DNA AND PLPC CONCENTRATED SOLUTIONS: A SAXS STUDY OF lNTERACTlNG SYSTEMS. L. Q. Amaral, V. Castelletto and R Itri, Instituto de Fisica, Universidade de Sao Paulo, Caixa Postal 66318, 05389-970-Sao Paulo, SP, Brazil

Results obtained by smal1 angle X-Ray scattering (SAXS) in systems with interactions are presented, with emphasis on information obtained from both the particle form factor P(q) and interference function S(q). DNA fragments in water represent a solution of rod particles with cylindrical symmeny while PLPC (palmitoyl lisophosphatidil choline) in water form spheroidal micelles. DNA solutions are studied¹ in the isotropic (I) phase in the semidilute regime until the I-cholesteric phase transition. The interference peak position q_m as a function of concentration fits an universal curve with exponent 1/2 for an effective rod length L=340Å . The form factor of the DNA fragments is obtained from the less concentrated solution, where interference effects are not present in the measured range, and it is in good agreement with the B form of DNA. The interference curves for the more concentrated solutions are experimentally derived by dividing the corrected SAXS curve by the, particle form factor. The Indirect Transformation Method in Reciprocal Space is used to get the desmeared SAXS curve. Modeling of the interference peaks with Gaussian functions compares wel1 with recent theories for interparticle interactions on solutions of rodlike polyelectrolytes. The peak broadening [[beta]] expressed as [[beta]]L in function of concentration also fits universal curve with exponent 1/2. The fitting shows that the short-range order for rods in the semidilute region has correlation length slightly above first neighbors. In the case of inhomogeneous globullar micelles as PLPC it is also possible to obtain² the particle distance distribution function p(r) for lower concentrations. For higher concentrations the SAXS curve is fitted to the smeared product P(q)S(q), with S(q) calculated in the mean spherical approximation. The lipid system PLPC/water is an example of isotropic solution leading to a cubic phase. Analysis of SAXS in terms of P(q) and S(q) show that the micelle form remains stable and with small anisotropy. This is in contrast with detergent/water systems, where there is micellar growth and the isotropic solution leads to an hexagonal phase.³

1- V. Castelletto,R. Itri and L.Q.Amaral, Macromolecules 28, 8395 (1995).

2- R. Itri and L.Q. Amaral, J. Applied Cryst. 27, 20 (1994).

3- R. Iki and L.Q. Amaral Phys. Rev. E47, 2551 (1993).