fluorescence characterization of the cyclodextrin/pyrene complex interaction with chiral alcohols...

Fluorescence Characterization of the Cyclodextrin/Pyrene Complex Interaction with Chiral Alcohols and Diols

J O D I M. S C H U E ' I T E , * A. Y V E ' I T E W I L L , t R E Z I K A. AGBARIA, t and I S I A H M. W A R N E R t 1 : Department of Chemistry, Emory University, Atlanta, Georgia 30322

The ehiral properties of cyclodextrins (CDs) facilitate the formation of diastereomeric complexes with a number of pesticides and pharmaceuticals which are also frequently composed of one or more chiral centers. The roles of chirality and structural volume in CD binding to a homol- ogous series of linear, chiral alcohols and diols are evaluated by com- paring the trend in the pyrene fluorescence I / l l l band ratio and the hydrophobici~ for the CD/pyrene complex with CD/pyrene complexes incorporating achiral alcohols. Stronger hydrophobicity is observed for complexes capped by chiral alcohols relative to complexes formed with a similar achiral counterpart, suggesting the importance of the alcohol chiral center. Furthermore, the diols induce a more hydrophobic environment than their alcohol counterparts with the ~0-CD/pyrene complex, while the converse is the case for the 7-CD systems. The systems in- volving 7-CD were also compared by use of pyrene fluorescence lifetime measurements.

Index Headings: Pyrene; Cyclodextrins; Chiral alcohols and diols; Fluo- rescence.

INTRODUCTION

Cyclodextrins (CDs) are cyclic oligosaccharides formed by the connection of individual glucopyranose units through a(1-4) glycosidic oxygen bridges. These glycosidic oxygens impart a degree of hydrophobicity to the interior of the cavity, while the primary and secondary hydroxyl groups lining the smaller and larger edges of the cavity, respectively, serve to increase the hydrophilicity of the CD molecule in aqueous environments. Cyclo- dextrins have been studied extensively for their ability to selectively incorporate various hydrocarbon molecules on the basis of dimensional as well as hydrophobic consid- erations. 1

An interest in methods which will rapidly and efficient- ly detect and remove hazardous chemicals from polluted sources has arisen with the increasing awareness of and concern for environmental preservation and safety. The ability of CDs to enhance some of the photophysical prop- eriies ofpolynuclear aromatic hydrocarbons (PAHs) serves as a potential tool for the detection and consolidation of environmental contaminants. In particular, the enhance- ment in photoluminescence properties ofpyrene has been studied as an effective model of the type of polynuclear aromatic constituents which often comprise environmental contaminants. 2 In addition, previous studies in this research group have demonstrated the ability of small concentrations of alcohols or amines to dramatically increase the hydrophobicity of the complexes formed be-

Received 8 October 1993; accepted 3 February 1994. * Present Address: Isis Pharmaceuticals, Carlsbad, CA 92008. t Present Address: Department of Chemistry, Louisiana State Univer-

sity, Baton Rouge, LA 70803. Author to whom correspondence should be sent.

tween pyrene and CDs, specifically,/3- and ~-CD. 3-5 The studies engaging alcohols are used as a reference in the present investigation.

Chirality plays an intrinsic role in natural recognition mechanisms of the body and the environment. 6 It is generally accepted that discrimination between individual isomers requires a three-dimensional contact with the chiral selector. 7-9 However, this simplistic definition is rather limited in its applicability to different stereospecific environments. For this reason, the chiral recognition properties ofcyclodextrins have been investigated in this work with the intention of further elucidating the stereospecific mechanisms of association involved during complexation of CDs with environmentally important com- pounds. In this report, the importance of modifier chirality and hydrophilicity to the interaction of the modifier with the guest molecule and CD cavity is specifically ad- dressed. For this purpose, a series of chiral alcohols and diols are examined.

EXPERIMENTAL

Apparatus. Fluorescence emission spectra (uncorrect- ed) were acquired on a Spex Model F2T211 Fluorolog-2 spectrofluorometer equipped with a thermostated cell which was maintained at 20.0 _+ 0.1°C. Samples were measured in a 1-cm quartz cell with the use of excitation and emission bandwidths of 5.16 nm and 1.72, respectively. Emission spectra were collected with an excitation wavelength of 335 nm. A Photon Technology Interna- tional (PTI) LS-100 spectrophotomer was employed to obtain fluorescence lifetime data which were analyzed by use of an iterative curve-fitting procedure based on the Marquardt algorithm.I° The lamp was filled with nitrogen and operated at a pressure of 16.5 mm Hg. Excitation and emission wavelengths were set at 335 and 390 nm, respectively. The profiles of aerated solutions were taken at ambient room temperature.

