Molecular Recognition Capabilities of a Nucleolipid Amphiphile (3',5

Sep 8, 2000 - Monolayer behavior of a nucleolipid amphiphile, (3',5'-distearoyl)-2'-deoxythymidine, on pure water, aqueous 1 mM and 5 mM adenosine sol...
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Langmuir 2000, 16, 7701-7707

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Molecular Recognition Capabilities of a Nucleolipid Amphiphile (3′,5′-Distearoyl)-2′-Deoxythymidine to Adenosine at the Air/Water Interface and Langmuir-Blodgett Films Studied by Molecular Spectroscopy Chun Li, Jianguo Huang, and Yingqiu Liang* Institute of Mesoscopic Solid State Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China Received February 8, 2000. In Final Form: July 3, 2000 Monolayer behavior of a nucleolipid amphiphile, (3′,5′-distearoyl)-2′-deoxythymidine, on pure water, aqueous 1 mM and 5 mM adenosine solutions was investigated by means of surface pressure-molecular area (π-A) isotherms, which indicate that both hydrogen-bonding and stacking interactions are present between the (3′,5′-distearoyl)-2′-deoxythymidine monolayer and adenosine in subphase, and the recognition of this amphiphile to adenosine is more complete in higher subphase concentration. The ultravioletvisible (UV-vis), Fourier transform infrared (FTIR) transmission, FTIR-attenuated total reflection (ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopic results indicate that the adenosine molecules in the subphase can be transferred onto solid substrates by Langmuir-Blodgett (LB) technique as a result of the formation of Watson-Crick base-pairing at the air/water interface. The regular and closed packing of the constituent molecules facilitates the photodimerization of the thymine moieties in the headgroup under ultraviolet irradiation even at room temperature. FT-SERS technique, for the first time, was introduced into the research area of molecular recognition occurring at an interface system. High-quality FT-SERS spectra of single LB monolayers obtained in the 1750-500 cm-1 region, together with the FTIR-ATR results in the NH-stretching high-frequency region, provide direct evidences for the formation of multiple complementary hydrogen bonds between the base pairs.

Introduction Molecular recognition is an important concept in many biological processes. The most notable example of biological recognition that is based on hydrogen bonding is complementary base pairing in nucleic acids.1 The mutual recognition of complementary bases in DNA and RNA by means of multiple hydrogen bonding is the most efficient mechanisms of accumulating, storing, reproducing, and evolving genetic information.2 Much attention has been centered on achieving selective molecular recognition and binding in artificial systems during the last two decades.2-7 Analysis and mimicry of these processes not only will facilitate better understanding of the corresponding biological functions and processes but also can be significant in the development of chemical sensors. Experimental and theoretical studies on complementary base recognition have been performed in nonaqueous media.8-13 In contrast, monomeric nucleic acids cannot * Author to whom correspondence should be addressed. (1) Saenger, W. Principles of Nucleic Acid Structure; SpringerVerlag: New York, 1984. (2) Lehn, J. M. Supramolecular Chemistry; VCH: Weinheim, 1995. (3) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (4) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (5) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (6) Lawrence, D. S.; Jiang, T.; Levitt, M. Chem. Rev. 1995, 95, 2229. (7) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371. (8) Katz, L.; Penman, S. J. Mol. Biol. 1966, 15, 220. (9) Hamlin, R. M., Jr.; Lord, R. C.; Rich, A. Science 1966, 148, 1734. (10) Williams, G. W.; Williams, L. D.; Shaw, B. R. J. Am. Chem. Soc. 1989, 111, 7205. (11) Pranata, J.; Wierschke, S. G.; Jorgensen, W. L. J. Am. Chem. Soc. 1991, 113, 2810. (12) Pistolis, G.; Paleos, C. M.; Malliaris, A. J. Phys. Chem. 1995, 99, 8896.

