pubs.acs.org/Langmuir © 2009 American Chemical Society
Influence of Tunable External Stimuli on the Self-Assembly of Guanosine Supramolecular Nanostructures Studied By Atomic Force Microscope Yinli Li,† Mingdong Dong,‡ Daniel E . Otzen,§ Yuheng Yao,† Bo Liu,*,† Flemming Besenbacher,*,‡ and Wael Mamdouh*,‡ † ‡
Institute of Photo-Biophysics, Physics and Electronics Department, Henan University, 475004, Kaifeng, China, Interdisciplinary Nanoscience Center (iNANO), Centre for DNA Nanotechnology (CDNA), and Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark, and §Interdisciplinary Nanoscience Center (iNANO), Center for Insoluble Protein Structures (inSPIN), University of Aarhus, DK 8000 Aarhus C, Denmark Received February 22, 2009. Revised Manuscript Received May 21, 2009
The self-assembly of guanosine (G) molecules on solid surfaces is investigated by tapping-mode atomic force microscopy (AFM) upon controlling and introducing external factors (stimuli) to the G stock solution such as incubation time, presence/absence of metal cations, and mechanical shaking. Surprisingly, at different stages of incubation time at room temperature and in the absence of any metal cations in the G stock solution, which are known to be one of the governing factors in forming G-nanostructures, two assembly pathways resulting into two distinct supramolecular nanostructures were revealed. Astonishingly, by introducing a mechanical shaking of the tube containing the G stock solution, one-dimensional (1D) wires of G molecules are observed by AFM, and very interestingly, novel “branched” supramolecular nanostructures are formed. We have also observed that the later branched G nanostructures can grow further into a two-dimensional (2D) thin film by increasing the incubation time of the G stock solution at room temperature after it is exposed to the external mechanical stimuli. The self-assembled nanostructures of G molecules are changed significantly by tuning the assembly conditions, which show that it is indeed possible to grow complex 2D nanostructures from simple nucleoside molecules.
Introduction The self-assembly of biomolecules is a powerful approach for creating novel supramolecular nanostructures and molecular architectures. Biomolecules, including polysaccharides, peptides, nucleic acids, and proteins, in general self-organize to form welldefined structures that often are stabilized by weak, noncovalent interactions.1-3 Hydrogen bonding between DNA or RNA nucleobase (NB) molecules is one of the main interactions that controls the conformation and biochemical activity of nucleic acids,4 and the exquisite specificity between NB molecules embrace their unique role in biological processes because of their ability to store, transfer and reproduce genetic information.4-7 NB molecules have recently been used in connection with novel biosensors to functionalize surfaces with ss-DNA oligomers,8,9 and furthermore to steer the self-assembly of DNA-based artificial molecular constructions.10-12 Guanine is one of the most interesting NB molecules, and together with its sugar and sugar-phosphate derivatives, they *Corresponding author. E-mail:
[email protected] (B.L.); wael@inano. dk (W.M.);
[email protected] (F.B.). (1) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128–4158. (2) Xu, Y.; Ye, J.; Liu, H.; Cheng, E.; Yang, Y.; Wang, W. X.; Zhao, M.; Zhou, D.; Liu, D. S.; Fang, R. X. Chem. Commun. 2008, 49–51. (3) Sivakova, S.; Rowan, S. J. Chem. Soc. Rev. 2005, 34, 9–21. (4) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737–738. (5) Mamdouh, W.; Kelly, R. E. A.; Dong, M. D.; Kantorovich, L. N.; Besenbacher, F. J. Am. Chem. Soc. 2008, 130, 695–702. (6) Lee, J. C.; Gutell, R. R. J. Mol. Biol. 2004, 344, 1225–1249. (7) Arnott, S. Nature 1984, 312, 174–174. (8) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757–1760. (9) Fritz, J.; Baller, M. K.; Lang, H. P.; Rothuizen, H.; Vettiger, P.; Meyer, E.; Guntherodt, H. J.; Gerber, C.; Gimzewski, J. K. Science 2000, 288, 316–318. (10) Samori, B.; Zuccheri, G. Angew. Chem., Int. Ed. 2005, 44, 1166–1181. (11) Sessler, J. L.; Jayawickramarajah, J. Chem. Commun. 2005, 1939–1949. (12) Seeman, N. C. Angew. Chem., Int. Ed. 1998, 37, 3220–3238.