Materials. Pyrene (99 + %) was purchased from Aldrich (Milwaukee, WI) and used as received. The (R)-(-)- and (S)-(+)-l,3-butanediol (98%), anhydrous (_+)-l,3-butanediol (99+%), (R)-(-)- and (S)-(+)-2-butanol (99%), (R)- ( - ) - and (S) - (+) -2-pentanol (99%), (_+)-2-butanol (99 + %), and (_+)- 2 -pentanol (99 + %) were also supplied by Aldrich. The/3- and ,y-CD were gifts ofG. A. Reed of American Maize Products (Hammond, IN).

Method. Preparation of Fluorescence Samples. A 1 x 10 -7 M stock solution of pyrene was prepared for fluorescence measurements by pipetting 50 ~L o f a 5 × 10 -4 M cyclohexane solution containing the PAH into a 250- mL flask. The cyclohexane was evaporated under a dry nitrogen stream, and the residue was then dissolved in

Volume 48, Number 5, 1994 0003-7028/94/4805-058152.00/0 © 1994 Society for Applied Spectroscopy

APPLIED SPECTROSCOPY 581

9.5E+005 r ~

III

0

O.OE+O00 360

I I I - - a

~ i V V ...... b

~ w - - ' - - C

378 396 414 432 450

W a v e l e n g t h (nm)

FiG. 1. Fluorescence emission spectra of pyrene in (a) 7-CD/0.05 M (S)-(+)-2-butanol; (b) 7-CD/0.05 M (S)-(+)-l,3-butanediol; (c) 7-CD only; (d) 0.05 M (S)-(+)-2-butanol only; (e) pure H20; and (f) 0.05 M (S)-(+)- 1,3-butanediol only.

deionized water (Continental System, Atlanta, GA). The aqueous stock was allowed to equilibrate overnight before being transferred to vials containing a total of three mil- liliters of solution. Appropriate amounts of individual modifiers were added to this volume in order to maintain a concentration of 0.05 M for the pure isomers. Thus, a 0.10 M solution was required for the racemic modifiers. In some cases, 0.10 M of the pure isomer was added in an effort to assess the importance of general modifier concentration.

RESULTS AND DISCUSSION

It is generally proposed that alcohols hydrogen-bond with the primary hydroxyl groups which line the smaller edge of the CD structure. By so doing, the alcohol increases the hydrophobicity of the microenvironment ex- perienced by the pyrene probe molecule. Since it is the hydroxyl groups lining the periphery of the CD cavity which are responsible for chiral recognition, it was expected that the chirality of the modifiers would provide a basis for examining interactions with the interior of the cavity, lit was also expected that optimization of these interactions would depend largely upon the geometrical compatibility of the alcohol relative to the void volume remaining after complexation with the guest, as well as the preference of the individual modifier isomers for a particular orientation.

The fluorescence emission spectrum of pyrene is characterized by five bands in the region between 360 and 400 nm (Fig. 1). Traditionally, the I/III or 372/382-nm band ratio is used to monitor the microenvironment of pyrene. The 382-nm band is particularly sensitive to changes in media polarity and tends to increase with decreases in the microenvironmental polarity. Consequent- ly, the I/III ratio decreases with increasing hydrophobicity. Addition of 0.05 M (S)-(+)-2-butanol to the ~-CD/ pyrene complex dramatically enhances the intensity of the third band and affords further spectral definition of the IV and V bands (spectrum a). Interestingly, although (S)-(+)-l,3-butanediol induces similar changes (spectrum b), the relative intensity and resolution are some- what reduced as compared to the effect of (S)-(+)-2-bu-

582 Volume 48, Number 5, 1994

TABLE I. Peak ratios for 0.10 ~M pyrene fluorescence measured in the presence of 10.0 mM cyclodextrin and various chiral alcohols.

