PubMed · 14153109
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
R VANROSSUM. 1964-05-30. [RADIOACTIVE FALLOUT].. https://pubmed.ncbi.nlm.nih.gov/14153109/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
We have developed conditions for recording the low-frequency S(2)/S(1) Fourier transform infrared difference spectrum of hydrated PSII samples. By exchanging PSII samples with buffered (18)O water, we found that a positive band at 606 cm(-)(1) in the S(2)/S(1) spectrum in (16)O water is clearly downshifted to 596 cm(-)(1) in (18)O water. By taking double-difference (S(2)/S(1) and (16)O minus (18)O) spectra, we assign the 606 cm(-)(1) mode to an S(2) mode and also identify a corresponding S(1) mode at about 625 cm(-)(1). In addition, by Sr and (44)Ca substitution experiments, we found that the 606 cm(-)(1) mode is upshifted to about 618 cm(-)(1) by Sr(2+) substitution but that this mode is not affected by substitution with the (44)Ca isotope. On the basis of these results and also on the basis of studies of Mn model compounds, we assign the 625 cm(-)(1) mode in the S(1) state and the 606 cm(-)(1) mode in the S(2) state to a Mn-O-Mn cluster vibration of the oxygen-evolving complex (OEC) in PSII. This structure may include additional bridge(s), which could be another oxo, carboxylato(s), or atoms derived from an amino acid side chain. Our results indicate that the bridged oxygen atom shown in this Mn-O-Mn cluster is exchangeable and accessible by water. The downshift in the Mn-O-Mn cluster vibration as manganese is oxidized during the S(1) --> S(2) transition is counterintuitive; we discuss possible origins of this behavior. Our results also indicate that Sr(2+) substitution in PSII causes a small structural perturbation that affects the bond strength of the Mn-O-Mn cluster in the PSII OEC. This suggests that Sr(2+), and by inference, Ca(2+), communicates with, but is not integral to, the manganese core.
Melittin is a naturally occurring hexacosa peptide which forms an amphiphilic helix in methanol, a random coil in water, and a tetramer of helices at basic pH or in the presence of a high salt concentration. The monomeric structure in methanol has been well characterized by proton NMR (Pastore et al. (1989) Eur. Biophys. J., 16, 363-367). In the present paper, chemical shifts of the backbone alpha-carbons of melittin in methanol were determined by mapping previously published alpha-proton shifts (Bazzo et al. (1988) Eur. J. Biochem., 173, 139-146), to natural abundance (1H)13C cross peaks appearing in the 2D heteronuclear multiple-quantum NMR spectrum. Changes in chemical shifts consequent to stepwise increases in the percentage of water in a mixed methanol/water solvent system were observed in similar spectra. The alpha-carbon shifts varied more smoothly than the corresponding alpha-proton shifts and were found to correlate with the transition from the helix to the random coil conformer in parallel with changes in the circular dichroism spectrum. Chemical shifts of this peptide are interpreted with regard to the current database of assignments in proteins of known 3D structure (Wishart et al. (1991) J. Mol. Biol., 222, 311-333). The N-terminal region of the peptide shows increased flexibility at lower methanol concentrations, as evidenced by the merger of the alpha-proton resonances of G1 (at 40% and 15% methanol) and G3 (at 15% methanol). Conformational exchange rates for G1 and G3 were estimated by comparison of the experimental spectra with simulated spectra and found to be as large as 4000 s-1 for G1 in 40% and 15% methanol and 600 s-1 for G3 in 15% methanol. Overall, these 1H and 13C chemical shift data support the description of monomeric melittin in methanol currently evolving in the literature and suggest a structure composed of a linked pair of helices with different structural stabilities, each of which experiences dynamical fraying at its free terminus.
Explore the source record for details and available documents.