Figure 22 DNA-DNA spacing as a function of p in a series of theoretical and experimental results. The theoretical results correspond to (top to bottom) ^ = 0.3, 0.4, 0.5, 0.6, and 0.8; all results are presented for a screening length of 50 A (corresponding to ca. 4 nm of bathing salt solution). The experimental results correspond to ^ = 0.3 (squares), 0.5 (circles), and 0.7 (triangles), and were performed with no added salt. (Theoretical results adapted from Refs. (125) and (145); experimental results adapted from Ref. 101.)

(AFM) study by Fang and Yang (133,134) of DNA adsorption on supported lipid bilayers. In these experiments, DNA was first adsorbed on dipalmitoyldimethylammoniumylpropane (DPDAP) or distearoyl-DAP (DSDAP) CL bilayers, assumed to be in the gel phase. After equilibration and saturation of the surface, the DNA bulk solution was removed, and the surface was put in contact with solution of various concentrations of NaCl. After further equilibration, the salt solution was removed and the surface imaged by AFM. Plasmid and linear DNA similarly treated showed similar results.

Striking, fingerprint-like images of DNA adsorbed on the surface were revealed (Fig. 23). The typical domain size for the aligned, smecticlike order is usually several hundred Angstroms, reflecting the DNA's intrinsic persistence length. These structures are expected to be like those found in L°a complexes: the domain size, inferred from x-ray scattering is quite similar (103-104). Furthermore, it was found that the surfaces are often overcharged when DNA is adsorbed, (i.e., the number of DNA fixed charges exceeds the number of lipid charges). This can be anticipated on the basis of theoretical studies of a similar problem: adsorption of charged globular proteins (yet another macroion) on oppositely charged membranes (135). In both cases the driving force for adsorption is similar to that driving lipoplex formation, namely, counterion release. In Lca complex formation, much of the DNA can interact with the 2 sandwiching bilayers. In contrast, topology dictates that adsorbates on a single lipid bilayer will always pos-

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