form hydrogen-bonded complementary pairs in bulk water due to strong competitive binding of water molecules.1,14 Recently, several types of simulated systems, micelles,15-17 vesicles,18,19 and monolayers20-27 at the air/water interface, have been developed to study the complementary binding of nucleobases. In these mesoscopic phases water is not readily accessible or displays unique properties that are different from those of bulk water.28 The proper hydrophobic microenviroment of these aggregates has been demonstrated to be effectiveness for the formation of hydrogen-bonded complementary base pairing. However, (13) Tsiourvas, D.; Sideratou, Z.; Haralabakopoulos, A. A.; Pistolis, G.; Paleos, C. M. J. Phys. Chem. 1996, 100, 14087. (14) Fersht, A. R. Trends Biochem. Sci. 1987, 12, 301. (15) Novick, J. S.; Chen, J. S.; Noronda, G. J. Am. Chem. Soc. 1993, 115, 7636. (16) Novick, J. S.; Cao, T.; Noronda, G. J. Am. Chem. Soc. 1994, 116, 3285. (17) Berti, D.; Barbaro, P.; Bucci, H.; Baglioni, P. J. Phys. Chem. B 1999, 103, 4916. (18) Onda, M.; Yoshihara, K.; Koyano, H.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 8524. (19) Berti, D.; Baglioni, P.; Bonaccio, S.; Barsacchi-Bo, G.; Luisi, P. L. J. Phys. Chem. B 1998, 102, 303. (20) Kitano, K.; Ringsdorf, H. Bull. Chem. Soc. Jpn. 1985, 58, 2826. (21) Ahlers, M.; Ringsdorf, H.; Rosemeyer, H.; Seela, F. Colloid Polym. Sci. 1990, 268, 132. (22) Kurihara, K.; Ohto, K.; Honda, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 5077. (23) Berndt, P.; Kurihara, K.; Kunitake, T. Langmuir 1995, 11, 3083. (24) Ding, D.; Zhang, Z.; Shi, B.; Luo, X.; Liang, Y. Colloids Surf. 1996, 112, 25. (25) Berti, D.; Franchi, L.; Baglioni, P. Langmuir 1997, 13, 3438. (26) Shimomura, M.; Nakamura, F.; Ijiro, K.; Taketsuna, H.; Tanaka, M.; Nakamura, H.; Hasbe, K. J. Am. Chem. Soc. 1997, 119, 2341. (27) Ra¨ler, U.; Heiz, C.; Luisi, P. L.; Tampe´, R. Langmuir 1998, 14, 6620. (28) Drost-Hansen, W.; Singleton, J. L. Fundamentals of Medical Cell Biology, Vol. 3A, Chemistry of the Living Cell; JAI Press: Greenwich, CT, 1992.

10.1021/la000181i CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000

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Figure 1. Schematic drawing of the chemical structure of (3′,5′distearoyl)-2′-deoxythymidine (1).

in most of these examples only macroscopic techniques were used to monitor the recognition processes.20,21,24,25 To elucidate the mechanisms involved in these processes it is necessary to probe the microscopic properties of these systems at a molecular level. In the present work, we report our studies on the molecular recognition capabilities of a nucleolipid amphiphile (3′,5′-distearoyl)-2′-deoxythymidine bearing thymine moiety as headgroup and two stearoyl tail chains to the complementary nucleoside adenosine at air/water interface, and the molecular structure of the corresponding LB films before and after the recognition effect occurred. The monolayer behavior of the nucleolipid amphiphile on pure water, aqueous adenosine solution was studied through surface pressure-molecular area (π-A) isotherms. Ultraviolet visible (UV-vis), Fourier transform infrared (FTIR) transmission, FTIR-attenuated total reflection (ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopic techniques were used to investigate the photodimerization of thymine moieties in LB matrixes and the structure of LB films deposited from different subphases. Experimental Section Materials. The synthesis and purification of nucleolipid amphiphile (3′,5′-distearoyl)-2′-deoxythymidine (the chemical structure is reported in Figure 1, designated as 1 hereafter) was reported elsewhere.29 Adenosine was obtained from Sigma and used without further purification. Water used for the subphase was deionized and doubly distilled. π-A Isotherms and LB Films Deposition. The experiments for monolayer spreading and the deposition of LB films were performed on a computer-controlled WM-1 LB trough system (National Laboratory of Molecular and Bioelectronics, Southeast University, China) with a Wilhelmy plate used as the surface pressure sensor. Two barriers compressed or expanded symmetrically at the same rate from two sides of the trough. Pure water (pH ) 6.1), aqueous 1.0 and 5.0 mM adenosine solution (pH ) 8.3, 8.5, respectively) were used as subphases, all of which did not give any appreciable values of surface pressure under compression in the examined area ranges. Monolayers were obtained by spreading amphiphile 1 chloroform solution with the concentration of 1 mM onto subphases. After spreading, 15 min was allowed for solvent evaporation, and then the monolayers were compressed at a constant rate of 25 cm2/min unless otherwise indicated. The subphase temperature was kept at 20 ( 1 °C. Each π-A curve was reproducible. For LB films fabrication, the monolayers were compressed constantly up to the surface pressure of 35 mN/m, and 30 min was spent to establish an equilibrium of the monolayers. The monolayers were then transferred by the vertical dipping method onto quartz plates (for UV-vis measurements), CaF2 plates (for infrared transmission measurements), Ge-attenuated total re(29) Huang, J.; Ding, D.; Zhang, Z.; Shi, B.; Liang, Y. Synth. Commun. 1997, 27, 681.

