Pyrene-fatty acid-containing phospholipid analogues: Characterization of monolayers and Langmuir-Blodgett assemblies

Abstract

The following pyrene-containing phospholipid analogues have been studied: 1-palmitoyl-2(pyren-1-yl)decanoyl-sn-glycero-3-phosphatidylcholine§ (PPDPC), -glycerol (PPDPG), and -PE (PPDPE), 1-palmitoyl-2(pyren-1-yl)hexanoyl-sn-glycero-3-phosphatidylmethanol (PPHPC), 1-octacosanyl-2(pyren-1-yl)hexanoyl-sn-glycero-3-phosphatidylmethanol (C-28-O-PHPM) and 1-octacosanyl-2(pyren-1-yl)octanoyl-sn-glycero-3-phosphatidylmethanol (C-28-O-POPM). These compounds were first characterized by compression isotherms and surface potential measured at an air/water interface, after which conditions for their transfer onto Cd arachidate-coated quartz glass slides were sought. The transferred layers were investigated for their absorption in the range of 300–650 nm. All compounds revealed spectra characteristic for pyrene. It was further found that with the slide at an angle of 45° to the beam, the ratio of absorption of light polarized parallel to that polarized perpendicular to the layer plane, A ∥/ A ⊥, was strongly dependent on the chemical structure of the lipid derivative; however, the surface pressure at which the film was maintained during the deposition had less effect. In fluorescence emission the excimer fluorescence intensity ( I e) exceeded by several times the intensity of emission from the monomeric pyrene species ( I m). The most significant new findings of the present study came from preliminary fluorescence decay measurements of transferred layers of PPDPG. These were obtained using time-resolved photon counting with the sample held in vacuo and at a temperature of 10, 200, 250, 280, or 300 K. The observed rapid non-exponential decay of monomer fluorescence at 380 nm was rather insensitive to temperature. The decay of excimer emission at 380 K appeared to exhibit three components of approximate lifetimes of (a) below 0.1 ns, (b) 43 ns and (c) 83 ns. The relative proportions of the component lifetimes were sensitive to temperature. Notably, in the temperature range of 10–300 K the decay of excimer emission was observed immediately following the nanosecond flash, i.e. the kinetics of excimer formation from monomeric pyrene could not be resolved with the instrument used. Therefore, we conclude that the organization of pyrene in the phospholipid liquid crystal monolayer on quartz to be such that redistribution of electronic excitation to yield excimers is allowed within less than 0.1 ns following the formation of the excited monomeric species. This indicates the very close proximity of the pyrene moieties, yet without significant ground state interaction, as suggested by the similarity to the absorption spectra of pyrene in organic solvents.