Langmuir 1994,10, 3195-3200
3195
Saccharide Recognition by Amphiphilic Diboronic Acids at the Air-Water Interface and the Relationship between Selectivity and Stoichiometry Rainer Ludwig,+Yutaka Shiomi, and Seiji Shinkai" Shinkai Chemirecognics Project, Research and Development Corporation of Japan, Aikawa 2432-3, Kurume, Fukuoka 830, Japan Received December 17, 1993. In Final Form: June 9, 1994@ Amphiphilic diphenylmethane-3,3'-diboronic acids form stable monolayers which selectively bind monosaccharides. The selectivity order expressed as a difference in the collapse pressure between the uncomplexed and complexed boronic acid monolayer is governed by the formation of complexes consisting of two diboronic acid molecules bridged by one sugar molecule. The selectivity can be changed to correlate with the order ofthe associationconstants ofthe 1:l complexes in homogeneous solution by adding polycations to the subphase or by diluting the monolayer with amphiphilic quaternized ammonium ions.
Introduction The specific interaction between phenylboronic acids and saccharides or related compounds has been studied over the past years.'-15 A number of phenylboronic and diphenylmethane-3,3'-diboronica c i d V possess a high sensitivity toward sugars together with pronounced selectivities for mono- and disaccharides due to facile complexation between boronic acids and 1,2- or 1,3-diols. Recently, association constants between a diphenylmethane-3,3'-diboronic acid and several saccharides were determined in homogeneous solution.6 Compound la, for example, shows a high selectivity for D-glucose (log Km8, = 4.3, Table 1)compared with D-galactose,D-mannose, or disaccharides, and the resulting complex has a 1:l stoichiometry. In the case ofmonoboronicacid 2, however, the complex involves two molecules of 2 bridged by one saccharide molecule.' Amphiphilic phenylboronic acids were shown to form stable monolayers at the air-water interface, which selectively recognize monosaccharides at micromolar levels by the formation of ester^.^-^ In the present study, we compare the selectivity order of lb-d toward monosac'Present address: FU Berlin, Institut f. Anorg. u. Analyt. Chemie, Fabecktr. 34-6, 14195 Berlin, Germany. Abstract published inAdvanceACSAbstracts,August 15,1994. (1)Tsukagoshi, K.; Shinkai, S. J. Org. Chem. 1991,56, 4089. (2) Shinkai, S.; Tsukagoshi, K.; Ishikawa, Y.; Kunitake, T. J.Chem. @
Commun., Chem. Commun. 1991, 1039. (3) Ludwig, R.; Ariga, K.; Shinkai, S. Chem. Lett. 1993, 1413. (4)Ludwig, R. ; Harada, T.; Ueda, K.; James, T. D.; Shinkai, S.J. Chem SOC.,Perkin Trans. 2 1994, 697. (5) James, T. D.; Harada, T.; Shinkai, S. J. Chem. SOC.,Chem. Commun. 1993, 857. ( 6 )Shiomi, Y.; Kondo, K.; Saicho, M.; Harada, T.; Tsukagoshi, K.; Shinkai, S. Supramol. Chem. 1993,2, 11. (7) Shiomi, Y.; Saicho, M.; Tsukagoshi, K.; Shinkai,S.J.Chem. SOC., Perkin Trans. 1 1993, 2111. (8) Kondo, K.; Shiomi, Y.; Saicho, M.; Harada, T.; Shinkai, S. Tetrahedron 1992, 48, 8239. (9) Huttunen, E. Ann. Acad. Sci. Fenn., Ser. A2 1984,201,l;Chem. Abstr. 102(14):120917a. (10)Wulff, G.;Heide, B.; Helfmeier, G. J.Am. Chem. SOC.1986,108, 1089. (11)Lochmuller, C. H.; Hill, W. B. ACS Symp. Ser. 1986,297,210; Chem. Abstr. 105(2):17540k. (12) Shinbo, T.; Nishimura, K.; Yamaguchi, T.; Sugiura, M. J.Chem. SOC.,Chem. Commun. 1986,349. (13)D'Silva, C.; Green, D. J. Chem. SOC.,Chem. Commun. 1991, 227. (14)Yoon, J.; Czamik, A. W. J. Am. Chem. SOC.1992, 114, 5874. (15)Mohler, L. K.; Czarnik, A. W. J. Am. Chem. SOC.1993, 115, 2998.
