Melody L. Mitchell

The interfacial structure of phospholipid monolayer films: an infrared reflectance study

Abstract Monolayer films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water (A/W) interface have been investigated in situ using external reflection-absorption IR spectroscopy. The IR spectra of the monolayer films were monitored as a function of the two-dimensional molecular area of the phospholipd molecules on the surface of a Langmuir trough. Examination of the conformation-sensitive CH stretching bands of the molecules hydrocarbon chains reveals the existence of two phase transitions during film compression. The first transition corresponds to the main liquid-expanded-to-liquid-condensed monolayer phase transition (at ∼58–78 A2 molecule−1), while the second transition occurs at ∼40–57 A2 molecule−1 and is indicative of a transition to a solid-condensed phase under increasing pressure. The headgroup bands from the phosphate ester group were also monitored during film compression. The asymmetric phosphate stretching band of the DPPC monolayer film is composed of two bands, one at ∼1220 cm−1 and the second at ∼1257 cm−1. These bands are also seen, and frequency shifts noted, for the interaction of the soluble cations Ca2+ and Pr3+ with the phospholipid phosphate group. The band splitting and frequency shifts suggest a mixture of hydration states for the lipids phosphate group. The equilibrium between these hydration states can be changed by film compression and/or cation interaction with the lipid headgroup.

Design and Interfacing of an Automated Langmuir-Type Film Balance to an FT-IR Spectrometer

We report the design of a surface analysis accessory that allows for the use of external reflection FT-IR spectroscopy to investigate the structure of insoluble monolayer films in situ at the air/water interface. The Langmuir film balance constructed for this purpose is optimized to contain the monolayer film at the focal point of the infrared beam, as well as to generate high surface pressures within the film. The optical interfacing of the film balance to an FT-IR spectrometer is also described. Results are presented for a representative surface pressure/molecular area isotherm and infrared spectrum for a phospholipid monolayer film.

In situ FT-IR studies of langmuir-blodgett monolayers

The liquid-expanded (LE) to liquid condensed (LC) phase transition of monomolecular films of 1,2-dipalmitpol-5-sn-glycero-3-phosphocholine (DPPC) on water substrates was examined via single reflectance FT-IR Spectroscopy in the CH2 stretching region. A continuous decrease in frequency observed in these bands as a function of decreasing molecular area indicates that the DPPC monolayer transition involves a conformational change in the hydrocarbon chains and is biphasic in nature.

Understanding The Interfacial Structure Of Aqueous Phospholipid Monolayer Films Via External Reflection FT-IR Spectroscopy.

Monolayer films of dimyristoyl-phosphatidic-acid (DMPA) at neutral and basic pH exhibit first-order phase transitions in their pressure-area curves. In situ external reflection FT-IR studies in the CH, stretching bands over this phase transition region exhibit a --6 cm-1 shift similar to that observed in previous studies of dipalmitoyl-phosphotidylcholine (DPPC)1. The acid form of DMPA at pH 3.0 does not exhibit the first order phase transition, but a ~1cm-1 frequency shift is observed in the liquid condensed phase and is also present in the neutral pH form. A solid-solid phase transition is proposed. Examination of the polar headgroup region (1300-960 cm-1)for acidic, neutral, and basic forms of DMPA give characteristic bands of each protonation state of PO3.