**2.2.1 DNA-pDADMAC characteristic ratios, (N/P)c and (N/P)\***

Different to the chitosan system, the DNA-pDADMAC polyplexes exhibited two distinct characteristic ratios: the previously observed (N/P)c (i.e., the ratio from which DNA is compacted) and (N/P)\*, an additional ratio from which the polyplexes adopt the most compact structure. Similarly to the DNA-chitosan polyplexes, (N/P)c was found to be dependent on the polycationic charge density, whereas (N/P)\* proved to be a function of pDADMAC valence. Table 4 summarizes the characteristic ratios for each DNA-pDADMAC system together with the average size of the polyplexes formed (discussed in the next section). Our main findings can be described as follows.

(N/P)c was determined by means of conductometry. pDADMAC aliquots were injected to DNA and buffer solutions and the change in conductivity was recorded. Successive pDADMAC injections produced exactly the same outcome observed for the chitosan system; namely, a linear increment in conductivity for the buffer solution and an inflection in the conductivity plot for the DNA one (plots not shown). The ratio at which the conductivity inflection occurred is reported as (N/P)c (see section 2.1.1 for a complete explanation).

It is clear from table 4 that (N/P)c is governed by the charge density provided that polyplexes formed with pDADMAC homopolymers have complexation ratios lower than that of the polyplex formed with coDADMAC. This result confirms what we observed for chitosan not only with respect to the role of charge density but also regarding the (N/P)c values obtained which are very similar for both chitosan- and pDADMAC-based polyplexes (see section 2.1.1).


Reproduced from (Alatorre-Meda et al., 2010b) with permission of AMERICAN CHEMICAL SOCIETY in the format Journal via Copyright Clearance Center.

Table 4. Characteristic N/P ratios and RH of the DNA-pDADMAC polyplexes. (N/P)c and (N/P)\* stand for the DNA compaction ratio (determined by conductometry) and for the ratio from which the size of the polyplexes remain constant (determined by DLS), respectively. RH is the average of the recorded values in the range (N/P)\* ≤ N/P ≤ 10.

(N/P)\* was found by DLS as a characteristic ratio from which the polyplexes adopt the most compact structure. As observed from table 4, the value of (N/P)\* follows a decreasing trend with pDADMAC valence of the polyplexes, with p(1,619) and p(1,2786) showing the highest and the lowest (N/P)\* values of 4 and 1, respectively. The interplay between (N/P)c and (N/P)\* can be reasoned in terms of the different complexation states of DNA mediated by pDADMAC (Fischer et al., 2004).