0743-7463/94/2410-3195$04.50l0
Chart 1. Phenylboronic Acids Used for This Study R
I
R
I
?
I
B(OH)z
(CH~~IS-CH~
I
,
l a : R=Methyl l b : R = 2-Octyldodecyl IC : R = 4-rert-Butylbenzyl
Id : R = H2C-C=C-C-C-C=C H 1 H2HzH
CH3
3 I
(Geranyl)
CH3
charides in the monolayer system with those in homogeneous solution and in two-phase solvent extraction. Furthermore, we investigate the influence of inert amphiphilic diluents and of polyions in the subphase on the selectivity, because we previously found molecular assemblies consisting of boronic acid and quaternized ammonium ions to have a n improved sensitivity compared with the monolayers of pure boronic acid.3 The aim of the work was to prepare monolayers which selectively recognize monosaccharides by combining the diphenylmethane-3,3'-diboronic acid moiety, which specifically binds monosaccharides in solution, with long alkyl chains in order to obtain stable films. The results are interpreted in terms of binding constants, the stoichiometry of the complexes, and their lipophilicity. Chart 1 shows the structures of the phenylboronic acids used for this study. The additives 3-9 (Chart 2) were used in order to influence the selectivity order of monolayers of lb.
Experimental Section Chemicals. Diboronic acids lb-d were synthesized in a manner similar to that described for la.8 In the case of lb, a mixture of bis(2-hydroxy-5-bromophenyl)methane(10,5.0g, 14 mmol), powdered potassium carbonate (4.8 g, 42 mmol), 2-octyldodecyl p-toluenesulfonate (15.8 g, 35 mmol), and N,Ndimethylformamide (20 mL) was stirred at 80 "Cfor 2 days under nitrogen. After removal ofthe solvent in umuo, water was added to the residue. The mixture was neutralized with 2 M hydrochloric acid and extracted with chloroform. The extract was washed with water and dried over anhydrous magnesium sulfate. After filtration, the solvent was removed, and the residue was purified by silica gel column chromatography. From hexane elution the intermediate product bis[2-(2-octyldodecyloxy)-5bromophenyllmethane (11) was obtained (10.75 g, 11.7 mmol,
0 1994 American Chemical Society
Ludwig et al.
3196 Langmuir, Vol. 10, No. 9, 1994 Chart 2. Additives Used in This Study 3 Dimethyldioctadecylammonium bromide +/Ci8H37
H3C,
N H1C’
4: Dimethylpalmitoylarmde
Br ‘CIBHV
0
5b:Octadecanoic acid
Sa: Octadecanol
6: Poly(4-vinyl-N-methylpyridinium iodide)
( n = 24)
7: Poly(ally1aminr) (n=17-18)
AH:
OH
CHI 8. Poly(acry1ic acid)
J n
9: Poly(diallyldimethylammonium chloride) (n = 350 - 700)
H3C’
‘CH3
83.7%)as a colorless oil and identified: ‘H NMR(a, CDCl3, TMS), 0.88 (t, 12H, CH3), 1.0-1.8 (m, 66H, -CH2 and -CHI, 3.7-3.9 (m, 6H, Ar-CH2 and -O-CHz), 6.7 (d, 2H, Ar-HI, 7.1-7.3 (m, 4H, Ar-H); SIMS+(3-nitrobenzyl alcohol matrix): mlz 918 (M+). In order t o convert compound 11 to the boronic acid lb, a 1.6 M solution ofbutyllithium in hexane (4.9 mL, 7.83 mmol) was added to a mixture of 11 (3.0 g, 3.26 mmol) and anhydrous tetrahydrofuran (50 mL) a t below -70 “C under nitrogen. After being stirred for 2 h, this mixture was dropped into a mixture of trimethyl borate (14.9 mL, 131 mmol) and anhydrous tetrahydrofuran (50 mL) at below -70 “C. After the mixture was stirred for 1h, its temperature was raised to room temperature within 5 