J. Phys. Chem. 1983,87,23-25
23
Inclusion Complexes between Amphiphllic Molecules and Carboxymethylamyiose in Aqueous Solution: Ketone Type I 1 Photoelimination Behavior as a Probe of the "Guest" Microenvironment' Y. Hul,' J. R. Winkle, and D. 0. Whitten' Department of Chemistry, University of North Carolina, Chapel Mi, North Carolina 27514 (Received: October 7, 1982)
This paper reports a study of complex formation between two amphiphilic ketones and carboxymethylamylose and the photoreactivity of the ketones in their respective inclusion complexes. The binding constants are found to increase as the chain length of the linear ketone (n in PhCO(CH,),COOH equals 3 and 10) increases. The CMA-included ketone is found to exhibit relatively low quantum efficiencies for the type I1 photoelimination process, the only reaction observed in each case. The results suggest that the ketone environment in the amylose cavities is relatively hydrophobic and conformationally restrictive; the photochemical reactivity observed is similar to that occurring for related ketones incorporated into bilayer vesicles below the phase transition temperature.
Aliphatic or aryl-alkyl ketones possessing y-hydrogen atoms and lowest n,r* singlet or triplet states typically undergo Norrish type I1 photoelimination as their chief photoreaction with moderate to high quantum efficienc i e ~ . ~ This - ~ reaction, which has been well studied in a variety of different media, is strongly dependent on solvent polarity; it is now fairly well accepted for aryl-alkyl ketones that, in all but the most rigid media, hydrogen atom abstraction occurs with nearly unit quantum efficiency for most of these ketones! Differences in the overall reaction efficiency in most media are attributed to solvent interactions with the intermediate biradical which affect strongly the partitioning between the major paths of cleavage and reverse hydrogen atom abstraction to regenerate ground-state starting material. The high sensitivity of this reaction to medium polarity and microviscosity has made it a useful probe of the microenvironment in microheterogeneous media such as aqueous surfactant solutions; for example, several ketones have been shown to give very high quantum efficiencies for the type I1 photoelimination in aqueous micellar media, indicating that the ketone chromophore resides in a highly polar site.*" In contrast studies with the same, or similar, ketones in aqueous solutions of surfactants forming bilayers give much lower quantum efficiencies, comparable to those obtained for the same ketones in hydrocarbons such as hexane or benzene, suggesting a much more hydrophobic site for the ketones in these media." (1) "PhotochemicalReactivity in Organized Aeaemblies",34;paper 33. Hui, Y.; Russell, J. C.; Whitten, D. G., submitted for publication. (2)Visiting scientist from Shanghai Institute of Organic Chemistry, Academia Sinica, People's Republic of China. (3)Wagner, P. J.; Kelso, P. A.; Kemppainen, A. E.; Zepp, R. G. J.Am. Chem. SOC.1972,94,7500. Wagner, P. J.; Kelso, P. A,; Kemppainen, A. E.; McGrath, J. M.; Schott, N. H.; Zepp, R. G. ibid. 1972,94,7506. (4)For a review, see Wagner, P. J. Acc. Chem. Res. 1971,4,168,and references therein. (5) Lewis, F. D.; Johnson, R. W.; Kory, D. R. J.Am. Chem. Soc. 1974, 96,6100. (6)Wagner, P. J. Pure Appl. Chem. 1977,49,259. (7)Small, R. D.; Scaiano, J. C. J.Phys. Chem. 1977,81,2126. (8)Small, R. D.;Scaiano, J. C. J.Phys. Chem. 1977,81,828. (9)Turro, N. J.; Liu, K. C.; Chow, M. F. Photochem. Photobiol. 1977, 26,413. (10)Worsham, P.R.; Eaker, D. W.; Whitten, D. G. J. Am. Chem. SOC. 1978,100,7091. (11)Winkle, J. R.; Worsham, P. R.; Whitten, D. G., submitted for publication.