Modifier present

Cyclo- (S)-(+)-2- (S)-(+)- 1,3- dextrin None a butanol butanediol

None 1.58 ± 0.06 1.52 ± 0.02 1.51 ± 0.14 ~-CD 0.89 ± 0.02 0.62 ± 0.01 0.73 ± 0.03 y-CD 0.62 ± 0.02 0.56 ± 0.03 0.56 ± 0.03

No alcohol present.

tanol. This observation is likely to be a consequence of the more hydrophilic nature and/or bulkiness of the diol, which produces a significant change in microenvironmental polarity relative to the exterior water molecules. In general, the polarity effects of~-CD are more extensive due to increased hydrophobicity and reducec! guest mobility within the limited ~-CD cavity.

It is important to note that increases in intensity may not necessarily reflect a more nonpolar or even protective environment. For example, band I in spectrum c of Fig. 1 represents pyrene in the presence of 3 mM 7-CD. The overall intensity of this band is lower with respect to pyrene alone, even though it is generally accepted that the CD molecule protects the pyrene molecule. On the other hand, the addition of (S)-(+)-2-butanol produces an increase in band I relative to pyrene in water. Thus, the I/III ratio is expected to provide a more suitable gauge of fluctuations in the microenvironmental polarity sur- rounding pyrene.

Table I summarizes the influence of the chiral modifiers on the I/III peak ratios for pyrene in the absence and presence of 10 mM CD. The decrease in the I/III ratio with the addition of/~- or 7-CD suggests the formation of an inclusional complex since it is the interior of the CD which provides the nonpolar and more protective region. Interestingly, the chiral diol itself affects the I/III ratio ofpyrene. This pattern is in contrast to the behavior of the chiral 2-butanols and to the results of Mufioz de la Pefia et al., 3 where the influence of tert-butyl alcohol was reported to be negligible. It is possible that a distinction arises from the added dimension of the modifier in the present study. It appears that the diol is better able to interact with the pyrene molecule in comparison to the butyl alcohol counterpart. The additional hydroxyl group of the diol may enhance the disruption of the sphere of hydration enclosing the pyrene molecule in a manner similar to a surfactant monomer. Hydrogen bonding be- gins to provide a larger contribution to the binding mechanism for the diol. As a consequence of an increased number of hydroxyl binding sites, a stronger association of the diol with bulk solvent molecules occurs, providing more effective protection or shielding of the pyrene fluo- rophore from nonradiative interactions with water molecules and various dissolved quenchers. The magnitude of interaction of 7-CD with pyrene in the presence of the various chiral alcohols, as indicated by the I/III ratio values in Table I, is not evident at higher CD concentrations. This result may be attributed, in part, to the 1:1 7-CD : pyrene complex/° which would permit consider- able interaction of pyrene with the bulk solvent phase. The discrepancy also appears to be a function of the CD purity. It should be noted that the usual CD purification

4o E

1 .80

1 .38

0 . 9 7 "

0 . 5 8

A

A ~ A A

0 . 0 0 0 0 . 0 0 3 0 . 0 0 7 0 . 0 1 0

[13-CD] in Molar

• No Modif ie r

0 (R)-( - ) -BuOH

• (±)-BuOH

A (S)- (+)-BuOH

o I n.

1 .80 .

1 .38

0 . 9 7 •

0 . 5 5

B

• No Modif ie r

O (±)-Diol

A (S)-(+)-Diol

• (R)-(-)-Diol

0 . 0 0 0 0 . 0 0 3 0 . 0 0 7 0 . 0 1 0

[p-CD] In Molar

FiG. 2. P l o t o f t h e I / I I I b a n d r a t i o fo r ~ - C D / p y r e n e in t h e p r e s e n c e o f v a r i o u s (A) 2 - b u t a n o l s ( B u O H ) a n d (B) 1 , 3 - b u t a n e d i o l s (Diol) .

procedures used in this laboratory were not possible for "r-CD due to its greater water solubility.