h. HCl(2 M) was added to the mixture under cooling with an ice bath, followed by stirring for 18 h. This mixture was concentrated in vacuo and extracted with hexane after water was added. The extract was washed with water and dried over anhydrous magnesium sulfate. After filtration, the solvent was removed and the residue purified by silica gel column chromatography. From hexane elution dehydrated polymeric l b (1.7 g, 2.09 mmol, 64.1%)was obtained as a colorless oil: IR(neat), 1350 cm-1 (YB-0). Apart of l b (200 mg, 0.246 mmol) was characterized by converting it to the corresponding dioxaborinane 12 upon refluxing with 1,3-propanediol(45mg, 0.59 mmol) in toluene (10 mL) in the presence of molecular sieve 4A. 12 was identified by its spectral evidence: ‘HNMR (a,CDCls), 0.88 (t, 12H, -CH3), 1.0-2.2 (m, 70H, -CH2 and -CHI, 3.81 (d, 4H, -O-CH2), 3.96 (s, 2H, Ar-CHz), 4.08 (t, 8H, -O-CH2), 6.80 (d, 2H, Ar-H), 7.2-7.6 (m, 4H, Ar-HI; SIMS+ (matrix: 2-nitrophenyl octyl ether), mlz 928 (M+);IRbeat), 1320 cm-l (YB-o). In the case of lc, the intermediate bis{2-[(4-tert-butyl)benzyloxy]-5-bromopheny1)methane (13) was obtained in 80.5% yield from the reaction of 10 (2.0 g, 5.6 mmol) with 4-(tert-butyl)benzyl chloride (4.07 g, 22.3 mmol) in dry N,N-dimethylformamide (20 mL) in the presence of K2CO3 (3.1 g, 22.3 mmol) and catalytic amounts of KI and identified by its spectral evidence and elemental analysis. lH NMR (CDCls),1.32 (s, 18H, CH3), 3.93 (s,2H, ArCH2), 4.95 (s, 4H, -O-CH2-Ar), 6.7-7.34 (m, 14H, Ar-H); IR(neat), no absorption a t 3300 cm-l; elemental analysis, anal. C 64.45%,H 5.95%,calcd C 64.6%,H 5.9%; mp 164 “C. Compound 13 (2.16 g, 3.3 mmol) was converted to ICas described above, and the product was separated into a hygroscopic polymeric
compound IC’(0.5 g, 0.9 mmol) [IR(neat), 1350 cm-I (YB-01, no absorption a t 3300 cm-l; mp 62 T I and a crystalline monomeric product IC”(0.55 g, 1mmol) [IR(neat), similar to the polymer, but additional 3300 cm-l (strong, YO-H); mp 170 T I . The ‘H NMR spectra (a,CD3COCD3)of IC’and IC”are almost identical: 1.29 (s,18H, CH3),4.1(~,2H, Ar-CH2), 5.07 (s,4H, -0-CH2Ar), 6.9-7.8 (m, 14H,Ar-H). Two peaks (crystalline modification only) at 5.7 (s, 2H) and 6.8 (s, 2H) ppm disappear after D2O is added and are ascribed to -BOH. The two forms can be converted into each other. For subsequent experiments, IC’dissolved in CDCl3was used for liquid-liquid extraction, while IC”was used for solid-liquid extraction into acetone-&. In the case of Id, precursor 10 (2.0 g, 5.6 mmol) was treated with geranyl bromide (4.8 g, 22.1 mmol) in acetone as solvent in the presence ofKzCO3 (3.1g, 22.3 mmol). After being recrystallized twice from hexane, the intermediate bis(2-geranyl-5-bromophenyl)methane 14 (1.59 g, 2.5 mmol, 44.6%) was obtained: ‘H NMR (a, CD&OCD3), 1.58-1.86 (m, 18H, CH3), 2.0-2.1 (m, 8H, -CH2), 3.85 (s, 2H, Ar-CHz), 4.55 (d, 4H, -O-CHz), 5.1 (t, 2H, =CH), 5.45 (t, 2H, =CH), 6.8-7.3 (m, 6H, Ar-HI; IR (neat), 1674 cm-l(vc=c); elemental analysis, anal. C 62.25%,H 6.66%,calcd C 62.86%,H 6.71%;mp60.5-60.9 “C. The intermediate 14 (1.0 g, 1.59 mmol) was converted to Id (white solid, 0.11 g, 0.2 mmol, 12%) as described for 1b and identified by its spectral evidence and elemental analysis: ‘H