MOLECULAR-DYNAMICS SIMULATION ANALYSIS OF A SODIUM DODECYL-SULFATE MICELLE IN AQUEOUS-SOLUTION - DECREASED FLUIDITY OF THE MICELLE HYDROCARBON INTERIOR
Ad. Mackerell, MOLECULAR-DYNAMICS SIMULATION ANALYSIS OF A SODIUM DODECYL-SULFATE MICELLE IN AQUEOUS-SOLUTION - DECREASED FLUIDITY OF THE MICELLE HYDROCARBON INTERIOR, Journal of physical chemistry, 99(7), 1995, pp. 1846-1855
Structural and dynamic properties of a sodium dodecyl sulfate micelle
were studied in aqueous solution via a molecular dynamics simulation u
sing periodic boundary conditions. Results are presented for both the
average structure and dynamic properties of the micelle. Over the cour
se of the simulation the micelle remained spherical with a radius of g
yration in agreement with experiment. Motions of individual lipid head
groups were significant, with calculated changes of up to 7 Angstrom
occurring with respect to the micelle center of mass and 8 Angstrom pa
rallel to the surface of the micelle. These motions were reminiscent o
f a piston in a cylinder or the movement of the head groups along the
surface of the micelle. The micelle hydrocarbon interior is predicted
to be less fluid than a pure alkane based on decreased dihedral transi
tion rates and an increased free energy barrier to dihedral rotation o
f the aliphatic tails as compared to pure dodecane. This result contra
sts calculations on a dipalmitoyl phosphatidylcholine Lipid bilayer wh
ere the fluidity of the hydrocarbon interior was similar to that of pu
re hexadecane (Venable, R. M.; Zhang, Y.; Hardy, B. J.; Pastor, R. W.
Science 1993, 262, 223-226). The predicted decrease in fluidity should
be taken into account when micelles are used as model systems for lip
id bilayers. The overall relative trans to gauche populations, however
, are equivalent for the micelle and dodecane. Interactions between wa
ter and the micelle involve the sulfate head groups while the interior
of the micelle is void of water. It is predicted, however, that the t
erminal methyl group of the hydrocarbon chain, in specific instances,
may be located at the micelle surface and exposed to solvent. Interact
ions of the sodium ions with the micelle sulfate head groups occur pri
marily via the second hydration shell of the sulfate; no stable sodium
to sulfate contact pairs were observed.