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have attracted a lot of attention in the last two decades because of their exquisite internal hydrogen bonding motifs. Therefore, guanine has been considered as an excellent model system by means of which one can form a variety of self-assembled nanostructures including 2D networks that consist of honeycomb arrangements,13 quartets,14-23 ribbonlike patterns21,24and “sheetlike” nanostructures.25,26 The guanosine (G) molecules (Figure 1 a) are nucleosides comprising a guanine molecule attached to a ribose (ribofuranose) ring via a β-N9-glycosidic bond. Individual G molecules can assemble to form a G-supramolecular structure as depicted in the schematic illustration in Figure 1. The formation of (13) Tanaka, H.; Kawai, T. Mater. Sci. Eng., C 1995, 3, 143–148. (14) Kelly, R. E. A.; Kantorovich, L. N. J. Mater. Chem. 2006, 16, 1894–1905. (15) Mamdouh, W.; Dong, M. D.; Kelly, R. E. A.; Kantorovich, L. N.; Besenbacher, F. J. Phys. Chem. B 2007, 111, 12048–12052. (16) Perdigao, L. M. A.; Staniec, P. A.; Champness, N. R.; Kelly, R. E. A.; Kantorovich, L. N.; Beton, P. H. Phys. Rev. B 2006, 73, 195423-1–195423-7. (17) Tanaka, H.; Nakagawa, T.; Kawai, T. Surf. Sci. 1996, 364, L575–L579. (18) Sowerby, S. J.; Heckl, W. M.; Petersen, G. B. J. Mol. Evol. 1996, 43, 419– 424. (19) Furukawa, M.; Tanaka, H.; Kawai, T. J. Chem. Phys. 2001, 115, 3419– 3423. (20) Giorgi, T.; Lena, S.; Mariani, P.; Cremonini, M. A.; Masiero, S.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P.; Gottarelli, G. J. Am. Chem. Soc. 2003, 125, 14741–14749. (21) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem.;Eur. J. 2000, 6, 3242–3248. (22) Sowerby, S. J.; Stockwell, P. A.; Heckl, W. M.; Petersen, G. B. Origins Life Evol. Biosphere 2000, 30, 81–99. (23) Tao, N. J.; Derose, J. A.; Lindsay, S. M. J. Phys. Chem. 1993, 97, 910–919. (24) Pham, T. N.; Griffin, J. M.; Masiero, S.; Lena, S.; Gottarelli, G.; Hodgkinson, P.; Fillip, C.; Brown, S. P. Phys. Chem. Chem. Phys. 2007, 9, 3416–3423. (25) Yoshikawa, I.; Sawayama, J.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 1038–1041. (26) Yoshikawa, I.; Yanagi, S.; Yamaji, Y.; Araki, K. Tetrahedron 2007, 63, 7474–7481.
