Influence of micellar size on the structure of surfactant-DNA complexes

A. V. Radhakrishnan, S. Madhukar, A. Chowdhury, V. A. Raghunathan
Phys. Rev. E. 2022-06-24; 105(6):
DOI: 10.1103/PhysRevE.105.064504

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1. Phys Rev E. 2022 Jun;105(6-1):064504. doi: 10.1103/PhysRevE.105.064504.

Influence of micellar size on the structure of surfactant-DNA complexes.

Radhakrishnan AV(1), Madhukar S(1), Chowdhury A(1), Raghunathan VA(1).

Author information:
(1)Raman Research Institute, Bangalore 560 080, India.

We have studied the structure of complexes of the cationic surfactant
dodecyltrimethylammonium bromide (DTAB) with DNA as a function of surfactant to
DNA base molar ratio (R) and salt concentration. Small-angle x-ray scattering
data show the formation of nematic gels at lower and higher salt concentrations,
irrespective of the value of R. Two crystalline phases are observed over
intermediate salt concentrations; a square (S) phase for R>3 and a hexagonal
(H_{S}) phase for lower R. Electron density maps of these phases show
intercalated structures, with DTAB micelles sandwiched between long DNA strands.
The composition of these complexes, estimated using elemental analysis, indicates
that the micelles are not very long, and they occupy only about half of the
interstitial volume between the DNA strands. This phase behavior is strikingly
different from that of complexes of DNA with longer chain surfactants
cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide
(TTAB), which show only a hexagonal (H) phase over similar ranges of R and salt
concentration, the H_{S} structure observed in the present study being a
sqrt[3]×sqrt[3] superlattice of the H structure. Madelung energies of the S and H
structures, calculated from the electrostatic interaction between their
cylindrical constituents, suggest that the former is preferred in DTAB-DNA
complexes due to the smaller micellar radius of DTAB. The propensity of DTAB to
form short micelles seems also to favor the H_{S} phase at lower R. These results
illustrate the important role of micellar size in determining the structure of
these two-dimensional macro-ion crystals.

DOI: 10.1103/PhysRevE.105.064504
PMID: 35854566

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