Properties of amphiphilic oligonucleotide films at the air/water interface and after film transfer
Graphical abstract
Introduction
Oligonucleotide (ODN) molecules consisting of deoxyribonucleic acid monomers are particularly promising candidates for the design of new materials and macromolecular assemblies [1]. The highly specific binding through Watson–Crick base-pairing of oligonucleotides facilitates convenient creation of defined assemblies by molecular recognition. One strategy is to induce defined conformational changes by DNA hybridization to realize complex molecular machines in liquids [2], [3]. A popular example is the design of molecular tweezers, which can be actuated by DNA hybridization, as reported by Yurke et al. [4]. One class of such oligonucleotide hybrid materials are amphiphilic molecules like oligonucleotide block copolymers [5], [6] and oligonucleotides modified with small hydrophobic moieties [7], [8]. The partial hydrophobicity of the oligonucleotide hybrid materials induces microphase separation in aqueous solution leading to the formation of self-assembled aggregates like micelles and lipid vesicles [5], [9], [10]. The micelle shape is tuneable by hybridization with tailored complementary oligonucleotide sequences [11]. Furthermore, such oligonucleotide amphiphiles were introduced into vesicles composed of phospholipids. In this regard, the surface of lipid vesicles could be easily functionalized by hybridization [12]. More sophisticated functions mediated by the DNA motif and specific base pairing of complementary sequences were vesicle aggregation and fusion [13], [14], [15], [16]. In a similar manner oligonucleotides with lipid tails were doped in supported phospholipid bilayers allowing selective tethering of liposomes, enzymes, drugs or fluorescent probes and therewith fabricating highly functional surfaces [17]. Moreover, the use of hydrophobic moieties allows the organization of oligonucleotides at the air/water interface. Langmuir monolayers composed of single nucleosides functionalized with a lipid moiety have been investigated [18], [19], [20]. Recently, Caseli et al., have reported the use of diblock copolymers containing a poly(butadiene) block adsorbing to the air/water interface with the intention to study the interaction with cell membranes [21].
The molecule, which we have investigated in this regard is an amphiphilic oligonucleotide molecule (dU11) [22], which consists of a single stranded oligonucleotide (ssODN) 11mer containing two hydrophobically modified 5-(dodec-1-ynyl)uracil nucleobases at the terminal 5′-end of the oligonucleotide sequence (Fig. 1). Hereby, the alkyl chains of the dU11 act as arm floats for the oligonucleotide. The air/water interface was used as confinement for the aggregation of dU11 in order to prepare functional thin films in a subsequent transfer process onto a solid. However the alkyl chains are rather short and stable molecular monolayers at the air/water interface are not readily expected. Nevertheless we investigated the thin films after transfer onto a solid substrate with the aim to realize an orientation of the oligonucleotide moiety towards the air interface. Such an orientation would not be accessible in a classical Langmuir–Blodgett film scheme.
Section snippets
Molecules
dU11 was synthesized and purified as reported previously by Anaya et al. [22]. Purity was controlled by 31P-NMR (100 MHz, THF-d 8) and MALDI-TOF MS as well as by ion exchange chromatography and polyacrylamide gel electrophoresis. For details see the Supporting Information as well as [22], [23]. The self-assembly of these lipid-oligonucleotides in aqueous solution was investigated by light scattering experiments and revealed micelle sizes depending on the position and number of the hydrophobic
Film stability at the interface
In order to investigate the stability of the dU11 molecules at the air/water interface Π–A isotherms were recorded as compression–expansion cycles (Fig. 2). The isotherms showed a continuous increase of the surface pressure upon compression at A < 1390 Å2. The onset of all compression–expansion cycles is defined as the area per molecule which corresponds to a surface pressure Π = 0.5 mN/m. A weakly pronounced kink at Π = 7 mN/m was observed upon expansion (marked by an oval in Fig. 2). Furthermore, no
Conclusions
The dU11 molecules form semi-stable films at the air/water interface: Upon film compression the molecules exhibit relaxation phenomena like dissolution of dU11 molecules into the subphase but also reorganization into multiple layers at the air/water interface. SFM investigation of interfacial films transferred via Langmuir–Blodgett and surface lowering technique elucidated, that the formation of multiple layers at the air/water interface is one of the major relaxation mechanisms for films made
Acknowledgements
We acknowledge partial financial support from DFG (SFB 625 From Single Molecules to Nanoscopically Structured Materials). Furthermore we acknowledge Hans Riegler, Uwe Rietzler and Karlheinz Graf for support and discussions while perfroming experiments.
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