Particularly noteworthy are the significantly lower I/III ratios measured for CD/pyrene in the presence of the chiral modifiers. A comparison between the values reported by Mufioz de la Pefia et al. 3 and Zung et al. 4 and those of the present study suggests that the chirality of the modifier is an important factor in the extent of modifier interaction with the cavity. For example, the I/III ratios for/3-CD-pyrene in the presence of(S)-(+)-2-butanol and (S)-(+)-l,3-butanediol are 0.62 + 0.01 and 0.73 _+ 0.03, respectively. For the "r-CD/pyrene complex, both values are 0.56 + 0.03. The addition of 1-butanol to /3- and 7-CD complexes of pyrene produces I/III ratios of 0.563 and 0.65, 4 respectively, although it should be noted that these reference values were calculated from data collected at different slit width settings.

It has been established that spatial arrangement of the modifier functional groups governs, to a certain extent, the optimum organization of the modifier with the remaining void volume of the cavity. Zung et al. 4 demonstrated that 2-propanol forms a stronger complex with 7-CD/pyrene than does 1-propanol, even though the vol-

3 umes of the alcohols are virtually identical (72/~ ). This

finding may be further extended to incorporate the role that stereochemistry plays--in terms of hydroxyl group arrangement--in altering the interaction of the modifier with the CD cavity and pyrene guest molecule.

Although the study of I/III ratios at individual CD concentrations is informative, it is actually more appropriate to follow the trend in I/III ratios. The influence of modifier chirality and hydrophilicity on the microenvironmental polarity of the CD/pyrene inclusional complexes could be discerned. Figure 2A demonstrates a dramatic decrease and eventual leveling effect of the I/III ratio for/3-CD/pyrene in the presence of 0.05 M (S)-(+)- and (R)-(-)-2-butanol. Here, an isomeric concentration of 0.05 M was maintained so that a valid comparison could be made. Interestingly, 0.05 M of the enantiomeric alcohols is enough to induce an equivalent degree of change in the I/III ratio to that produced by 0.10 M of the racemic alcohol. However, there appears to be no significant distinction between the individual modifier effects. This observation may be attributed to the 2:1/3-CD : pyrene bar- rel-like complex, ll which effectively leaves only the smaller ends of the cavity available for further complexation with ternary components. The limiting factor is the minimal residual volume of the CD cavity rather than the modifier concentration. Another consideration is that deep insertion of the alkyl chain will be hindered as well, preventing the recognition of the modifier hydroxyl group by the CD hydroxyls--a mechanism which is necessary for distin- guishing between different chiral modifiers.

The initial, steep slope of the I/III ratio suggests that pyrene experiences a more nonpolar and protective microenvironment. The increased hydrophilicity and bulkiness of an additional hydroxyl group for the modifier increases the hydrophobicity around the pyrene (Fig. 2B) with/3-CD. Despite this consideration, the CD is able to distinguish between the individual diol isomers. The (S)- (+)-1,3-butanediol produces the most extensive decrease in I/III ratio, followed by the (R)-(-)-isomer. Yet, the initial declination of the (S)-(+)-isomer is clearly more gradual than the others. It was originally hypothesized that the effect of the racemic mixture should be intermediate between the effects of the individual enantio- mers. This suggestion can be verified by experiment for the data in Figs. 2A and 2B.

A comparison of the effects of the alcohols vs. diols in the/3-CD system suggests that the increased hydrophilicity of the diol is the regulating mechanism. Due to the restricted dimensions of the smaller edge of the cavity, it was expected that the interaction of the diol with the interior of the cavity would be very minimal. However, this proposal necessarily consigns the diol to the vicinity of the CD hydroxyl groups. Furthermore, the bulk aqueous phase becomes the most compatible location for the diol as a consequence of the additional hydroxyl group on the diol. The conditions for chiral recognition are, therefore, optimized.

The larger and less-confining ~-CD cavity permits the optimization of modifier orientation absent with the/3-CD cavity. This point is observed for both the alcohol and the diol in the 7-CD : pyrene complex (Fig. 3A and 3B). As a consequence of the greater void volume remaining in the 7-CD cavity after complexation with pyrene, as well as the generally increased hydrophilicity of the 7-CD