Published on Web 06/05/2009
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Figure 1. Top: Chemical structures of a guanosine molecule and its self-assembled G-sugar and sugar-phosphate derivatives: (a) guanosine, (b) G-quartet, and (c) G-wire in which G-quartets stack on top of each other. The black arrows indicate the growth process from one stage to the next by using a specific “factor” that is written above the black arrow. Bottom: Schematic cartoon of the growth process that is described in the top panel.40
G-nanostructures is mainly steered by the fact that two G-molecules will form a G-dimer stabilized by hydrogen bonding27 via two hydrogen bonding acceptors (N7 and O13) on its Hoogsteen face and two hydrogen bonding donors (N1 amide and N14 amino) on its Watson-Crick face.28 G-dimers will eventually come closer together to form hydrogen-bonded “G quartets” (Figure 1 b),29-31 which are further used to construct G 1D chains in solution by π-π stacking32-34 of the G-quartets on top of each other because of the strong van der Waals (vdW) attraction forces between the large G-planar surfaces27 (Figure 1c) contingent on a wide variety of experimental “factors” as illustrated in Figure 1. This hypothesis has been tested previously by the diffraction pattern of 50 -GMP (guanosine 5monophosphate) nanowire. Recently, the linear nanowire of the 50 -GMP quartets with metal ions has been studied by AFM,35 and it is found that the stability of GMP quartets depends strongly on which monovalent ions are present in the central cavity.32-34 It should be noted that the molecular adsorption model in Figure 1 has previously been proposed as the most accepted molecular model to account for the binding possibilities between guanosine molecules (as quartets for example).36-39 Although it is not (27) Donohue, J. Proc. Natl. Acad. Sci. U.S.A. 1956, 42, 60–65. (28) Davis, J. T.; Spada, G. P. Chem. Soc. Rev. 2007, 36, 296–313. (29) Gellert, M.; Lipsett, M. N.; Davies, D. R. Proc. Natl. Acad. Sci. U.S.A. 1962, 48, 2013–2018. (30) Otero, R.; Schock, M.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Angew. Chem., Int. Ed. 2005, 44, 2270–2275. (31) Setnicka, V.; Urbanova, M.; Volka, K.; Nampally, S.; Lehn, J. M. Chem.; Eur. J. 2006, 12, 8735–8743. (32) Sundquist, W. I.; Klug, A. Nature 1989, 342, 825–829. (33) Williamson, J. R.; Raghuraman, M. K.; Cech, T. R. Cell 1989, 59, 871–880. (34) Baumann, P. Nat. Struct. Mol. Biol. 2005, 12, 832–833. (35) Kunstelja, K.; Federiconib, F.; Spindlerc, L.; Drevensek-Olenikd, I. Colloids Surf., B 2007, 59, 120–127. (36) Fukushima, K.; Iwahashi, H. Chem. Commun. 2000, 895–896. (37) Moriwaki, H. J. Mass Spectrom. 2003, 38, 321–327. (38) Sakamoto, S.; Nakatani, K.; Saito, I.; Yamaguchi, K. Chem. Commun. 2003, 6, 788–789. (39) Vairamani, M.; Gross, M. L. J. Am. Chem. Soc. 2003, 125, 42–43.
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possible with the AFM to resolve the exact configuration of the individual molecules, AFM has been used successfully to monitor the nucleation and growth of the individual molecular wire by timelapse imaging and has thus proved as a very useful and versatile technique to reveal the most suitable conditions for following continuous dynamic process such as growth, nucleation, etc. Many external factors have been used to steer the self-assembly of biomolecular building blocks into larger nanostructures, and also the influence of these external stimuli on the self-assembly process resulting in a variety of different nanostructures has been reported. Examples of such external factors are: metal cations,33 temperature,41,42 concentration,43,44 incubation time,41 pH value,41,44pressure45 for gas phase, media solvent,46 etc. Figure 1 illustrates a general schematic example where the Gmolecules are used as building blocks in the presence of a wide variety of external factors (as indicated on top of the black arrows in Figure 1) to form G-quartets, which subsequently self-assemble into G-1D nanowires. It is thus of great importance to study the growth of simple G molecules (used as building blocks) into larger G-nanostructures and get more insights into the influence of such external factors and stimuli on the self-assembly processes. Here we investigate the formation of G-nanostructures from G molecules particularly in the absence of any metal ions, and upon (1) controlling the incubation time of the crystal in the G solution (40) Davis, J. T. Angew. Chem., Int. Ed 2004, 43, 668–698. (41) Dong, M. D.; Hovgaard, M. B.; Xu, S. L.; Otzen, D. E.; Besenbacher, F. Nanotechnology 2006, 17, 4003–4009. (42) Kailas, L.; Vasilev, C.; Audinot, J. N.; Migeon, H. N.; Hobbs, J. K. Macromolecules 2007, 40, 7223–7230. (43) Dong, M. D.; Xu, S. L.; Bunger, M. H.; Birkedal, H.; Besenbacher, F. Adv. Eng. Mater. 2007, 9, 1129–1133. (44) Zhao, Y.; Yokoi, H.; Tanaka, M.; Kinoshita, T.; Tan, T. W. Biomacromolecules 2008, 9, 1511–1518. (45) Wilton, D. J.; Ghosh, M.; Chary, K. V. A.; Akasaka, K.; Williamson, M. P. Nucl. Acids Res. 2008, 36, 4032–4037. (46) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317–325.