Figure 2. Surface pressure-molecular area (π-A) isotherms of amphiphile 1 (a) on pure water, (b) aqueous 1 mM adenosine solution, and (c) aqueous 5 mM adenosine solution. flection (ATR) plates (for FTIR-ATR measurements), and Agcoated glass slides (deposited by chemical precedure,30 for FTSERS measurements) from different subphases. The substrates used have been cleaned by successive sonification in chloroform, acetone, ethanol, and pure water for 5 min each. The typical dipping rate was 2 mm/min, and the transfer ratio of Z-type films was nearly unity (1.0 ( 0.1) in the upstroke mode. UV-Vis Spectra Measurements and UV Irradiation. UV spectra of the LB films deposited from various subphases were recorded on a Shimadzu UV-3100 spectrometer. The LB films deposited from different subphases were irradiated by a 254-nm light with a power of 1.2 W from a UV analyzer (XingYue model ZF-1, Shanghai). The quartz plates were placed horizontally at a distance of 15 cm from the light source. Infrared Spectra Measurements. Infrared spectra were recorded through a Bruker IFS66V spectrometer equipped with a DTGS detector. A variable angle ATR attachment was used for ATR measurement, the incidence angle was 45°, and the number of internal reflections was 25. All spectra were collected for 1000 interferograms with a spectral resolution of 4 cm-1. FT-Raman Spectra Measurements. FT-Raman spectra were detected on a Bruker RFS100 spectrometer with a Nd:YAG laser emitting at 1064 nm and an InGaAs detector working at liquid nitrogen temperature. All spectra were collected at 4 cm-1 resolution by accumulating 500 scans (measurement time 17 min) with a laser power of 75 mW.

Results and Discussion Monolayer Behavior of Amphiphile 1. Figure 2 compares the π-A isotherms of amphiphile 1 monolayers on various subphases. It is clear that the slop, molecular area, and collapse pressure depend on the types of subphases. Spreading the solution of 1 onto pure water (Figure 2a) gives a stable monolayer with a collapse pressure of 50 mN/m. The isotherm trace exhibits typical characteristics of a condensed monolayer where a titled condensed phase to a vertical condensed phase transition is observed. The limiting molecular area 50.9 Å2/molecule was estimated from the π-A curve by extrapolating the condensed region to zero pressure. This value is larger than twice the cross section, 19 Å2, of a single alkyl tail chain, which may arise from the separation between the two alkyl tail chains in amphiphile 1. On an aqueous subphase containing 1 mM adenosine (Figure 2b), amphiphile 1 also gives a stable monolayer with a collapse (30) Ni, F.; Cotton, T. M. Anal. Chem. 1986, 58, 3159.