i

1 . 7 0

1.~16

1 . 0 2

0 . 8 8

0 . 8 4

~~ A

0 . 0 0 ' ' ' ' 0 . 0 0 0 0 . 0 0 2 0 . 0 0 5 0 . 0 0 7 0 . 0 1 0 0 . 0 1 2

[ y - C D ] in Molar

No Modificr

• (±)-Diol

O (R)-(-)-Diol

• (S)-(+)-Diol

E

1 . 7 0

1 . 3 6

1 . 0 2

0 . 6 8

0 . 3 4

B

- - C ~ O 0

Z~ No Alcohol

• (R)-(-)-BuOH

o (S)-(+)-BuOI-I

• (±)-BuOH

0 . 0 0 ' ' ' ' 0 . 0 0 0 0 . 0 0 2 0 . 0 0 5 0 . 0 0 7 0 . 0 1 0 0 . 0 1 2

[ y - C D ] In Molar

FIG. 3. Plot of I/III band ratio for ,~-CD/pyrene in the presence of various (A) 1,3-butanediols (Diol) and (B) 2-butanols (BuOH).

interior, the interaction of the smaller, more nonpolar alcohol is weak. This observation is reflected in the gradual decrease in the I/III band ratio curves in the presence of the 2-butanols. Nevertheless, in spite of the larger dimensions of the "r-CD cavity, the differences in alcohol orientation due to chirality are more strongly emphasized. This result is probably due to the increased modifier and guest mobility. In this case, there is a distinct preference of the cavity for the (S)-(+)-isomer. However, the effect of the racemic mixture is not intermediate.

The bulkiness of the diols provides for efficient contact optimization with ~-CD (Fig. 3A). It is evident from the trends in the I/III ratio for 7-CD/pyrene that the R- ( - ) - isomer increases the hydrophobicity most strongly with this system. The dramatic initial decrease and eventual leveling of the I/III ratio supports this conclusion. There appears to be a less pronounced distinction between the influences of individual diols, however, and this result may be attributed to the tendency of the diol to be more fully inserted within the ~-CD cavity at some distance from the CD hydroxyl groups. Furthermore, the bulkiness of the diol contributes to its reduced mobility. From Fig. 3A is appears that the racemic diol renders the micro-

g E

1 . 7 0

1 .81

0 . 9 1

0 . 5 2 0 . 0 0 0

A

0 . 0 0 4 0 . 0 0 8 0 . 0 1 2

[ p - C D ] in Molar

• (R)-(-)-PcOH

zx (S)-(+)-PcOH

O (±)-PeOH

1 . 7 0

1 . 2 8

0.87

0 . 4 6 0 . 0 0 0

B

0 . 0 0 5 0 . 0 1 0 0 . 0 1 5

• (±)-PeOH

(S)-(+)-PcOH

0 (R)-(-)-PcOH

[ v - C D ] (M)

FzG. 4. Influence of 2-pentanols (PeOH) on the I/III band ratio of pyrene in the presence of (A) ~-CD and (B) ~-CD.

environment around pyrene more hydrophilic than is the case in the absence of alcohol. One plausible explanation for this observation is that the high alcohol concentration results in a possible competitive equilibrium among the isomers for the CD interior--limiting the pyrene to the exterior of~-CD. Another explanation may be a tendency of the individual isomers to aggregate with each other, thus minimizing chiral recognition of the isomer binding sites. The bulkiness of the aggregate would prevent effective interaction of the modifier with the limited remaining volume of the complex.

The relative importance of alkyl chain length vs. chirality was also assessed through the addi t ion of 2-pentanols. The effects of the 2-pentanol isomers and the racemic mixture are compared for the 13- and ~-CD complexes in Fig. 4. The trend displayed in plot A is reminiscent of those observed for the 2-butanols. The interaction of the 2-pentanols is definitely stronger, as evidenced by the I/III ratio--which, at its lowest point, reaches approximately 0.52 vs. 0.61 for the 2-butanols. The decline of the I/III ratio for the ~-CD system is also rather steep initially. The lowest level achieved in this system is 0.44 in the presence of(R)-(-)-2-pentanol. This level is significantly lower than that reported for l-pentanol


TABLE II. Fluorescence lifetimes of pyrene in the presence of 10 -2 M 3'-CD and various 2-butanols.

Alcohol r," (ns) r2 b (ns)

None 138 _+ 4 277 _+ 9 (+)-2-Butanol 153 + 8 358 _+ 5 (R)-(- ) -2-butanol 162 + 5 367 + 5 (S)-(+)-2-butanol 168 + 10 391 _+ 8

Lifetime for uncomplexed (free) pyrene. b Lifetime for complexed pyrene.