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at room temperature, and (2) introducing external stimuli such as mechanical shaking to the G stock solution. The self-assembly of G molecules on the solid surface (mica) is revealed by AFM in ambient conditions, and two distinct assembly pathways were explored.
Experimental Section Materials and Methods. Guanosine (G) was purchased from
Sigma Aldrich Co. with a purity of g98%, the solvent is bufferfree deionized water, and the substrate is freshly cleaved mica surface (Ted Pella, Inc.). The concentration of the prepared G solution is 8.510-4 mg/mL, and the incubation time at room temperature (RT) ranges from 1 h to 10 days. The G molecules were first dissolved in deionized water and incubated at RT during the full time scale of the AFM experiments. After incubation times of 1 h, 3 h, 1 day, and 4 days, respectively, a 5 μL sample solution was extracted out of the G stock solution and deposited onto a freshly cleaved mica surface and left to be air-dried for 2 min at RT. To study the influence of external stimuli to the selfassembled patterns of G formed on the mica surface, we performed a 2 min mechanical shaking of the G-stock solution at the beginning of the incubation period by using a minishaker (MS2 minishaker IKA, Germany), with a rotation speed of 1200 rpm. Subsequently, a series of time-lapse studies of self-assembled G nanostructures was conducted using AFM to visualize the self-assembled structures formed after each of the different incubation times (from 3 h up to 10 days). In parallel, UV absorption spectra were recorded to identify possible conformational changes of the stock solution at the molecular level. For comparison, a mixture of the separated G-stock solution with KCl has been prepared with a concentration of 0.1 mol/L KCl, and both shaking as well as nonshaking additional control experiments were carried out in the presence of K+ ions. Atomic Force Microscopy. AFM measurements were carried out using the commercial Agilent AFM 5500 (Agilent Technologies, USA) operated in tapping-mode (TM-AFM). Microfabricated ultrasharp silicon cantilevers (model NSG-01, NT-MDT Co., Russia) were used with a typical resonance frequency υ0 ≈ 149 kHZ, a spring constant of 5 N m-1 and a normal tip radius of approximately 10 nm. TM-AFM was performed in air under ambient conditions at scan frequencies of 1-2 Hz with minimal loading forces applied and optimized feedback parameters. All the recorded AFM images consist of 512 512 pixels, and several images were obtained at separate locations across the mica surfaces to ensure a high degree of reproducibility of the recorded molecular nanostructures. All the AFM images were analyzed by means of the commercial Scanning Probe Image Processor (SPIP) software (Image Metrology ApS, version 4.7.0, Lyngby, Denmark). Spectroscopy. UV Absorption spectra were recorded in the wavelength range 240-330 nm using a Pharmacia BioTech Ultrospec 1000 UV/Visible Spectrophotometer. Spectra were aligned using the zero baseline between 320 and 330 nm.