Recognition Capability of a Nucleolipid Amphiphile

pressure of 50 mN/m, but the π-A isotherm is clearly shifted toward a larger area with respect to that on pure water, indicating a decrease of the packing density caused by the interaction of the adenosine with the monolayer. A larger limiting molecular area 63.3 Å2/molecule was obtained on this subphase. Moreover, in isotherm b, a slight bend around 50 Å2/molecule was observed, which may be due to a reorientation resulting from the enlargement of a headgroup when the recognition effect occurred. Upon increasing the adenosine concentration to 5 mM (Figure 2c), the amphiphile forms a fairly stable monolayer that possesses only a vertical condensed phase with a limiting molecular area of 54 Å2/molecule and a more higher collapse pressure of about 65 mN/m compared to those on pure water and aqueous 1 mM adenosine. It is worth mentioning that the higher adenosine concentration in the subphase results in a distinct different monolayer behavior, which suggests the difference of interaction and arrangement between the molecules. It has been reported that for a nucleolipid amphiphile, 1′,-3′-bis(octadecyloxy)isopropyl orotate, with the analogous headgroup, the complementary nucleobase cooperatively binds to the monolayer through independent binding sites, i.e., hydrogen bonding and aromatic stacking.31 For the lower adenosine concentration (1 mM) in the subphase, minor complementary bases were recognized by the monolayer and the aromatic stacking effect is weaker due to the little contacted probability of adenosine units; whereas for the higher adenosine concentration (5 mM) in the subphase, the higher recognition ratio makes the stacking effect intensified. Considering the fact that the adenine unit compared to thymine unit shows a higher base-stacking tendency,32 the stronger interaction in the headgroups results in a close-packing of the alkyl tail chains, and thus increases the collapse pressure which may explain the formation of only a vertical condensed phase at higher adenosine concentration (5 mM). This is in agreement with the results reported in an earlier research,21 in which the nucleolipid amphiphiles with two different headgroups (thymine and adenine) and equal hydrophobic tails exhibit different surface properties, the amphiphile bearing adenine moiety as headgroup forms only a condensed phase, whereas the thymine-nucleolipid shows a liquid expanded phase as well. UV-Vis Spectra of LB Films Deposited from Different Subphases. Five- monolayer amphiphile 1 LB films deposited from pure water, aqueous 1 mM, and 5 mM adenosine solution exhibit a broad absorption band with a maximum absorption around 270, 268, and 265 nm, respectively (Figure 3a-c), which are close to those of aqueous thymidine (266 nm)33 and adenosine (260 nm)33 solution. However, with increasing adenosine concentration, the distinct increase in intensity can be observed. It is well-known that in aqueous solution adenosine has a larger extinction coefficient than thymidine.33,34 So it is concluded that the special recognition of the monolayer to adenosine is established at the air/water interface, the deposition of the monolayers onto solid substrate cannot destroy the hydrogen bonding between thymine moiety in the headgroup and the adenosine in water subphase, and the adenosine is also transferred onto quartz plate along with amphiphile 1 monolayer. The stronger absorp(31) Kawahara, T.; Kurihara, K.; Kunitake, T. Chem. Lett. 1992, 1839. (32) Overberger, C. G.; Morishima, Y. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 1247. (33) Voelter, W.; Records, R.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. Soc. 1968, 90, 6163. (34) Callis, P. R. Annu. Rev. Phys. Chem. 1983, 34, 329.

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Figure 3. UV spectra of 5-monolayer amphiphile 1 LB films deposited on quartz substrates from (a) pure water, (b) aqueous 1 mM adenosine solution, and (c) aqueous 5 mM adenosine solution.

Figure 4. Structure of the photoproduct of thymine moieties in nucleic acid upon ultraviolet irradiation.

tion maximum intensity in Figure 3c than that in Figure 3b indicated that, in higher subphase concentration, more adenosine molecules were recognized by amphiphile 1 monolayer and were transferred onto the solid substrates. Photodimerization of Thymine Moieties in LB Film Matrixes. The dimerization of thymine moiety, induced by UV-light, is one of the main lethal and mutagenic effects to DNA,35 which has been extensively studied in bulk solution and matrixes.36,37 It would be interesting to study this photoreaction in two-dimensional organized aggregates. The photoproduct of thymine moieties in DNA upon UV-irradiation has the cyclobutanetype structure38 as shown in Figure 4, whose formation can be followed by the intensity changes of the characteristic absorption band near 270 nm. Shown in Figure 5 are the UV spectra of 5-monolayer amphiphile 1 LB film deposited from pure water recorded at different times after (35) Setlow, R. B. Science 1966, 153, 379. (36) Wang, S. Y. Photochemistry and Photobiology of Nucleic Acids; Academic Press: New York, 1976. (37) Beukers, R.; Berends, W. Biochim. Biophys. Acta 1960, 41, 550. (38) Weinblum, D.; Johns, H. E. Biochim. Biophys. Acta 1966, 114, 450.

