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A pH titration study on the ionic bridging within lipopolysaccharide aggregates.

R T Coughlin, A A Peterson, A Haug

    Biochimica Et Biophysica Acta
    |December 19, 1985
    PubMed
    Summary
    This summary is machine-generated.

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    Lipopolysaccharide (LPS) aggregate rigidity in Escherichia coli is pH-dependent, influenced by ionizable groups and ionic bridging. Metal ion type also affects LPS head-group mobility and packing.

    Area of Science:

    • Biochemistry
    • Microbiology
    • Physical Chemistry

    Background:

    • Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram-negative bacteria, crucial for structural integrity and interaction with the environment.
    • The aggregation state and physical properties of LPS are influenced by environmental factors such as pH and ionic composition, affecting bacterial physiology.

    Purpose of the Study:

    • To investigate the impact of pH and divalent/monovalent cations on the packing and head-group motion of lipopolysaccharide aggregates from rough strains of Escherichia coli.
    • To elucidate the role of ionizable groups and ionic bridging in modulating LPS aggregation and physical properties.

    Main Methods:

    • Electron spin resonance (ESR) spectroscopy was employed to measure lipopolysaccharide head-group motion.

    Related Experiment Videos

  • Differential scanning calorimetry (DSC) was used to analyze phase transitions and their enthalpies (ΔHt).
  • Experiments were conducted across a range of pH values (5 to 12) and with different salt forms (sodium and magnesium).

    • Main Results:

    • Lipopolysaccharide (LPS) head-group motion exhibited significant pH dependence, indicating the presence of multiple ionizable groups within the LPS structure.
    • LPS aggregates from rough (Ra) and heptose-less (Re) mutants showed increased rigidity at acidic pH (5) compared to alkaline pH (10.5).
    • The type of cation salt influenced LPS mobility: magnesium salts were less mobile than sodium salts at neutral pH (7.0), but similar at high pH (12).
    • Phase transition enthalpy (ΔHt) for Re LPS sodium salt peaked at pH 7.5 and diminished at acidic and basic pH.
    • Ionic bridging between phosphate groups was identified as a key factor in rigidifying LPS head-group interactions, particularly in neutral to acidic conditions.

    Conclusions:

    • Fixed charges in the core and lipid A regions of LPS play a critical role in determining head-group motion and aggregation state.
    • pH-dependent ionic interactions, including ionic bridging, significantly alter the physical properties and packing of LPS aggregates.
    • Understanding these physicochemical properties is essential for comprehending LPS structure-function relationships and bacterial outer membrane stability.