NMR (a,CD3COCD31, 1.35 (s, 6H, CH3), 1.6 (d, 12H, CH3), 2.0 (m, 8H, CHz), 3.9 (s, 2H, Ar-CHz), 4.5 (d, 4H, -O-CH2), 5.1 (t,2H, =CH), 5.47 (t,2H, =CHI, 6.8-7.6 (m, 6H, Ar-H); W (0.3 mM in CH30WK2C03 at pH 11.3,e = 2.0 a t 296.5 nm; IR (neat, cm-’), 812(vc~),1022 + 1255(vc-o-caa), 1348(v~-0),1507 1603(~,~), 3300(vo~);elemental analysis, anal. C 70.48%, H 8.30%, calcd C 70.73%, H 8.27%. Poly(4-vinylpyridine) was converted to poly(4-vinyl-N-methylpyridinium iodide) (6).The product was 100%quaternized as characterized by lH NMR, and the average molecular weight was 2500 before quaternization as determined by static light scattering in water. Polyallylamine hydrochloride (Aldrich, low MW)was converted to the free amine 7 with Amberlite 1RA 402. From the 1H NMR spectra of mixtures between phenylboronic acids and 6 or 7 a t a concentration ratio and pH similar t o the conditions in the monolayer system, it was concluded that covalent bonds between boron and nitrogen atoms are not formed in the monolayer system. Saccharides (Fluka, for microbiology) were used without further purification. Water was ion-exchanged and bidistilled (17.5 M Q cm). The subphase was buffered with K2COylKHCO3, and the ionic strength was 0.1. Solutions were prepared freshly and used within 1 h. Benzene was used as spreading solvent. Decomposition and epimerization reaction in the saccharide-containing solutions were neglectable under the experimental condition^.^ Equipment. Pressure-area (n-A)isotherms were measured with a computer-controlled FSD-50 type film balance (United System Integrators) at 293 & 0.1 K, according to the Wilhelmy method. The maximum area was 2.5 nm2 molecule-’ and the barrier speed 0.4 mm s-l. In the n-A graphs every second measurement point is depicted. Data for surface pressure and molecular area are the average from at least two reproducible measurements. W-reflection spectra of monolayers during compressionwere measured with a MCPD-110 type spectrometer (Otsuka Electronics) equipped with a photodiode array, a deuterium lamp, and optical fibers, one tip of which was placed 2 mm above the air-water interface. Secondary ion mass spectra (SIMS) were measured by using a M-2500 mass spectrometer (Hitachi). The LB films of saccharide complexes of l b for SIMSmeasurements (280 layers) were prepared by vertical dipping of a stainless steel plate (42 mm2) with a modified surface.
+
Results and Discussion
Sugar Recognition by Monolayers of lb. Figure 1 shows typical n-Aisotherms of lb at pH 11.2in the absence and presence of various amounts of monosaccharides. Compound lb forms a liquid expanded monolayer, which collapses at A, = 0.85 nm2 molecule-I, corresponding t o the area expected for four parallel-oriented alkyl chains. This value increases only slightly when monosaccharides a r e added to the subphase in t h e concentration range u p
Saccharide Recognition by Diboronic Acids
Langmuir, Vol. 10, No. 9,1994 3197
-
27
2 26
z
E 25
!
.-
0.4
0.6
0.8 1.0 1.2 Area/ nm2 molecule-1
Figure 1. Pressure-area (n-A) isotherms of monolayers of l b at the air-water interface at 293 K. Curve a, 11 mM (M =
moles per liter) D-glucose; b, 1.1 mM D-glucose; c, 1.1 mhf D-fI-UCtOSe; d and e, no saccharide.Curves a-d: pH 11.2.Curve e: pH 6.1.