0022-365418312087-0023$0 1.50lO
It has recently been shown that amylose and carboxymethylamylose can form inclusion compounds with a variety of organic molecules ranging from fairly large aromatic molecules such as 2-p-toluidinylnaphthalene-6sulfonate to various functionalized molecules bearing linear hydrophobic chains.12-" For the latter it has generally been found that binding constants increase with chain length of the hydrocarbon chain in such a way as to suggest important hydrophobic interactions as major driving forces for complex formation. It has been suggested that the inclusion process is accompanied by conformational changes of the amylose polymer chain from random coils to interrupted helices.14J6 An interesting feature of the helical cavities is that the size can be adjusted according to the substrate. Thus a 6-unit/turn helix with an internal cross-sectional area of 16 A2is indicated as the most stable structure and the host for aliphatic hydrocarbons and benzene derivatives.18 A "7-helix'' having 38 A2 as the cross-sectional area can accommodate naphthalene derivatives. While at least part of the driving force for inclusion complex formation is hydrophobic, it is clear that the inner cavity contains exposed glucoside oxygen and hydrogen which would be expected to be of considerably higher polarity than hydrocarbon. In the present paper we report a study of complex formation between two amphiphilic ketones, 1 and 2, and
1
2
carboxymethylamylose in aqueous solution and a study of their photoreactivity within these inclusion complexes. Results of this study indicate that the driving force for complex formation between these substrates and the amylose is largely hydrophobic and that the included guest in the helical cavities shows relatively low quantum yields (12)Nishimura, N.; Janado, M. J.Biochem. 1975,97,421. (13)Nakatani, H.; Shibata, K.-I.;Kondo, H.; Hiromi, K. Biopolymers 1977.16.3263. (14)Hui, Y.;Gu, J. Acta Chim. Sin. 1981,39,309. (15)Hui, Y.;Gu, J.; Jiang, X. Acta Chim. Sin. 1981,39,376. (16)Hui,Y.;Chen,X.;Gu, J.;Jiang,X.ISci.ISin.)(Engl.IEd.),(Ser~ 19g2.125 698 I (17)Hui, Y.;Wang, S.; Jiang, X. J. A m . Chem. SOC.1982,104,347. (18)Sundararajan, P. R.; Rao, V. S. R. Biopolymers 1969,8, 313. ~~
..--I
0 1983 American Chemical Society
24
The Journal of Physical Chemistry, Vol. 87, No. 1, 1983
Letters
TABLE I: Dissociation Constants of the Inclusion Complexes Formed between 1 and 2 and CMA in Aqueous NaCl Solution, 22 OC compd
Kd* .5.07
1
2
K,
M)
M)= 1.69 0.094
0.150
a Kd = Kd*/n, the dissociation constant of inclusion complexes based o n the macromolecules, where n is the number of binding sites per macromolecule.
TABLE 11: Quantum Yields of T y p e I1 Photoreaction Obtained as a Function of Concentration of Amylose and LY ~
amylose
(10-‘M) a
4
i:
6 I
L-ilOSe
’7
0 0.181 0.272
01
@ll
CY
0 IT
0
1.00
0.09
0.99 0.99 0.96 0.90
0 0.49 0.56 0.60 0.63 0.73
1.00 0.62 0.44 0.51 0.45 0.43 0.36
0.363 0.454 0.909
0.12 0.15 0.18 0.28
1.81
0.39
K
-
0.81 0.75
+ CMA -’l I4’ Kd*
0.80
-
K-CMA
for the type I1 photoelimination. The reactivity observed is very similar to that in bilayer vesicles below the phase transition temperature, indicating that the cavity environment is both relatively hydrophobic and sufficiently conformationally restrictive to reduce reactivity. Amylose (Aldrich Chemical Co.) was treated with chloroacetic acid and sodium hydroxide to yield soluble carboxymethylamylose (CMA, degree of polymerization, DP = 680) as previously des~ribed.’~The degree of substitution (number of COO- per glucose residue) was determined to be 0.12.20 The ketone substrate-guest compounds 1 and 2 were synthesized by Friedel-Crafts acylation of benzene with the appropriate half-ester-half-acid chloride as described in detail e1sewhere;’l both of the pure keto acids were characterized for several methods. IrraM) in the presence of diation of the ketones (1 X varying CMA concentrations in triply distilled aqueous solution of sodium chloride (2%) were carried out in argon-deaerated solutions with (filtered) light (254 nm); the type I1 photoelimination efficiencies were measured by monitoring the acetophenone produced by gas chromatography. Both ketones 1 and 2 are sufficiently soluble in water as their sodium salts to allow preparation of M solutions. For each of the ketones it was found that addition of CMA to aqueous solutions of the ketone resulted in a decrease in the extinction coefficient for the T,T* transition at 247 nm. The change in absorbance, which was more pronounced for 2 than for 1, suggests that interaction between the ketone and CMA is occurring; if we assume this interaction is formation of a discrete complex as indicated by eq 1,we can use the change in e to evaluate (19) Sloan, J. W.; Mehltretten, C. L.; Senti, F. R. J. Chem. Eng. Data 1962, 7, 156. (20) The degree of polymerization and the degree of substitution of
CMA were determined by viscosimetry and conductance titration, respectively; see Acta Chim. Sin. 1978, 36,61.
(1) -
114 v
Figure 1. Double reciprocal plots for evaluating dissociation constant: (0) 1; ( 0 )2.