Results and Discussions Figure 2 shows a variety of different self-assembled G-nanostructures that were formed after different incubation times at RT, and after an incubation time of 1 h, nanoparticles (NP) with diameters less than 70 nm are formed as depicted in Figure 2A. However, upon increasing the incubation time to 3 h, some “Gwires” with an approximate length of more than 500 nm were formed (Figure 2B), which may be due to a vertical stacking of multiple “G-quartets”29-31 as suggested in the schematic assembly pathway in Figure 1. More interestingly, these G-wires grew further in length to more than 1 μm by increasing the incubation time to 1 day (Figure 2C), and even to longer than 10 μm after an incubation time of 4 days (Figure 2D), respectively. 13434 DOI: 10.1021/la900640f
G quadruplexes have been monitored using UV absorption spectra.47 Interestingly, the accumulation of micrometer-long G-wires over a time-scale of days coincides with a decrease in the intensity of the absorption spectra (Figure 3A and 3B). The decrease in absolute absorption at 252 nm is mirrored by the decrease in the ratio I252/I268 between the intensity of the peak at 252 nm and the shoulder around 268 nm (shown in Figure S1 in the Supporting Information), which indicates a subtle change in the molecular environment of the G molecule in the transition from the G-NPs to the G-wires. The most interesting observation is the transition from G-NP to longer G-nanowire structures such as those observed in Figure 2B-D which most importantly occurs in the absence of any metal cations that in the past have been considered to be one of the most important factors in the formation of such G-wires structures.33 We have found that the growth speed of these G-wires is much slower in the absence of any metal ions as compared to the situation when K+ ions are present (as depicted in Figures S2 and S3 in the Supporting Information) and that of the “GMP-wires” when metal cations are present as reported earlier.35,48 In the absence of metal ions, the noncovalent π-π interactions in these G-wire systems will occur only when the G-quartets are oriented in the correct “stacked orientation”, which is suggested to assist their growth into large G-wire structures (as depicted in the schematic illustration in Figure 1c). Biomolecules are known to be sensitive to any external stimuli which may affect their dynamics and assembled patterns at the nanoscale. The mechanical shaking technique appears to be one of the methods that can affect the way biomolecules interact with each other. Thus, by introducing mechanical shaking to the G-stock solution using a minishaker for 2 min, surprisingly, completely new branched G-nanostructures were formed after 3 h of incubation time (as indicated in Figure 4A in green ovals). These branched G-nanostructures are clearly distinct from the G-wire structures (Figure 2B) observed after the same incubation period for the G stock solution at room temperature that has not been exposed to any mechanical shaking. More interestingly, these branched G-nanostructures (Figure 4A) grew further into more densely packed structures (Figures 4B and 4C) after 6 and 7 days of incubation, respectively. After 10 days of incubation, the G structures nearly covered the mica surface, resulting in the formation of a G-thin film (Figure 4D). Remarkably, these profound structural rearrangements did not lead to any changes in the G-UV absorption spectra (Figure 3B). Interestingly, after shaking the G-solution, the G-wire structures appear to have a linear configuration compared to the coillike configuration for the G-samples in the solution which has not been shaken, which indicates that the two kinds of self-assembled G-structures have different mechanical properties. We can thus conclude that the coil-like G-wires show more flexibility without shaking than the linear G-wires after shaking. In addition, the AFM images indicate a higher resistance of the chains to bending, which results in the formation of the linear type G-wire structures after the shaking. Therefore, the linear G-wires have longer persistence length than the coil-like G-wires. Figure 4A depicts many coexisting G-branched structures, indicated by the green ovals, each consisting of a number of branches, and these G nanostructures are depicted in further details in Figure 5. The most common feature that is observed in Figure 4A is the G-wire structure, which either remains as an (47) Krishnan-Ghosh, Y.; Whitney, A. M.; Balasubramanian, S. Chem. Commun. 2005, 24, 3068–3070. (48) Samori, P.; Pieraccini, S.; Masiero, S.; Spada, G. P.; Gottarelli, G.; Rabe, J. P. Colloids Surf., B 2002, 23, 283–288.
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Figure 2. AFM topography images of G with different incubation times at RT. the incubation times of A, B, C and D are 1 h, 3 h, 1 day, and 4 days, respectively. The scale bars are 200 nm.
Figure 3. (A) UV absorption spectra of nonshaken G at different incubation times. (B) The change in the UV absorption spectra at 252 nm of nonshaken and shaken G stock solution with time, respectively.