- 29-281 E 21 .
b
5
1.4
A
None
7 8 9 1 0 1 1 1 2 pH of Subphase
6
I
D-Fructose D-Glucose
240 0'
D-Fructose D-Glucose
Figure 3. Collapse pressure ncof l b as a function of the pH in the absence and presence o f 5.5 mM monosaccharides.
Ll
20
b
I
I
I
4
I
260
280
300
320
340
Wavelength /nm
!/ 1
10
50
Monosaccharide/ m M
Figure 2. Collapse pressure n,of monolayers of l b as a function of the concentration O f D-glucose or D-fructose in the subphase at pH 11 on a logarithmic scale.
to 27 mM. However, the limiting area at zero pressure (1.03nm2 molecule-l) increases significantly in dependence on concentration and structure of the added saccharide. The order of influence is methyl-a-glucoside < D-glucose < D-mannose < D-fructose. The increase in the molecular area of lb upon binding monosaccharides is less pronounced compared with the case for monoboronic acid 2. The higher rigidity of lb compared with 2 causes this difference. The collapse pressure, however, increases considerably when increasing amounts of saccharides are added to the subphase. This functionality is depicted in Figure 2 on a logarithmic scale for D-glucose and D-fructose. The plot is linear within the investigated concentration range. The extrapolated sensitivity is about 1 mM for D-glucose and 200 pM for D-fructose. This sensitivity is lower compared with the sensitivity of monolayers of 2,2,3 and of solutions of la.6 The reason for the lower sensitivity of compound lb may be the lower flexibility compared with that for la and 2. The binding of saccharides to boronic acids depends on the pH not only in homogeneous solution6but also a t the air-water interface a s well. Figure 3 shows how the binding of monosaccharides to lb is affected by the pH of the subphase. At higher pH, the boron atom becomes ~p~-hybridized.~J I n the W-reflection spectra of the monolayer (Figure 4) this change in hybridization is observed as a decrease in the absorption peak at 245 nm accompanied by a shift of the second peak from 265 to 275 nm. The boron atom must be tetravalent for the recognition of saccharides in aqueous media, which is achieved by a pH > ca. 10 or the presence of a n auxiliary, like R4N+, a t pH 7.3 No absorption is observed after spreading (curve 0). Upon compression to 1.2nm2molecule-1, two absorption peaks arise (curve l), the intensity of which remains constant when the surface pressure increases from zero to 10 mN m-l (curve 2). From this fact, it is concluded
Figure 4. UV-reflection spectra of monolayers of lb. Curve 0: after spreading (2 nm2molecule-l, pH 5). Curve 1: at 1.2 nm2 molecule-l (zero mN m-l, pH 5). Curve 2: at 10 mN m-l (pH 5).Curve 3: at 10 mN m-l(pH 10).Curve 4: at 10 mN m-l (pH 11).Curve 5: reference (before spreading). Table 1. Collapse Pressure nc (mN m-l) of lb and 2 in the Absence or fiesence of 11 mM Saccharide in the Subphase at 293 K and Association Constants for 1:l Complexes of la with Those Saccharides n, of lb z c of 2 pH 10.0 pH 11.4 pH 11.4 log Kass.for laa ~
D-fructose D-mannose
D-galactose D-glucose methyl-a-glucoside
none
22.6 20.6 20.4
28.4 25.5
19.8 18.8
24.3 20.0 20.8
19.7
25.0
27.6
not detectableb
20.9 23.0 20.9 10 10
1.78 3.34 4.28
not detectableb
1:l complexes in methanovwater at pH 11, data from ref 6. CD-silent.
