Orientational Order of the Polyene Fatty Acid Membrane Probe trans-Parinaric Acid in Langmuir−Blodgett Multilayer Films

Abstract

trans-Parinaric acid, a polyene fatty acid, is widely used as a fluorescent membrane probe in photophysical and biophysical studies. We have worked on the orientational order of this molecule in Langmuir−Blodgett multilayer films of both gel-phase dipalmitoylphosphatidylcholine and arachidic acid at room temperature. Namely, orientational density probability functions were calculated using the maximum entropy method. Because liquid-crystal lipids do not deposit forming multilayers by Langmuir−Blodgett techniques, we have used Brownian dynamics methods to simulate a trans-parinaric acid molecule in a liquid-crystal bilayer environment. Orientational density probability functions are also achieved under the ergodic hypothesis. In the gel phase, trans-parinaric acid is highly ordered (narrow density probability function) and almost perpendicular (78°) to the multilayer planes. In the liquid-crystal phase, the density probability function broadens and the average orientation is not so close to the multilayer normal axis. The highly ordered system detected in the gel phase is in agreement with the preference for the gel phase in the partitioning of this probe among different phases, which is a very peculiar behavior. If its inclusion was not a favorable process, i.e., not adjusted to the membrane geometry, it could not have such a strict alignment. The change in orientational distribution upon phase transition is in agreement with published data on the dynamics of the probe in the nanosecond time range.

Using UV‐Vis. Linear Dichroism to Study the Orientation of Molecular Probes and Biomolecules in Lipidic Membranes

“…(sin(ω) and the relative refractive index, n, are introduced for experimental artefacts correction [50] -see Section 2.2.) The term sin(ω)A ω /A ω=π/2 is the dichroic ratio (sin(ω)A ω − A ω=π/2 is sometimes named "linear dichroism" but the designation is more commonly used to mean the methodology itself instead of this experimental parameter).…”

“…It can be interpreted as the time during which the transition moment has a particular, fixed orientation relative to the director. G, f (α) and n are correction factors [47,50] (see Section 2.2). The subscripts in I ij refer to the position of the excitation (i) or emission (j) polarizers in the lab frame (v -vertical; h -horizontal).…”

Sources and Bioactive Properties of Conjugated Dietary Fatty Acids

“…A conjugated C18:4 fatty acid known as α-parinaric acid can also be found in the oils of certain plant seeds and algae (Table 1). This c9, t11, t13, c15-18:4 conjugated isomer along with its t9, t11, t13, t15-18:4 counterpart (β-parinaric acid) have previously been utilized as probes to characterize lipid-lipid and lipid-protein interactions [106][107][108][109], and as sensitive markers to characterize lipid peroxidation in eukaryotic systems [110][111][112][113]. There is also evidence to indicate that α-parinaric acid may possess cytotoxic properties towards cancer cells, including human monocytic leukaemia cells and malignant gliomas, mediated through increased cellular lipid peroxidation [83,84].…”

Overview of Common Spectroscopic Methods to Determine the Orientation/Alignment of Membrane Probes and Drugs in Lipidic Bilayers

“…LD methodologies have been applied in a large number of studies involving different classes of molecules such as membrane probes [7,8], polyene macrolide antibiotics [31,32], multifunctional peptides [33,34] and DNA-ligand systems [35].…”

“…Most times our notion of probes orientation relative to the membrane surface is guided by chemical intuition based on what is known for the bilayers structure, mainly the conformation of the acyl chain of lipids [5,6]. Nevertheless, although sometimes chemical intuition proves realistic [7], others is illusory [8]. Depending on the membrane property to be reported or the phenomena involved (e.g.…”