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Figure 5. UV spectra of 5-monolayer amphiphile 1 LB films deposited from pure water on quartz substrate after being irradiated by a 254 nm ultraviolet light for (a) 0 h, (b) 3 h, (c) 11 h, and (d) 19 h.

it was subjected to UV-light at 254 nm. After being irradiated for 3 h, the intensity for the 270 nm absorption band decreased considerably (Figure 5b) compared with that of the nonirradiated one (Figure 5a). A 19-h irradiation makes the absorption maximum tend to be a constant value, which indicates the photoreaction is completed. It is well-known that to form cyclobutane dimers, the thymine moieties must be able to come into close contact. From crystallographic data, for dimerization, the maximum distance between the 5,6-double bond of two thymine rings should be at most 4.2 Å.21 It was also reported that thymine dimers are generated upon UV irradiation after its solution is frozen,37 because under this condition the molecular arrangement becomes more regular compared with that in the room-temperature solution. Thus, this photoreaction should depend on the packing of the thymine moieties. Therefore, it can be concluded that the thymine headgroups are regularly close-packed in quasi-crystalline LB film, which facilitates the photodimerization of the thymine moieties even at room temperature. Shown in Figure 6 are the UV absorption spectra of 5-monolayer amphiphile 1 LB film deposited from aqueous 1 mM adenosine subphase recorded at different time after being subjected to ultraviolet irradiation. After 20-h irradiation, a constant absorption maximum intensity is obtained, which indicates that the photoreaction is completed, and the absorption maximum shifts to 263 nm (Figure 6d). The absorption maximum shown in Figure 6d should be the characteristic absorption band of adenosine. Figure 7 depicts the UV absorption spectra of different irradiation times for a 5-monolayer amphiphile 1 LB film deposited from aqueous 5 mM adenosine subphase. It took 85 h for the photoreaction to be completed, and the absorption maximum kept near 265 nm. By comparing Figures 7e and 6d, it is clear that the absorption maximum intensity in the former is rather larger than that in the latter, which provides further evidence that more adenosine molecules were recognized and transferred. In a word, in a higher adenosine

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Figure 6. UV spectra of 5-monolayer amphiphile 1 LB films deposited from aqueous 1 mM adenosine solution on quartz substrate after being irradiated by a 254 nm ultraviolet light for (a) 0 h, (b) 2 h, (c) 8 h, and (d) 20 h.

Figure 7. UV spectra of 5-monolayer amphiphile 1 LB films deposited from aqueous 5 mM adenosine solution on quartz substrate after being irradiated by a 254 nm ultraviolet light for (a) 0 h, (b) 17 h, (c) 38 h, (d) 59 h, and (e) 85 h.

concentration subphase, more adenosine molecules were recognized by amphiphile 1 monolayer, and a more strong stacking interaction exists in the headgroups, which results in the formation of a more condensed monolayer at the air/water interface. FTIR Spectra of Amphiphile 1 LB Films. Figure 8 exhibits FTIR transmission spectra of (a) 1-, (b) 2-, (c) 3-, (d) 4-, and (e) 5-monolayer amphiphile 1 LB films deposited

Recognition Capability of a Nucleolipid Amphiphile

Figure 8. FTIR transmission spectra of (a) 1-, (b) 2-, (c) 3-, (d) 4-, (e) 5-monolayer amphiphile 1 LB films deposited on CaF2 substrates from pure water.