Figure 4. AFM topography images of G molecules with different incubation times at room temperature after introducing mechanical shacking to the G-stock solution. The incubation times are 3 h, 6 days, 7 days, and 10 days for A-D, respectively (the scale bars: 200 nm.) The branched G-supramolecular structures are indicated in green ovals in A and given several numbers that will be used later in Figure 5.
individual wire structure or further develops to form larger branched G-nanostructures. Figure 4 also depicts that the coverage of G-nanostructures appearing on the mica surface is increased with the increase in incubation time (from 3 h to 10 days). Figure 5 depicts magnified high-resolution AFM images of several branched G-nanostructures revealed in Figure 4A, and their corresponding height profiles. It is well-known that the AFM images may be perturbed by the final size of the AFM tip; an effect that is often referred to as “AFM tip convolution”, and therefore, the height value is a more accurate measure of the fibril size. The main observation one can clearly see is that these branched G-nanostructures appear to consist of a central part “core” Langmuir 2009, 25(23), 13432–13437
(which appears as a central bright spot in the AFM image) to which some “branches” are attached. After 3 h of incubation, the height of this central core is determined to be approximately 3.4 ( 0.5 nm, which is almost twice as high as the average height of the G-branches connected to the core. On the other hand, after 6 days of incubation, the height of the central core decreases and becomes closer to that of the average height of the G-branches as shown in Figure 4C. After 10 days of incubation, the branched G-nanostructures was observed to grew together to form a G-thin film, and in this case, the height difference between the core and the branches disappeared and the overall height became more homogeneous in all parts of the G-thin film as shown in Figure 4D. A detailed analysis of the average heights of the DOI: 10.1021/la900640f
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Figure 5. Magnified insets of the branched G-nanostructures shown in Figure 4A and their respective height profiles of i-vi. The scan size of all images is 270270 nm2.
branched G-nanostructures is depicted in Table 1. Furthermore, the individual G-wires structures tend to merge together as can clearly be seen in images C and D in Figure 4 to form the G-thin film, and the surface coverage of these supramolecular structures increased from 11 to 71%. Furthermore, we have surprisingly observed that by mechanical shaking the G-stock solution without the presence of K+ ions, supramolecular branched nanostructures are revealed in the AFM images (Figure 4), and these branched nanostructures are distinctly different from the initial supramolecular G-nanowires structures prepared without shaking involved (Figure 2). Therefore, upon introducing mechanical shacking of the G-stock solution, the G-molecules that have an interim conformations similar to polypeptide chains49 most probably bind together through a different binding motif, resulting in smaller G-filaments that grow further to a layer of G-nanostructures on the mica surface upon gradually increasing the incubation time. The general mechanism underlying the G-wire formation depicted in the schematic presentation in Figure 1 shows that G-molecules bind together to form G-dimers, which then subsequently form the G-quartets structures that grow further into G-wires by π-π interaction in solution.50 The interesting question is that regarding the mechanism behind the formation of the branched G-nanostructures. When the mechanical shaking of the G-stock solution is introduced (Figure 4), G-wires such as those observed in Figure 2 with increasing length are no longer (49) Frokjaer, S.; Otzen, D. E. Nat. Rev. Drug Discovery 2005, 4, 298–306. (50) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679.
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Table 1. Detailed Analysis of the Branched G-Structures items time 3h 6 days 7 days 10 days
average height of the branches (nm)
average height of the core (nm)
1.9 ( 0.2 1.8 ( 0.2 1.7 ( 0.2 1.5 ( 0.2
3.4 ( 0.5 2.7 ( 0.4 2.2 ( 0.2 1.5 ( 0.2
observed. The fact that the core of the branched G-nanostructures have a higher height than the corresponding attached branches for short incubation times and this height difference between the core and the branches of the branched G-nanostructure vanishes upon increasing the incubation time (Table 1), seems to imply that the branched G-nanostructures are formed via another pathway than that of the G-wires structures depicted in Figure 1, which also involves another molecular environment as suggested by the absorption spectra (Figure 3). The high-resolution AFM images in Figure 5 suggest that the short G-filaments (which may consist of several G-molecules) play the key role in this assembly process, and provide the basis for the new pathway where they act as nucleation sites in solution. The initial short G-filaments (i, Figure 5) seem to join together in solution to form “G-dibranched units,” which in the AFM images appear with two different conformations: either as a “linear” G-dibranched unit (ii, Figure 5), or as a “V-shape” G-dibranched unit (ii, Figure 5). Furthermore, larger G-structures can clearly be revealed such as the G-tribranched unit (iii, Figure 5) or even larger branched G-nanostructures with multiple branches (iv, v, and vi, Figure 5). Langmuir 2009, 25(23), 13432–13437
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These branched assemblies can further diffuse dynamically in time, which apparently reduces the difference in height between the core and the branches of the G-nanostructures, as depicted in Table 1 and Figure 5.