TABLE I lL Fluorescence lifetimes of pyrene in the presence of 10 -2 M v-CD and various 1,3-butanediols (BuDiol).

Alcohol r, a (ns) z2 b (ns)

None 138 _+ 4 277 _+ 9 (R)-(-)-BuDiol 154 +_ 7 343 _+ 18 (S)-(+)-BuDiol 109 +_ 1 337 _+ 3 (_+)-l,3-BuDiol 119 _+ 3 261 _+ 2

" Lifetime for uncomplexed (free) pyrene. b Lifetime for complexed pyrene.

(0.61) 4 in the presence of 10 mM 3,-CD, suggesting the importance of the modifier chirality or at least the location of the hydroxyl group along the chain. In general, the additional methyl group serves only to weaken the ability of the -y-CD to differentiate on the basis of chirality. The increased bulkiness and nonpolarity of the 2-pentanol directs the modifier more deeply inside the cavity where contact of 2-pentanol with the cavity will tend to be more extensive than is the case for the cor- responding 2-butanol molecule. Thus, the distinction in the trends induced by the 2-pentanol on the basis of cavity dimensions is not as apparent. A gradual increase in the I/III ratio was observed for the (R)-(-)-2-pentanol curve beyond 0.003 M 3,-CD. However, a common trend is clearly depicted for the 2-pentanols in general.

Since chirality of the modifier was determined to be a contributing factor to the hydrophobicity of the CD/pyrene complexes, it was suspected that optimum modifier concentration may differ between individual isomers and the racemic mixture. The concentration of modifier was varied from 0.0 to 0.40 M. For the fl-CD/pyrene system, the (R)-(-)-2-butanol induces the strongest decrease in the I/III ratio. Even so, the difference does not appear to be very significant. At 0.36 M alcohol, a slight increase in the I/III ratio is observed for the (R)-(-)-isomer and racemic form, suggesting an extraction of the pyrene from the cavity into the increasingly nonpolar bulk phase. This pattern of leveling and gradual increase is not observed for the 3,-CD complex. The larger cavity of 3,-CD requires greater concentrations of alcohol to achieve the low ratios characteristic of the/3-CD system. Although the effects of the diols on hydrophobicity with t3-CD are generally weaker, clear distinction between the diol isomers can be evaluated. A similar phenomenon is indicated for the diol systems containing 3,-CD. It is evident from the leveling and the increase in the I/III ratio at about 0.24 M modifier that the (S)-(+)- 1,3-butanediol provides a more nonpolar environment than the (R)-(-)-isomer or racemic diol. This observation further supports the possibility of pre- parative contaminants in the (S)-(+)-isomer.

Again, it is interesting that the gradual leveling and subsequent increase in the I/III ratio do not occur for the ~,-CD complex. A feasible explanation for this result may be attributed to the tendency of the diol to aggregate inside the -r-CD and, yet, be excluded from interaction inside the B-CD complex. As a consequence, the y-CD environment and the bulk aqueous phase in the presence of diols are actually quite similar, neither providing the stronger driving force for interaction with pyrene. Pato- nay et al. 12 attributed the reduction in hydrophobicity characterizing the pyrene microenvironment inside y-CD to the proximity of the hydroxyl groups along the hydro-

carbon backbone of 1,3-propanediol. In contrast, the distance between hydroxyl groups of 1,5-pentanediol limits their interaction with pyrene. The structural configuration of the guest molecule governs the location of the probe inside the cavity. In turn, the depth of probe insertion and stoichiometry of the complex influence the degree of modifier contact within the cavity.