on CaF2 substrates from pure water. Two strong bands near 2917 and 2849 cm-1 are attributed to the antisymmetric and symmetric CH2 stretching vibrations of long hydrocarbon chains, respectively. It has been demonstrated for long-chain hydrocarbon molecules that the frequencies of the antisymmetric and symmetric CH2 stretching vibrations are conformation-sensitive due to the perturbation by Fermi-resonance interaction with the methylene bending vibration.39-41 Low wavenumbers at 2917 and 2849 cm-1 correspond to a highly ordered alltrans conformations,39-41 which indicates that the hydrocarbon chains of amphiphile 1 in the LB film take on a close-packed all-trans conformation at room temperature. The CH2 scissoring mode is well-known to be extremely sensitive to interchain interactions.42 A singlet peak at approximately 1467 cm-1 is indicative of a hexagonal subcell packing43 where each hydrocarbon chain freely rotates around its axis oriented perpendicular to membrane surface. For multiplayer LB films as shown in Figure 8, all the spectra are very similar to each other except for the band intensities which increased linearly with the number of monolayers, suggesting that the conformation, subcell packing, and molecular orientation of the nucleolipid amphiphile in the LB films change little as a function of the monolayer number. For the LB films transferred from aqueous 5 mM adenosine solution (Figure 9), similar results were obtained, the hydrocarbon chains are also in a close-packed all-trans conformation with a hexagonal subcell packing. However, in the 1750-1500 cm-1 region, no distinguished difference can be observed before and after the recognition effect occurred, which should be due to the insufficient sensitivity of the infrared transmission method. Figure 10 compares the FTIR-ATR spectra of 5-monolayer amphiphile 1 LB films transferred from pure water (39) Snyder, R. G.; Hou, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (40) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (41) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (42) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (43) Cameron, D. G.; Casal, H. L.; Gudgin, E. F.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 596, 463.

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Figure 9. FTIR transmission spectra of (a) 1-, (b) 2-, (c) 3-, (d) 4-, (e) 5-monolayer amphiphile 1 LB films deposited on CaF2 substrates from aqueous 5 mM adenosine solution.

Figure 10. FTIR-ATR spectra of 5-monolayer amphiphile 1 LB films deposited from (a) pure water and (b) aqueous 5 mM adenosine solution.

and aqueous 5 mM adenosine solution, respectively. For LB film deposited from pure water (Figure 10a), the thymine moieties are hydrated or bound together by hydrogen bonds, as indicated by the νNH peak near 3188 cm-1.44 As shown in Figure 10b, the spectrum of LB film fabricated from 5 mM adenosine does not show the adenine bands in the NH2 stretching region, i.e., at 3289, 3239, and 3110 cm-1, attributed to polymeric structures of the adenine derivative44 in the bulk phase, whereas two new bands appear at 3436 and 3200 cm-1. These two new bands are assigned to the existence of a non-hydrogen-bonded NH group of the adenine moiety.44,45 As also can be seen in Figure 10b, two new bands appear near 1653 and 1598 cm-1 which were ascribed to the coupling of CdC and CdN stretching vibration of the adenine ring and the adenine ring stretching vibration,44,46 respectively. At the (44) Kyogoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. Soc. 1967, 89, 496. (45) Kyogoku, Y.; Lord, R. C.; Rich, A. Nature 1968, 218, 69.

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Figure 12. Schematic illustration of the complementary multiple hydrogen bonds between thymine group and adenosine.

Figure 11. FT-Raman spectrum of a single layer amphiphile 1 LB film deposited on glass substrate (a); FT-SERS spectrum of a single layer amphiphile 1 LB film deposited on silver island film substrate from pure water (b); and aqueous 5 mM adenosine solution (c).

same time, the band near 1691 cm-1 assigned to CdO (for the C4 position) stretching mode44 increased its intensity obviously due to the overlap of the bonded NH2 scissoring mode in adenine moiety.13 The band near 1563 cm-1 also increased its intensity due to the coupling of C-C and C-N stretching vibration mode of the adenine ring.46,47 These spectral alternations can be explained as a result of the formation of hydrogen bonds between thymine and adenine bases at air/water interface, and further demonstrate that the adenosine was transferred along with the monolayer onto the solid substrates. FT-SERS Raman Spectra of Amphiphile 1 Deposited from Different Subphases. Figure 11a shows the FT-Raman spectrum of a single layer amphiphile 1 LB film deposited on glass substrate, no scattering band can be observed. Figure 11b,c shows FT-SERS spectra of amphiphile 1 as a single layer LB film deposited on chemical procedured silver island film substrates from pure water and aqueous 5 mM adenosine solution, respectively. In the 1750-500 cm-1 region, high quality FT-SERS spectra were obtained, and the enhanced bands of the related groups were clearly seen. For an amphiphile 1 LB film deposited from pure water subphase (Figure 11b), the enhanced bands of thymine moiety near 1642 cm-1 [C(4)dO stretching], 1601 cm-1 [a combination of C(4)dO and C(5)dC(6) stretching], 1575 cm-1 (C-N stretching), 1165 cm-1 [a combination of CH deformation and C(2)-N(3) stretching], 827 cm-1 (C-N symmetric stretching), and 615 cm-1 (N-CdO deformation) are found.48,49 By comparing Figure 11c (aqueous 5 mM adenosine subphase) and Figure 11b (pure water subphase), four new pronounced enhanced peaks attributed to adenine moiety appear at 734 cm-1 (adenine ring (46) Stepanian, S. G.; Smorygo, N. A.; Sheina, G. G.; Radchemnko, E. D.; Yakovleva, V. D.; Rusavskaya, T. N.; Studentsov, E. P.; Ivin, B. A.; Blagoi, Yu. P. Spectrochim. Acta 1990, 46A, 355. (47) Lord, R. C.; Thomas, G. J., Jr. Spectrochim. Acta 1967, 23A, 2551. (48) Otto, C.; van den Tweel, T. J. J.; de Mul, F. F. M.; Greve, J. J. Raman Spectrosc. 1986, 17, 289. (49) Aamouche, A.; Ghomi, M.; Couombeau, C.; Grajcar, L.; Baron, M. H.; Jobic, H.; Berthier, G. J. Phys. Chem. A 1997, 101, 1808.

