNA and RNA as regards the 2'OH group. The structure formation caused by these hydrogen bonds results in a stiffening of the RNA backbone. The tendency to form these hydrogen bonds increases in the order poly (U), poly(C), poly (A). This order of secondary structure stabilization is due to an interplay between the influences of (1) the 2'OH hydrogen bonds and (2) the base residues' stacking. Furthermore, the coupling of the antisymmetric stretching vibration of the greater than PO2- groups with a vibration involving the 2'OH group can result in a doublet structure of the band at about 1240 cm-1 if cations with strong fields are present. This probably shows that these cations can turn the greater than PO2-groups-which are usually turned outward at the backbone, as shown by construction of molecular models- towards the basic residues. Thus they cause stiff monohelices which are right-handed screws.

Birefringence↗
PubMed1972

Proton dispersion forces. Secondary-structure stabilizing forces between the hydrogen bonds of the polynucleotides.

In the double helix formed by the semiprotonated polycytidylic acid (poly C), both strands are linked via NH(+)...N hydrogen bonds. It is a known fact that such symmetrical hydrogen bonds with a double minimum potential well are extremely polarizable. This polarizability causes interaction effects, in particular the proton dispersion forces between such hydrogen bonds. These forces result in a shift of the energy levels and a continuum is observed in the infrared (IR) spectra of solutions in which such hydrogen bonds are present. The continuum occurs in the IR spectrum of the semiprotonated poly C, when the former is present in coiled state. If the double helix forms, an extremely broad band of the NH stretching vibration is observed instead of the continuum, since in the double helix all hydrogen bonds are oriented equally to one another and polarize each other mutually to a strong degree. The proton dispersion forces between the hydrogen bonds balance a considerable part of the electrostatic repulsion of the protons and hence enable the double helix to form. It is conceivable that an unsymmetrical double minimum potential well is present in the NH...N bonds in the DNA and RNA. Such bonds may likewise be considerably more polarizable than electron systems and thus, in this case too, proton dispersion forces would contribute to helix stabilization.

Cytosine Nucleotides↗