Research Papers: Conduction

Molecular Heat Transfer in Lipid Bilayers With Symmetric and Asymmetric Tail Chains

[+] Author and Article Information
Takeo Nakano

e-mail: nakano@microheat.ifs.tohoku.ac.jp

Gota Kikugawa

e-mail: kikugawa@microheat.ifs.tohoku.ac.jp

Taku Ohara

e-mail: ohara@ifs.tohoku.ac.jp
Institute of Fluid Science,
Tohoku University,
2-1-1 Katahira, Aoba-ku,
Sendai, Miyagi 980-8577, Japan

1Corresponding author.

Manuscript received September 29, 2012; final manuscript received January 21, 2013; published online May 16, 2013. Assoc. Editor: Leslie Phinney.

J. Heat Transfer 135(6), 061301 (May 16, 2013) (8 pages) Paper No: HT-12-1531; doi: 10.1115/1.4023572 History: Received September 29, 2012; Revised January 21, 2013

Intramolecular energy transfer in polymer molecules plays a dominant role in heat conduction in polymer materials. In soft matter where polymer molecules form an ordered structure, the intramolecular energy transfer works in an anisotropic manner, which results in an anisotropic thermal conductivity. Based on this idea, thermal energy transfer in lipid bilayers, a typical example of soft matter, has been analyzed in the present study. Nonequilibrium molecular dynamics simulations were carried out on single component lipid bilayers with ambient water. In the simulations, dipalmitoyl-phosphatidyl-choline (DPPC), dilauroyl-phosphatidyl-choline (DLPC), and stearoyl-myristoyl-phosphatidyl-choline (SMPC), which have two alkyl chains with 16 C atoms for each, 12 C atoms for each, and 18 and 14 C atoms, respectively, were used as lipid molecules. The thermal energy transfer has been decomposed to inter- and intramolecular energy transfer between individual molecules or molecular sites, and its characteristics were discussed. In the case of heat conduction in the direction across the membranes (cross-plane heat conduction), the highest thermal resistance exists at the center of the lipid bilayer, where lipid alkyl chains face each other. The asymmetric chain length of SMPC reduces this thermal resistance at the interface between lipid monolayers. The cross-plane thermal conductivities of lipid monolayers are 4.8–6.5 times as high as the ones in the direction parallel to the membranes (in-plane) for the cases of the tested lipids. The overall cross-plane thermal conductivities of the lipid bilayers are reduced to be approximately half of those of the monolayers, due to the thermal resistance at the interfaces between two monolayers. The lipid bilayer of SMPC with tail chains of asymmetric length exhibits the highest cross-plane thermal conductivity. These results provide detailed information about the transport characteristics of thermal energy in soft matter, which are new materials with design flexibility and biocompatibility. The results lead to their design to realize desired thermophysical properties and functions.

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Grahic Jump Location
Fig. 1

Molecules of the three species of lipids examined in the present study: (a) DPPC, (b) DLPC, and (c) SMPC

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Fig. 2

Simulation systems for the steady thermal energy transfer in the directions (A) parallel and (B) perpendicular to the bilayer membrane plane

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Fig. 3

Temperature distributions along the membranes according to the in-plane heat flux in the lipid systems of (a) DPPC, (b) DLPC, and (c) SMPC. Only half of the systems are shown because of the system symmetry. Temperatures of total, lipid, and water are plotted by the solid line, boxes, and circles, respectively.

Grahic Jump Location
Fig. 4

Temperature and density distributions across the membrane for the cross-plane heat flux in the systems of (a) DPPC, (b) DLPC, and (c) SMPC. Only half of the systems are shown because of the system symmetry. Temperatures of total, left lipid monolayer, right lipid monolayer, and water are plotted by the solid line, triangles, inverted triangles, and circles, respectively. Density of each component is displayed by the short-dashed, dashed, and dotted lines, respectively.

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Fig. 5

Contributions of transport of thermal energy due to translational motion of molecules (E1), and inter- and intramolecular energy transfer due to molecular interactions (E2) to the total heat fluxes in the case of in-plane heat conduction in the systems of DPPC, DLPC, and SMPC lipid bilayer membranes with water. The portion indicated as “lipid-lipid” includes intermolecular energy transfer between lipid molecules (E2a) and intramolecular energy transfer within each lipid molecule (E2b).

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Fig. 6

Contributions of inter- (E2a, red bars) and intramolecular (E2b, green bars) energy transfers to the heat flux in the DPPC, DLPC, and SMPC lipid bilayer membranes. The ratios to the total heat flux due to interaction (E2) are displayed.

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Fig. 7

Distribution of total heat flux (bars) and contributions of molecular thermal energy transport (E1, boxes), thermal energy transfer due to molecular interaction (E2, circles), inter- (inverted triangles) and intramolecular (triangles) energy transfers to the heat flux in the systems of a lipid bilayer of (a) DPPC, (b) DLPC, and (c) SMPC and water. Only the left half of the system is displayed because of the system symmetry.




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