-~
2
1
K,* and the extent of complex formation where CMA represents a single binding site on the CMA macromolecule such that the total “concentration” of binding sites is n[CMA], where n is the number of binding sites per CMA molecule. Starting with an initial concentration K of ketone and A of CMA, eq 2 can be developed, where At is nAKAt Aabs = n A K Kd*
+ +
the change in e between free and complexed ketone and Aabs is the absorbance change in absorbance a t an initial [CMA] = A. The reciprocal of eq 2 gives eq 3 which -= K- ( 1 +1 Aabs At
K + nAKd*
)
(3)
predicts a linear relationship between K/Aabs and 1/A.21 Such a relationship (Figure 1) is obtained for both ketones and to a good approximation both 1 and 2 give the same intercept. From the slope/intercept values obtained from the plots and the estimated values of n (from the dimensions of 1 and 2), the values of Kd and Kd* listed in Table I are obtained. The relative values seem reasonable in view of the anticipated hydrophobic components to the inclusion complex formation; the fact that both ketones give the same intercept suggests that the complexed carbonyl experiences a similar environment in the CMA inclusion complexes for both ketones. The quantum yields for type I1 photoelimination were measured for both ketones as a function of [CMA]. In each case the quantum yield for the type I1 elimination was found to be unity in pure water and to decrease upon addition of CMA. Table I1 lists quantum yields obtained for the two ketones as a function of A and a , the fraction of each ketone complexed as calculated from the measured Kd values. A plot of 4Dvs. a (Figure 2) (using data for both ketones) gives a reasonable straight line with an intercept (CU = 1) of 411 = 0.14. These results indicate that incorporation of the ketone into the inclusion complex produces a dramatic reduction in the efficiency of the type I1 photoelimination. This reduction could be due to one or more of a variety of
The Journal of Physical Chemistry, Vol. 87,No. 1, 1983 25
Letters
in upon forming the complexes can be attributed to reduced mobility of either the starting ketone or the biradical intermediates in the amylose complex. No other products than the type I1 products have been detected and it appears that no net bleaching of the carbonyl chromophore occurs.22 Studies with trans-stilbene derivatives have shown that fluorescence increases at the expense of trans-cis photoisomerization upon forming similar complexes with amylose in dimethyl sulfoxidewater solution; these studies clearly indicate a n increase in “microviscosity” in the inclusion complexes.23 Since, for the ketones, no additional photoreactions are observed upon forming the inclusion complexes, it may well be that the chief effect of the increased “microviscosity” is not to eliminate or even greatly reduce intramolecular hydrogen atom abstraction but, rather to hold the diradical formed in this process in a geometry such that reverse hydrogen atom abstraction is favored. It is interesting in this regard to note the “net” photoreactivity of ketones 1 and 2 in the inclusion complexes is lower but fairly close to that observed for other surfactant ketones incorporated as intrinsic probes in bilayer vesicles below the phase transition temperature.ll Thus, even though there is clear evidence that the amylose and CMA inclusion complexes with moderately hydrophobic molecules are formed reversibly in a dynamic process, the results show that the complexes are sufficiently tight or ordered to markedly modify reactivity, especially in cases where a short-lived intermediate must undergo substantial conformational changes. We are currently extending our studies of photoreactivity in inclusion complexes to other reactions including bimolecular processes. Acknowledgment. We are grateful to the U.S. Department of Energy (Contract DE-A-505-81ER10815.A000) for support of this research. Registry No. 2, 18017-71-5; acetophenone, 98-86-2. @
I
.c
41 I 0.
0.
0.
0.
~
0.2
0.4
0.6
0.8 1
1.0
(“1
Flgure 2. Linear relationship between 411 and a: (0) 1; (0)2. Correlation coefficient y = 0.985, intercept = 1.08, slope = -0.916. Quantum yield of complexed c h r q e = intercept slope = 0.14.
+
effects. First it is useful to point out that the extrapolated value is substantially lower than those obtained for structurally similar ketones in aromatic or aliphatic hydrocarbons ($qI 0.254.40).In addition the absorption spectra of the ketones in the complex do not show the blue shift one would anticipate for a highly hydrophobic environment.’l Thus it would appear that the effects observed for the CMA complexes cannot be attributed solely to a hydrophobic environment in the complex. This seems even more clearly indicated in view of the expectation that the cavity interior contains exposed oxygen and 0-H groups. It appears likely that at least some of the reduction
-
(21) This is an extended form for a 1:n complex from the equation used for 1:l complex formation based on spectrometric data, for details see: Van Etten, R. L.; Clowes, G. A.; Sebastian, F.; Bender, M. L. J. Am. Chem. SOC.1967,89, 3262.
J
~
~
(22) There is an increase in absorbance at 247 nm as the irradiation proceeds. This can be attributed to release of the product acetophenone from the cavities into the aqueous solution as it is formed.