that the molecules in the monolayer are well-ordered even at a low surface pressure and that lb is suitable for forming self-assembled monolayers. The selectivity order expressed as the difference in the collapse pressure n, of monolayers of lb in the absence and presence of saccharides is shown in Table 1. This order is similar to that of monoboronic acid 2 but exactly opposite of the order of association constants K,,,,of 1:l complexes between the very similar diboronic acid la and those saccharides in homogeneous solution a t 298 K. In order to determine the reason for the discrepancy between the selectivity in homogeneous solution and a t the airwater interface, the SIMS- spectra of LB films of lb prepared on a subphase containing 11 mM D-fructose or D-glucose at pH 11.2 were measured (Figure 5a and 5b). In the case of D-fructose, a peak a t mlz 1904 corresponds to the complex with the stoichiometry of 2:l (1b:saccharide). It contains two boronic acids (2*lb,2*844)with partly sp3-hybridization(2*17),minus two hydrate water molecules which are released during the complex formation, and one potassium counterion. A peak at mlz 1876 for the D-glucose complex corresponds to the same stoi-
3198 Langmuir, Vol. 10,No.9,1994 10or
I
,
*
Ludwig et al.
7
-
i
c-
ire h
!
I
Y
'g
0
50
u
Y
t- C
I
10
3ui
{ 1 c D-glucose > D-galactose > D-mannose. Similar to the results observed for 2 and lb, monolayers of compound Id show highest selectivity toward D-fructose. In Figure 7, the collapse pressure of monolayers of Id is plotted versus the concentration of D-glucose and D-fructose in the subphase. The plot is linear as in Figure 2; however, the slope is negative. The example shows that the interaction of phenylboronic acid monolayers with monosaccharides influences the shape of the n-Aisotherms to a large extent and can either increase or decrease the compressibility. Influence of Amphiphilic Additives on the Selectivity of Monolayers of lb. In an attempt to change the stoichiometry of the complexes between lb and monosaccharides a t the air-water interface from 2:l to 1:1, dimethyldioctadecylammonium bromide (3) was mixed with lb, because it has been established earlier that the recognition of sugar by phenylboronic acids
Langmuir, Vol. 10, No. 9, 1994 3199
Saccharide Recognition by Diboronic Acids -
1
-
I
d'
SO
10
Monosaccharidel mM
Figure 7. Influence of the concentration of monosaccharides on ncof Id at pH 11 and 293 K. Fructoge :
o
I
-Acid m a
I
h
: l b Ib Ib l b l b Ib Ib l b Ib Id
.- - 6 . - - -
7
8
9
- _ - -
h la
- - _
3 4 5 pHofS&@xise:IO 112 10.8 10.1 10 10.7 10.9 10.9 11.4 11 11.4 SzxhkkmM:55 55 11 28 11 2 8 028 I I 55 55 I I
m
b
Figure 8. Selectivity of phenylboronic acids toward monosaccharides, expressed as the change in collapse pressure in the presenceand absence of additivesto the monolayer(a)or to the subphase (b).
becomes more sensitive in the presence of quaternized ammonium ions.3 At a molar ratio of 1:4 (lb:3) the monolayer is very stable with the collapse occurring at 48 mNm-l. The z-Aisotherms of this mixture are influenced to a greater extent by D-glUCOSecompared with D-fructose. At a 0.28 mM level, D-glUCOSe decreases the collapse pressure, while D-fructose cause no change. The limiting area increases from 0.7 to 0.74 nm2 molecule-' in the presence of D-glucose but only to 0.72 nm2 molecule-' in the presence of D-fructose. A phase transition occurs at 21.8 mN m-l in the absence of saccharides. This value increases to 24.5 mN m-l in the presence Of D-glUCOSe and to 23.2 mNm-l in the presence of D-fructose. At a 2.8 mM level the order is the same. From these results it is concluded that the presence of 3 increases the sensitivity of the monolayer of lb toward D-glucose, while the sensitivity towards D-fructose is reduced. The selectivity order now correspondsto the order of association constants of the 1:1 complexes determined in homogeneous solution, and it is assumed that the complex with the 1:l stoichiometry predominates at the air-water interface. The influence of additives on the selectivity is depicted in Figure 8. The Influence of Inert