breathing mode),501333 cm-1 (adenine ring skeletal vibration), 1370 cm-1 [C(2)-H bending],51 and 1451 cm-1 [a combination of N(1)-C(2) and N(3)-C(2) stretching].48 Moreover, the bands near 1601, 927, and 615 cm-1, attributed to thymine moiety, are almost absent in Figure 11c. These changes also indicate that amphiphile 1 monolayer at the air-water interface is capable of interacting with dissolved adenosine in the subphase, and the adenosine is also transferred onto solid substrate along with the surface monolayer, which is consistent with the results of UV-vis and infrared spectra. According to the surface-enhanced Raman selective rule, that the scattering intensity of vibrational modes of a moiety in direct contact with the metal surface is preferentially enhanced,51 it can be deduced from Figure 11c that the complementary base pairing is close to the silver surface, whereas no enhanced information was detected in the high-frequency region indicates that the alkyl chains remain far from the silver surface. The above spectral features indicated that the molecular recognition occurred between the amphiphile 1 monolayer and adenosine substrate in the subphase through multiple complementary hydrogen bonds, together with the FTIR-ATR results, a schematic illustration is shown in Figure 12. These results show that FT-SERS technique will be a powerful tool for studying molecular recognition capability between the complementary bases in interface systems because of its very high sensitivity. Conclusion π-A isotherm alternations indicate that, in high subphase concentration, the recognition of amphiphile 1 monolayer to complementary nucleoside is promoted cooperatively by base stacking. Various spectroscopic results indicate that the special recognition of the amphiphile 1 monolayer to adenosine is established at the air/water interface, the deposition of the monolayers onto solid substrate cannot destroy the hydrogen bonding between thymine moiety in the headgroup and the adenosine in water subphase, and the adenosine is also transferred onto solid substrates along with amphiphile 1 monolayer. The ordered and closed packing of the molecules in quasi-crystalline LB film provides favorable conditions for the formation of thymine dimers under ultraviolet irradiation even at room temperature, which makes further investigations on the photochemical properties of DNA by way of chemistry possible. FT-SERS technique was successfully introduced into the research area of molecular recognition between nucleolipid amphiphile monolayers and the complementary base substrates in the subphase. High-quality FT-SERS spectra of 1-layer LB films provide abundant recognition infor(50) Ervin, K. M.; Koglin, E.; Sequaris, T. M.; Valenta, P.; Nurnberg, H. W. J. Electroanal. Chem. 1980, 114, 179. (51) Watanade, T.; Kawanami, O.; Kaoh, H.; Honda, K.; Nishimura, Y.; Tsuboi, M. Surf. Sci. 1985, 158, 341.

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mation, which indicates that this technique will become a powerful tool for studying molecular recognition capability because of its very high sensitivity. By combining the infrared and FT-SERS spectral results, a schematic illustration of the multiple hydrogen bond between complementary bases is given.

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Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (29873022) and a major research project grant from the State Science and Technology Commission of China. LA000181I