Elsevier

Bioelectrochemistry

Volume 132, April 2020, 107416
Bioelectrochemistry

Molecular recognition between guanine and cytosine-functionalized nucleolipid hybrid bilayers supported on gold (111) electrodes

https://doi.org/10.1016/j.bioelechem.2019.107416Get rights and content

Highlights

  • Hybrid bilayer with nucleolipid an excellent tool to study molecular recognition.

  • Guanine forms complex with cytosine moiety in the polar head of nucleolipid monolayer.

  • The complex is stable in a broad range of electrode potentials.

  • Reflection absorption IR spectroscopy is a tool to study structure of the complex.

Abstract

A hybrid bilayer lipid membrane (hBLM), constructed with a 1-hexadecanethiol self-assembled interior leaflet and a 1,2-dipalmitoyl-sn-glycero-3-cytidine nucleolipid exterior leaflet, was deposited at the surface of a gold (111) electrode. This system was used to investigate the molecular recognition reaction between the cytosine moieties of the lipid head group with guanine molecules in the bulk electrolyte solution. Electrochemical measurements and photon polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) were employed to characterize the system and determine the extent of the molecular recognition reaction. The capacitance of the hBLM-covered gold electrode was very low (~1 μF cm−2), therefore the charge density at the gold surface was small. Changing the electrode potential had a minimal effect on the complexation between the cytosine moieties and guanine molecules due to small changes in the static electric field across the membrane. This behavior favored the formation of the guanine–cytosine complex.

Introduction

Molecular recognition reactions taking place at an electrode surface play an important role in the development of drug delivery or biosensor systems where the potential is commonly controlled [1]. The voltage controlled adsorption and co-adsorption of nucleobases at gold electrode surfaces have been extensively investigated [2], [3], [4], [5], [6], [7]. However, when the nucleoside bases are directly adsorbed onto the electrode, it is difficult to separate base-base interactions from the nucleobase-gold interactions. In a recent publication, a monolayer of 1,2-dipalmitoyl-sn-glycero-3-cytidine was deposited onto a gold (111) surface [8]. In this architecture, the two long acyl chains of the nucleolipid separate the cytosine moiety from the gold electrode surface eliminating the base-gold interactions and direct the cytidine head group towards the solution making it accessible for guanine base pairing [8]. Several studies have also demonstrated that molecular recognition can be conveniently investigated by spreading nucleolipid monolayers at the air-water interface [9], [10], [11], [12]. Recently, photon polarization modulation infrared reflection spectroscopy (PM-IRRAS) was employed to investigate the potential controlled orientation of DNA duplexes tethered to the gold electrode surface [13]. Accordingly, we have applied the combination of electrochemical and PM-IRRAS techniques to investigate the interaction of guanine with a 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer deposited on a gold (111) electrode surface. These studies revealed that the binding of guanine to the nucleolipid monolayer is strongly potential dependent [14]. However, the complex formation was strongly dependent on the charge density at the metal. In addition, guanine has a strong affinity to the gold electrode surface, while the physical adsorption interactions between the nucleolipid acyl chains and the gold surface are weak. Fig. 1 shows that the PM-IRRAS spectrum of the 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer with co-adsorb guanine in the 1800–1600 cm−1 region is significantly different than the transmission spectrum of a 1,2-dipalmitoyl-sn-glycero-3-cytidine vesicle solution incubated with guanine. This spectral region contains information about the C=O vibrational groups, which are involved in the cytosine-guanine complex formation. The spectral differences suggest that guanine may deeply penetrate into the 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer when the monolayer is physisorbed to the surface of an electrode.

To prove that the guanine molecules did not displace the nucleolipids within film, we have assembled a hybrid bilayer membrane (hBLM), using the components shown in Fig. 2, where the inner leaflet consisted of a 1-hexadecanethiol self-assembled monolayer (SAM) and the outer leaflet was formed by depositing a monolayer of 1,2-dipalmitoyl-sn-glycero-3-cytidine using the Langmuir Schaefer (LS) technique. The dipole moment of guanine is 6.5 D and hence the molecule is quite polar. The long aliphatic chain of 1-hexadecanethiol created a thick nonpolar region where the lipids are densely packed in a gel state. This hBLM is highly robust and eliminates the direct interaction between gold and guanine due to the strong affinity of the gold-sulfur bond. The objective of this paper was to characterize the interaction between cytidine and guanine using the hBLM system. This architecture ensures better lipid packing and allows the interaction between the two complementary bases to be studied at a broader range of potentials. Electrochemical measurements were used to monitor the stability of the hBLM and the PMI-IRRAS spectra to provide information about the orientation of the acyl chain, the cytidine nucleolipid as well as the interaction between cytidine and guanine. The information gained in this study is relevant for the development of future molecule-based sensors.

Section snippets

Reagents, solutions, electrodes and materials

1,2-dipalmitoyl-sn-glycero-3-cytidine diphosphate (16:0 CDP DG), was purchased from Avanti Polar Lipid and dissolved in chloroform to give a 1 mg mL−1 stock solution. The 1-hexadecanethiol obtained from Merck was dissolved in methanol to obtain a 1 mg mL−1 stock solution. Sodium fluoride powder (BioXtra, 99%) obtained from Sigma-Aldrich was cleaned in a UV ozone chamber (UVO cleaner, Jelight) for 15 min to oxidize and remove any organic impurities prior to electrolyte preparation. For

Electrochemical measurements

Differential capacitance (DC) curves were used to determine the behavior and stability of the hybrid 1-hexadecanethiol/1,2-dipalmitoyl-sn-glycero-3-cytidine bilayers, in the absence and presence of guanine, as a function of the applied potential. Fig. 3 compares the DC curves of the pure gold (111) electrode (1), the gold (111) electrode covered with a SAM of 1-hexadecanethiol (2) and the 1-hexadecanethiol/1,2-dipalmitoyl-sn-glycero-3-cytidine bilayer in presence (3) and absence (4) of guanine

Conclusions

We have compared the guanine-cytosine molecular recognition reaction of a hybrid bilayer, where the inner leaflet consisted of a SAM of the 1-hexadecane thiol and the outer leaflet of a cytosine-containing nucleolipid, with a physisorbed monolayer of the same nucleolipid molecule deposited directly onto the gold (111) electrode surface as described in [14]. In both films, guanine was present in the complexed and non-complexed forms, however, the molecular complex between guanine and the

Declaration of Competing Interest

Author declares that there is no conflicts of interest.

Acknowledgements

The FP and MR acknowledge research grants from Spanish Ministry of Economy and Competitiveness (CTQ2014-57515-C2-1-R) and Andalusian government (PAI-FQM202). JL acknowledges support of the Discovery grant from Natural Sciences and Engineering Council of Canada RG-03958. JAM acknowledges a FPU grant and a Visiting Academic grant from the Spanish Ministry of Science and Technology.

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