Amphiphiles. Alternatively, other inert amphiphiles were mixed with l b in different ratios. The response of the mixed monolayers upon the addition of D-fruCtOSe and D-glucose to the subphase at pH 11 was monitored as a change in zc.For that purpose, lb was diluted with up to a 10-fold excess of dimethylpalmitoylamide (4), octadecanol (5a),or octadecanoic acid (5b). The mixed monolayers retained a selectivity toward D-fructose in all cases. However, the sensitivity decreased considerably when the concentration of l b in the monolayer decreased. The z-A isotherms of lb mixed with 5 show a break point around 20 mN m-l. This indicates inhomogeneous monolayers as a reason for the
unaffected selectivity: In domains of lb, a saccharide molecule can still act as an intermolecular bridge preventing the formation of 1:l complexes. Interaction of Monolayers of lb with Polycations. A different approach to change the selectivity of lb toward monosaccharides was also successful by using the method of polyion ~omp1exation.l~ In order to reduce the formation of bridged complexes between lb and monosaccharides at the air-water interface by separatingthe molecules of lb from each other, polycations were added to the subphase at concentrations below the cmc. Poly(4-vinylN-methylpyridinium iodide) (6)as well as polyallylamine (7) were suitable for that purpose. Chart 3 depicts the interaction with lb. Under the experimental conditions (pH > lo), the boron atoms in lb are sp3-hybridized and a polycation can interact with lb at the air-water interface. The collapse pressure increases from 21 to 31 mN m-l in the presence of 0.001%per weight of 6, and the molecular area where the monolayer collapses increases from 0.85 to 0.96 nm2.Addition of 11 mM D-glucose or D-galactose increases the collapse pressure to 33.3 and 31.6 mN m-l, respectively, while addition of D-fruCtoSe causes a small decrease only. The influence of saccharides increases in the order of D-fructose < D-galactose < D-glUCOSe. This order corresponds to the order of association constants of the 1:1 complexesK,,, of the structurally equal compound l a in solution (Table 1). D-Fructose with a very low association constant binds to the monolayer to a smaller extent compared with D-glucose. The limiting molecular area does not change in the presence of monosaccharides and 6 in the subphase. This behavior is different from the one observed in the absence of 6, where 11 mM D-hCtoSe or D-glucose increases the limiting molecular area from 1.0 to 1.2 and 1.1 nm2, respectively. It is therefore concluded that the presence of polycation 6 changes the stoichiometry of the complexes from 1:2 to 1:l. In the latter case the molecular area of lb does not increase upon binding monosaccharides and forming an ester, because one monosaccharide molecule fits well into four binding sites. Figure 5c depicts the SIMS- spectrum of a LB film (200 layers) of lb prepared on a subphase containing 2 x per weight of 6 as well as 11 mM D-glUCOSe at pH 11. The mass spectrum differs from the one in Figure 5b. Two peaks at mlz = 1323 and 1341 (observed: 1324,1343) are ascribed to anionic l b (844 2(17)) glucose (180) water (18n, n = 1,2; from esterification) HC03- (61) two building blocks of cationic 6 (2(120)),or multiples of this supramolecular unit. A peak at mlz = 1703 (observed: 1701) stems from anionic l b (844 2(17)) glucose (180) - water (18) 4 building blocks of cationic 6 (4(120))+ HC03- (3(61)),or multiples. Peaks at mlz = 353,328,250, and 184 stem from calibration with Fomblin prior to the measurements. Thus the strong interaction between quaternized nitrogen and sp3-hybridized boron
+
+
+
+
+
+
+
(17) Shimomura,M.;Kunitake, T.Thin Solid Films 1985,132,243.
Ludwig et al.
3200 Langmuir, Vol. 10, No. 9, 1994 36
42
1
D-Fructose
2, D-Glucose
35 34
33 32 ..
31 38 I j / 0 1 10 50 Monosaccharide/ mM 7 D-Glucose. pH 11.0.001m% 6 0 D-Fructose, pH 11,O.OOl m% 6
A D-Glucose, pH 10.1.0.0005 m% 7
Figure 9. Influence of the concentration of monosaccharides onn, of l b in the presence of6or 7in the subphase.The triangle shows the slope in the absence of polymers.
..
