Quantification of oligonucleotide modifications of small unilamellar lipid vesicles.

We present a new method for quantification of the coupling efficiency between amphiphilic oligonucleotides and suspended small unilamellar lipid vesicles (SUVs). The method employs a supported (phospho)lipid bilayer (SLB)-modified sensor template, which upon exposure to a mixture of SUVs and amphiphilic DNA reacts neither with free SUVs nor with DNA-modified SUVs, but with free DNA only. Using calibration curves obtained by recording the concentration dependence of the initial binding rate of free amphiphilic DNA (in the absence of SUVs), it is demonstrated how concentration determinations of both free and bound DNA in the two-component mixture (amphiphilic DNA and lipid vesicles) can be obtained. The calibration curves and the binding analysis were obtained using a quartz crystal microbalance with dissipation (QCM-D) monitoring. The binding efficiency of DNA coupled to SUVs (Ø approximately 50 nm) with two cholesterol moieties revealed that the bivalent coupling is essentially 100% in the range of approximately 1 to approximately 35 oligonucleotides per vesicle, whereas reversible coupling was confirmed in the case of monovalent coupling. Coupling of DNA via two cholesterol moieties was obtained by prehybridization of two single-stranded DNA strands modified with single cholesterol moieties in their 3' and 5' ends, respectively, and the monovalent coupling was obtained using single-stranded DNA. In the latter case, the analysis of the amount of free DNA at different DNA-SUV ratios also allowed for a determination of the maximum number of available binding sites on the SUVs, shown to be in good agreement with data obtained for DNA coupling on planar surfaces. With the only requirement that the SLB-modified sensor template react with one of the components in the two-component mixture only, as verified through fingerprint analysis of frequency, f, and energy dissipation, D, QCM-D measurements, it is emphasized that the method is generic and offers a fast and reliable method for evaluations of biomolecular modifications of any type of colloidal nanoparticles.