The use of gemini surfactants in gene delivery has been driven by the structural versatility of these compounds, which can be attained by a combination of different spacers, tails, and headgroups. Great efforts have been focused on the inclusion of natural motifs, such as sugars, lipids, and amino acids, to improve their biocompatibility. In this chapter, structural characteristics of gemini surfactants are correlated with their biological impact on cells, in terms of adverse effects produced and capacity to deliver and promote expression of nucleic acids. In the latter process (transfection), emphasis will be given to morphological changes of surfactant-nucleic acid complexes. These are induced by interaction with membranes or triggered by alterations of pH and reducing conditions within the cells, which can play a role in complex dissociation and nucleic acid release. Examples from the literature will illustrate the ability of those complexes to circumvent biological barriers associated with the different steps leading to gene expression, with emphasis on membrane translocation and endosomal escape.
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