0.0001 0.001 0.01 0.1 Concentration of Polymer/ m% D-Glucose +6 c No Sugar + 7 D-Fructose +6 7 D-Glucose +8 o D-Glucose + 7 n D-Fructose +8 D-Fructose +7 7 No Sugar +8 3
Figure 10. Collapse pressure JC,of l b in the presence of monosaccharidesas a function ofthe concentrationof6(pH 11, 11.1 mM saccharide),7 (pH 10,27.8mM saccharide),or 8 (pH 10, 11.1 mM saccharide) at 293 K.
which was observed in monolayers3 is evident under MS conditions as well. The tetrahydrated monoanionic l b has a mlz value of 933 (observed: 9311,while a peak a t 701 (observed: 698)may originate from (1341 HC03-)/ 2. Though the presence of polymer reduced the signal intensities, only complexes with a stoichiometry of 1:l (1b:glucose) are observed. The polymer also promotes the LB deposition, the calculated transfer ratio being 1.0 for all layers. The collapse pressure nc of monolayers of l b in the presence of 0.001% per weight of 6 a t pH 11 is plotted uersus the concentration of monosaccharides in the subphase on a logarithmic scale in Figure 9. The n,value increases linearly a t higher saccharide concentration, with a slope lower than in the absence of PVI. The detection limits extrapolated for D-glucose and D-fructose are 0.3 and 0.5 mM, respectively. The concentration of 6 plays a n important role in changing the stoichiometry of the complexes, as seen in Figure 10. For the system containing D-glucose, n, increases steadily, while a maximum is reached upon the addition of increasing amounts of D-fructose. From the different influence of the two saccharides on the n-A isotherms it is concluded that the equilibria, which lead to the formation of 1:l and 2:l complexes, are shifted to different extents by the presence of a polycation.
+
8
9
10
11
12
pH of Subphase
Figure 11. Collapse pressure n,of l b in the presence of0.001% per weight of 9 and 27.8mM monosaccharide.
Upon the addition of 5 x per weight of 7 at pH 10.1,the selectivity order of l b is also reversed. The selectivity, expressed as a change in collapse pressure n,, is D-glucose > > D-fructose (Figure 8). The selectivity toward D-glucoseis more pronounced compared with that of compound 6 as auxiliary. The collapse pressure increases a t higher sugar concentration as can be seen in Figure 9,where the data for D-glucose are included as an example. The plot is linear as in the case of 6,but the slope is steeper. The influence of 7 on l b depends on the degree of its protonation and therefore on the pH. The influence on the recognition of monosaccharides declines a t higher pH and disappears a t pH > 11. This is different from the case for 6,where the interaction with l b requires only a minimum pH for sp3-hybridization. In order to verify the conclusions, polyanion 8 was added to 27.8 mM solutions of monosaccharides a t pH 10. As seen in Figure 10, the polyanion does not change the Selectivity. This is attributed to the absence ofinteractions with l b . Alternatively, 0.001% per weight of the rigid polycation 9 was added to the 27.8mM aqueous solutions of D-fructose and D-glucose. At various pH values, the monolayer retains its selectivity toward D-fructose as shown in Figure 11. The reason for this is the high rigidity of the polymer. Scanning Electron Microscopy Measurements. SEM pictures of LB films (20 layers) prepared under the same conditions as for MS were taken. From a subphase containing 11 mM D-glucose at pH 11, a homogeneous multilayer on the Au-covered Cu-plate was observed with a smooth surface over several hundred micrometers. A view into a crater in the surface showed a well-ordered multilayer structure. After 0.001% per weight 6 was added to the subphase, the surface of the LB multilayer was smooth and showed a wave structure.
Conclusions The recognition of monosaccharides by monolayers of phenylboronic acids was studied in detail. Phenylboronic acids recognize monosaccharides selectively and with a high sensitivity. The selectivity is determined by the stoichiometry of the complexes a t the air-water interface and the configuration of the saccharide molecules. The stoichiometry and the selectivity can be influenced by the addition ofpolycations to the aqueous subphase. LB films of the complexes were characterized, and these studies aim to a new saccharide sensing system. Acknowledgment. The authors appreciate the help of Dr. M. Murata in taking SEM pictures as well as technical assistance by Ms. Keiko Ueda and by Ms. Ritsuko Iguchi in performing SIMS measurements.
