1 research outputs found
Reduced Condensing and Ordering Effects by 7‑Ketocholesterol and 5β,6β-Epoxycholesterol on DPPC Monolayers
The exposure of organic-coated marine
aerosols containing cholesterol
(Chol) to radiation and/or an oxidizing atmosphere results in the
formation of oxidized derivatives or oxysterols and will likely change
aerosol surface properties. However, the intermolecular interactions
between oxysterols and other lipid components and their influence
on the surface properties of marine aerosols are not well-known. To
address this question, the interfacial behavior and domain morphology
of model Langmuir monolayers of two ring-substituted oxysterols, 7-ketocholesterol
(7-KChol) and 5β,6β-epoxycholesterol (5,6β-EChol),
mixed with 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine
(DPPC) were investigated by means of compression isotherms and Brewster
angle microscopy (BAM) over a broad range of surface pressures and
sterol molar ratios. Mixed DPPC/cholesterol (Chol) monolayers were
also measured for comparison. The results of compression experiments
showed that the condensing effect induced on mixed DPPC/sterol monolayers
at low surface pressures and for intermediate molar ratios (0.3 ≤ <i>X</i><sub>sterol</sub> ≤ 0.7) was weaker for oxysterols
than for Chol. Additionally, mixed DPPC/oxysterol monolayers exhibited
markedly smaller (∼2–3-fold) interfacial rigidity. Examination
of the excess free energy of mixing further revealed that DPPC monolayers
containing 7-KChol and Chol were thermodynamically more stable at
high surface pressures than those with 5,6β-EChol, indicating
that the strength of interactions between DPPC and 5,6β-EChol
was the smallest. Finally, BAM images in the LE–LC phase of
DPPC revealed that in comparison to Chol the addition of small amounts
of oxysterols results in larger and less numerous domains, showing
that oxysterols are not as effective in fluidizing the condensed phase
of DPPC. Taken together, these results suggest that the strength of
van der Waals interactions of DPPC alkyl chains with sterols follows
the sterol hydrophobicity, with Chol being the most hydrophobic and
oxysterols more hydrophilic due to their ketone and epoxy moieties.
The difference in the condensing ability and stability of 7-KChol
and 5,6β-EChol on DPPC likely originates from the distinct molecular
structure and position of oxidation on the steroid nucleus. As suggested
by recent MD simulations, depending on the oxidation position, ring-substituted
oxysterols have a broader angular distribution of orientation than
Chol in bilayers, which could be responsible for the observed reduction
in condensing ability