height distribution. These results show that by introducing external stimuli, it is possible to introduce a different pathway through which the G molecules can bind together, leading to the formation of a variety of new supramolecular nanostructures.
Conclusions
Acknowledgment. The authors acknowledge financial support from the Danish National Research Foundation and the Danish Ministry for Science, Technology, and Innovation through the iNANO Center and inSPIN, from the Danish Research Councils and from the Carlsberg foundation. B.L. also thanks the Ministry of Education of China foundation of Supported Program for New Century Excellent Talents in University (NCET-07-0256) for financial support. The authors are thankful to Dr. Menglin Chen for her suggestive discussions.
A wide variety of self-assembled supramolecular nanostructures of guanine and its derivatives have in the past been reported, such as quartets, 1D wires, and sheets, etc. These structures can be formed either by means of the self-assembly mechanism or by means of external experimental stimuli such as the presence of metal cations, effect of temperature, solvent, etc. Here, we have used AFM to study the growth of self-assembled guanine derivative: guanosine (G) molecules upon their adsorption on mica surfaces, and we have investigated the influence of the formation of nanostructures upon controlling the incubation time of the G-stock solution at room temperature, and introducing an external mechanical shaking to the G-stock solution, respectively. Surprisingly, we find that after 1 h of incubation time, and in the absence of any metal cations, G-NPs are formed that grew further (in solution) upon increasing the incubation time into more than 10 μm long G-nanowires. These findings clearly reveal the high tendency of G-molecules to form longer nanowires and, because no metal ions are present, hydrogen bonding and π-π interaction most likely stabilize these G-wires structures in solution. It is also found that the growth speed of these G-wires in solution is much slower in the absence of any metal ions as compared to the case when K+ ions are present in solution, which proves that metal ions are important in stabilizing the G-quartets as that in GMP quartets.32-34 Furthermore, we have very surprisingly observed that by introducing mechanical shaking to the G-stock solution, in the absence of any metal ions, supramolecular branched G-nanostructures appear that are distinctly different from the initial supramolecular G-nanowires structures. Upon increasing the incubation time to approximately 10 days, these branched G-nanostructures were transformed into a more homogeneous G-thin film like supramolecular structures with a homogeneous
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Supporting Information Available: Figure S1 shows the UV absorption ratio I252/I268 at different incubation times. A clear decease of absorption bands has been observed which indicate a transition from the G-NPs to the G-wires. Figure S2 shows AFM images of G structures with and without K+ ions, and also show that the average height of the G-wires without K+ ions is 1.3 ( 0.2 nm after 3 h of incubation time, and the length of these G-wires is less than 500 nm. When there are K+ ions present in the G solution, the average height of the G-wires is almost the same (1.2 ( 0.4 nm), but the length of the G-wire is more than 3000 nm. Our experimental results confirmed that the growing speed of the G-wire without metal ions present in the solution was much slower than that of the G-wire with metal ions. Figure S3 shows UV absorption of nonshaken G with K+ ions at different incubation times. In about 2 h incubation time with K+ ions, the UV absorption of G solution decreases to the same level of G without K+ions, which suggests the formation of G-wire without K+ ions and also show much slower growth rate compared to the G-wires with K+ ions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org
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