Conformational Analysis of Hexagonal Arrangements of Phosphatidylcholine Head Groups with Bound Waters by Molecular Mechanics

Yoshiro NAKATA*, Akira TAKAHASHI*1 and Toshiharu TAKIZAWA*1

Introduction

Methods

Figure 1. Chemical Structure of phosphatidylcholine head group and torsional rotation angle definitions.

Hexagonal arrangement of phosphatidylcholine head groups in a bilayer plane; It is assumed that the lipid molecules are packed in a hexagonal lattice, and that each lipid head group reorients randomly about the bilayer normal. The distance between neighboring molecules is 8.7A, which gives the surface area of 76A2 per molecule at a temperature near the main transition in the La phase for DPPC[10].
Two head molecules were placed in distance 8.7A as the same orientation in same plane. Then the intermolecular energy scan was performed by simultaneously varying C2-C3 bonds from 0 to 360 at 5 intervals. The minimum energy orientation was selected in Fig. 2 as the state I. The state II and III were obtained by rotating two molecules to 60 and 120 simultaneously around the C2-C3 bonds.

Figure 2. The calculated potential   Figure 3. Hexagonal arrangement 
function around C2-C3 bonds          model of phosphatidylcholine 
in two head group system.            head groups.

Results and Discussion

 
Figure 4a. The calculated potential   Figure 4b. The calculated potential
function around P=O3 bond.            function around P=O4 bond.

Figure 5. A hexagonal arrangement of the phosphatidylcholine head groups model.

We have studied experimentally the spin-lattice relaxation for the proton of water(HDO) bound to the head group in dipalmitoylphosphatidylcholine (DPPC)-D2O system. Based on the preferred conformations and orientations of the head groups, the relaxation rate of bound water can be calculated, which seems fairly agreeable to the experimental value[11].

References