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G Büldt

Publications and source records attributed to G Büldt.

8 recordsLinked to original sources
PubMed1991

Time-resolved X-ray diffraction study of structural changes associated with the photocycle of bacteriorhodopsin.

The time course of structural changes accompanying the transition from the M412 intermediate to the BR568 ground state in the photocycle of bacteriorhodopsin (BR) from Halobacterium halobium was studied at room temperature with a time resolution of 15 ms using synchrotron radiation X-ray diffraction. The M412 decay rate was slowed down by employing mutated BR Asp96Asn in purple membranes at two different pH-values. The observed light-induced intensity changes of in-plane X-ray reflections were fully reversible. For the mutated BR at neutral pH the kinetics of the structural alterations (tau 1/2 = 125 ms) were very similar to those of the optical changes characterizing the M412 decay, whereas at pH 9.6 the structural relaxation (tau 1/2 = 3 s) slightly lagged behind the absorbance changes at 410 nm. The overall X-ray intensity change between the M412 intermediate and the ground state was about 9% for the different samples investigated and is associated with electron density changes close to helix G, B and E. Similar changes (tau 1/2 = 1.3-3.6 s), which also confirm earlier neutron scattering results on the BR568 and M412 intermediates trapped at -180 degrees C, were observed with wild type BR retarded by 2 M guanidine hydrochloride (pH 9.4). The results unequivocally prove that the tertiary structure of BR changes during the photocycle.

Bacteriorhodopsins↗
PubMed1990

Water molecules and exchangeable hydrogen ions at the active centre of bacteriorhodopsin localized by neutron diffraction. Elements of the proton pathway?

Neutron diffraction is used to localize water molecules and/or exchangeable hydrogen ions in the purple membrane by H2O/2H2O exchange experiments at different values of relative humidity. At 100% relative humidity, differences in the hydration between protein and lipid areas are observed, accounting for an excess amount of about 100 molecules of water in the lipid domains per unit cell. A pronounced isotope effect was observed, reproducibly showing an increase in the lamellar spacing from 60 A in 2H2O to 68 A in H2O. At 15% relative humidity, the positions of exchangeable protons became visible. A dominant difference density peak corresponding to 11 +/- 2 exchangeable protons was detected in the central part of the projected structure of bacteriorhodopsin at the Schiff's base end of the chromophore. A difference density map obtained from data on purple membrane films at 15% relative humidity in 2H2O, and the same sample after complete drying in vacuum, revealed that about eight of these protons belong to four water molecules. This is direct evidence for tightly bound water molecules close to the chromophore binding site of bacteriorhodopsin, which could participate in the active steps of H+ translocation as well as in the proton pathway across this membrane protein.

Bacteriorhodopsins↗
PubMed1979

The influence of cholesterol on head group mobility in phospholipid membranes.

The dielectric dispersion in the MHz range of the zwitterionic dipolar phosphocholine head groups has been measured from 0--70 degrees C for various mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol. The abrupt change in the derived relaxation frequency f2 observed for pure DPPC at the gel-to-liquid crystalline phase transition at 42 degrees C reduces to a more gradual increase of frequency with temperature as the cholesterol content is increased. In general the presence of cholesterol increases the DPPC head group mobility due to its spacing effect. Below 42 degrees C no sudden changes in f2 are found at 20 or 33 mol% cholesterol, where phase boundaries have been suggested from other methods. Above 42 degrees C, however, a decrease in f2 at cholesterol contents up to 20--30 mol% is found. This is thought to be partly due to an additional restricting effect of the cholesterol on the number of hydrocarbon chain conformations and consequently on the area occupied by the DPPC molecules.

Cholesterol↗
PubMed1978

Zwitterionic dipoles as a dielectric probe for investigating head group mobility in phospholipid membranes.

For phospholipid membranes with zwitterionic head groups, the dipole can be considered as a specific label for tracing the changes in the dynamic behaviour of this region of the bilayer in its various phases. Measurements of the dielectric properties of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers in the frequency range 1--50 MHz show a dispersion which is attributed to the motion of the phosphocholine dipoles in the plane of the bilayers. When the temperature is varied, both the permittivity and loss factor increase sharply at the pretransition (35 degrees C) and the main transition (42 degress C). The relaxation time and amplitude were also determined for this dispersion and these further reflect the structural changes occurring with temperature. The relaxation times varied between 4 ns at 30 degrees C and 2.3 ns at 50 degrees C. Due to steric hindrances a restriction in the angle of head group rotation occurs at lower temperatures but is greatly reduced above the main transition.

Ions↗