Internal and interfacial structure of membranes studied using X-ray standing waves

Area of Science:

• Materials Science
• Biophysics
• Surface Chemistry

Background:

• The X-ray standing wave (XSW) method, developed in the mid-1960s, initially determined heavy atom positions in silicon and germanium crystals with sub-Angström resolution.
• Layered synthetic microstructures advanced XSW applications to biologically relevant structures on the tens of Angströms scale.

Purpose of the Study:

• To explore the utility of XSW for studying biological structures and interfacial phenomena.
• To establish the effective probing distance of XSW for layered systems.
• To profile ion distribution at the membrane/aqueous interface and investigate the diffuse double layer.

Main Methods:

• Utilizing X-ray standing wave (XSW) techniques.
• Employing Langmuir-Blodgett (LB) films for model membrane studies.
• Investigating XSW probing depth up to approximately 1000 Å.
• Profiling ion distribution at the membrane/aqueous interface by adjusting pH.

Main Results:

• XSW demonstrated sub-Angström resolution for heavy atom location in model membrane LB films.
• The heavy-atom layer's movement during thermotropic transitions was successfully tracked.
• XSW probing depth was found to be around 1000 Å without resolution loss.
• Reversible establishment of the diffuse double layer at the phospholipid membrane/aqueous interface was observed by pH adjustment.

Conclusions:

• XSW is a powerful tool for determining heavy atom positions in biological and model membrane systems with high resolution.
• The method's effective probing distance is substantial, enabling studies of thicker or more complex structures.
• XSW provides direct insights into ion distribution and interfacial phenomena at membrane surfaces, with potential for reversible control via pH.