New fluorescent probes reveal that flippase-mediated flip-flop of phosphatidylinositol across the endoplasmic reticulum membrane does not depend on the stereochemistry of the lipid

Glycerophospholipid flip-flop across biogenic membranes such as the endoplasmic reticulum (ER) is a fundamental feature of membrane biogenesis. Flip-flop requires the activity of specific membrane proteins called flippases. These proteins have yet to be identified in biogenic membranes and the molecular basis of their action is unknown. It is generally believed that flippase-facilitated glycerophospholipid flip-flop across the ER is governed by the stereochemistry of the glycerolipid, but this important issue has not been resolved. Here we investigate whether the ER flippase stereochemically recognizes the glycerophospholipids that it transports. To address this question we selected phosphatidylinositol (PI), a biologically important molecule with chiral centres in both its myo-inositol headgroup and its glycerol-lipid tail. The flip-flop of PI across the ER has not been previously reported. We synthesized fluorescence-labeled forms of all four diastereoisomers of PI and evaluated their flipping in rat liver ER vesicles, as well as in flippase-containing proteoliposomes reconstituted from a detergent extract of ER. Our results show that the flippase is able to translocate all four PI isomers and that both glycerol isomers of PI flip-flop across the ER membrane at rates similar to that measured for fluorescence-labeled phosphatidylcholine. Our data have important implications for recent hypotheses concerning the evolution of distinct homochiral glycerophospholipid membranes during the speciation of archaea and bacteria/eukarya from a common cellular ancestor.     

Phase relations, quasicontinuous spectra and subfemtosecond pulses in high-order stimulated raman scattering with short-pulse excitation

High-order stimulated Raman scattering for pumping by ps and sub-ps pulses is studied in the frame of a time-domain approach without the slowly varying envelope approximation. Formation of pulse trains with sub-fs durations in the ps-pump regime is demonstrated using external phase compensation. For sub-ps excitation a novel broadening mechanism of Raman lines is predicted that leads to a quasicontinuous spectrum and permits one to generate single sub-fs pulses. We predict also essential shortening of a delayed fs-probe pulse by the Raman oscillations without phase control.     

Zur kernmagnetischen Relaxation in Mehrbereichsystemen mit endlicher räumlicher Ausdehnung

General equations, describing the nuclear spin relaxation behaviour in heterogeneous systems, consisting of regions with different relaxation times and diffusion coefficients, are presented. For a special two region system, by means of analogue computing technique the influence of both diffusivity and height of the barriers between the regions on total relaxation is studied. It is shown, that using appropriate expressions for the mean lifetimes of the nuclei in the regions, the relaxation function of total magnetisation may be approximated by the formulae of ZIMMERMAN and BRITTIN . These relations may be used for a rough estimation of diffusivity in the systems under consideration.     

The preparation und unusual rearrangement of triply bridged μ-heteromethylenecobalt complexes. Molecular structure of μ-ethoxycarbonylmethylenebis[carbonyl-(η5-cyclopentadienyl)cobalt] (CoCo)

the μ-heteromethylenecobalt complexes, which have been synthesized by photolysis of (η⁵-C₅H₅)Co(CO)₂ in the presence of the corresponding diazoacetates at < ?90°C in tetrahydrofuran, undergo a novel type of intramolecular irreversible rearrangement yielding two isomers. The structure of one of the isomers has been established by X-ray analysis. A comparison of the structural parameters of all known μ-methylene-transition metal complexes shows that the internal angles α of the cyclopropane-like systems are far less sensitive to the compositions and structures of the molecules than the external angles β. The internal angles cannot be readily correlated with other features of the molecule.     

Organoboron compounds: XIX. Alkoxydialkylaminophenylboranes

The preparation and properties of a series of alkoxydialkylaminophenylboranes are reported and their ¹H and ¹³C NMR spectra discussed. The spectra indicate that there is restricted rotation about the boronnitrogen bond in each compound studied.