Long-Range Diffusion Coefficients in Two-Dimensional Fluid Media Measured by the Pyrene Excimer Reaction

Abstract

Diffusion-controlled bimolecular reactions in two-dimensional or quasi two-dimensional space are important in both surface and membrane reaction kinetics. A model reaction that has been extensively used to study the dynamics and structure of phospholipid membranes is the formation of the excimer of pyrene. We present experimental evidence that in multilamellar liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, in the fluid phase at temperatures above phase transition, T L , and for a probe/phospholipid ratio of 1:75, or lower, the fluorescence decay of 1-palmitoyl-2-(1-pyrenodecanoyl)-sn-glycero-3-phosphocholine and the timeresolved emission of the excimer follow a kinetic law specific for two-dimensional diffusion-controlled reactions. The parameters obtained from the application of this law to the experimental results, either from steady-state or from time-dependent fluorescence, give diffusion coefficients, e.g., D(25°C) ) 3.1 × 10 -8 cm 2 s -1 and an activation energy for diffusion, E a ) 35 kJ mol -1 , that are in accordance with the results of other independent techniques.

Molecular Mechanism of Lateral Diffusion of py10-PC and Free Pyrene in Fluid DMPC Bilayers

“…However, although in 3D the rate coefficient tends asymptotically to a constant value, the rate coefficient in 2D tends to zero at long times. The formalism for 2D diffusion-controlled kinetics was introduced by Razi Naqvi (1974), and we have proved that, in the case of 1-palmitoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (py 10 -PC) in fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, this formalism conduces to a selfconsistent kinetic analysis of pyrene monomer and excimer fluorescence (Martins et al, 1996). Moreover, we have also shown that, whatever the time range considered, and for a large range of diffusion coefficients, encounter distances, and 2D concentrations, Razi Naqvi's formalism is in perfect accordance with the results of the simulation of 2D bimo-lecular reactions using Monte Carlo techniques (Martins et al, 2000).…”

“…If so, this should imply that the effective 2D quencher concentration is doubled with respect to the case where the reaction takes place independently in each leaflet. We have proved in a previous work (Martins et al, 1996) that in fluid bilayers of POPC the pyrene moiety of probes pertaining to different sides can in fact interact, because only on this condition the parameters retrieved, namely M , from the analysis with the 2D formalism have physical meaning. This is apparently inconsistent with the value of 1.11 nm for the depth of the effective center of the pyrenyl moiety of py 10 -PC measured from the bilayer/water interface in POPC bilayers quoted by Sassaroli et al (1995).…”

“…Due to the profile of the order parameters of the acyl segments across a phospholipid bilayer this value is underestimated by ϳ0.3 nm. In the fluid phase the average length of the decanoyl segment is Ϸ1.0 nm (Cevc and Marsh, 1987), and the carbon C10 of the pyrenyl group is at Ϸ0.4 nm from the center of the ring system, resulting in a location for pyrene near the center of the bilayer (Martins et al, 1996). The fully extended length of the pyrenedecanoyl chain, from the carbon C1 to the hydrogen connected to C6 of the pyrenyl group, is ϳ1.99 nm, which compares with the maximum length of the myristoyl chain in the all-trans configuration, ϳ1.75 nm (Cevc and Marsh, 1987).…”

“…The diffusion coefficients obtained for this bulky group inside the bilayer are compared to the available data from FRAP (fluorescence recovery after photobleaching) and discussed in view of the different characteristics of the diffusing molecules. In addition to this analysis of the lateral diffusion mechanism, we also examine the validity of the pure 2D diffusion-controlled kinetic formalism, already validated for py 10 -PC/POPC (Martins et al, 1996) for excimer formation between free pyrene molecules in DMPC fluid bilayers.…”

Determination of Bulk and Surface Diffusion Coefficients from Experimental Data for Thin Liquid Film Drainage

“…Different experimental techniques are widely used for the determination of the bulk and surface diffusion coefficients (5)(6)(7)(8). An original simple idea for the simultaneous measurement of both diffusion coefficients is discussed in Ref.…”