Differences in the chirality and hydrophilicity of the individual modifier are more strongly emphasized in the complexation with ~r-CD than with B-CD, as is apparent from the trends in the observed I/III ratios. Moreover, the role of the racemic mixture is not the expected intermediate one of the individual isomers for the ~,-CD complex. Hence, these systems were further characterized by use of fluorescence lifetime measurements. The estimated lifetimes in the presence of 2-butanols and 1,3- butanediols are summarized in Tables II and III, respectively. Two lifetimes are observed in both the absence and presence of alcohols--a long-lived component attributed to the ~,-CD : pyrene complex and a short-lived component for free pyrene. Our values obtained for the pure ~,-CD system are in agreement with those given in the literature.L3 The differences in the free pyrene lifetime in the presence of each modifier do not appear to be very substantial and are consistent in the presence of other alcohols? 3 An increase in the lifetime of the complex is observed in the presence of 2-butanols and the (R)-(-)- and (S)-(+)-l,3-butanediols. This observation suggests the presence of ternary complexes formed between ~,-CD, pyrene, and the alcohol. Nelson et al. ~3 reported similar influences on the ~,-CD : pyrene complex with the addition of various linear, branched, and cyclic achiral alcohols. Presumably, the role of the alcohol is to shield pyrene from the bulk aqueous phase and dissolved quenchers, resulting in enhanced lifetimes of the complex. Interestingly, the trend in the observed lifetimes corre- lates with the hydrophobicity induced by the alcohol present. In the case of the 2-butanols, the highest lifetime was estimated for the (S)-(+)-isomer, which also produced the most pronounced decrease of the overall microenvironmental polarity. This result further supports the ability of cbiral recognition of these isomers with ~,-CD. For the 1,3-butanediols, one may observe no appreciable difference in the complex lifetimes in the presence of either the (R)-(-)- or (S)-(+)-isomers. This observation is also in agreement with the similar I/III ratios for these alcohols. The (+)-l,3-butanediol, which rendered a less hydrophobic environment for pyrene than was the case in CD alone, also influenced a reduction in the recorded lifetime. It is useful to note that lower lifetimes were observed in the presence of the 1,3-butanediols than with the 2-butanols. This observation is likely to be the result


of the enhanced hydrophilicity of the diol due to the presence of the extra hydroxyl group.

CONCLUSIONS

A number of cooperative mechanisms govern the interaction of guest molecules with CDs. The introduction of competitive modifiers, which differ in spatial distri- bution, polarity, chirality, or hydrophilicity, facilitates a better understanding of the importance of these individual factors to the hydrophobicity of the complexes formed. Since molecules such as pyrene are incapable of hydrogen bonding, the driving force for interaction appears to be regulated by van der Waals interactions and geometry. Thus, alcohols and diols do not diminish, but rather enhance, the hydrophobicity of the complexes formed with polynuclear aromatic molecules.

Functionalization of the modifier may, in certain cases, hinder the effective interaction of the modifier with the CD. For example, the increased bulky character of diols incorporating an additional hydroxyl group limits deep association of the modifier with the cavity. However, this factor is compensated for by the greater hydrophilicity of the modifier, which promotes interaction with CD hydroxyl groups. The diols, being more suited to optimizing larger residual volumes, are able to effectively compart- mentalize the ,y-CD/pyrene complex. A comparison between chiral and achiral modifiers suggests that chirality also contributes, in part, to the overall polarity of the complex. It can then again be emphasized that the CD chirality is responsible for recognition of the individual isomers.

ACKNOWLEDGMENTS

Funding for this study was provided by the National Institutes of Health (GM-39844) and the National Science Foundation (CHE- 9001412). The authors would also like to express their gratitude to G. A. Reed of American Maize Products for kindly donating the CDs used in this laboratory.

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2. J. W. Bridges, in Recent Advances in Chiral Separations, D. Ste- venson and I. D. Wilson, Eds. (Plenum Press, New York, 1990), pp. 1---4.

3. A Mufioz de la Pefia, T. T. Ndou, J. B. Zung, K. L. Greene, D. H. Live, and I. M. Warner, J. Am. Chem. Soc. 113, 1572 (1991).

4. J. B. Zung, A. Mufioz de la Pefia, T. T. Ndou, and I. M. Warner, J. Phys. Chem. 95, 6701 (1991).

5. A. Y. Will, A. Mufioz de la Pefia, T. T. Ndou, and I. M. Warner, Appl. Spectrosc. 47, 277 (1993).

6. C. Dagliesh, J. Chem. Soc. 137, 3940 (1952). 7. A. Pyrde, in Chiral Liquid Chromatography, W. J. Lough, Ed.

(Chapman and Hall, New York, 1989), pp. 23-35. 8. J. Szejtli, Cyclodextrins and Their Inclusion Complexes (Akad6miai

Kiad6, Budapest, 1982). 9. N. B. Elliott, A. J. Prenni, T. T. Ndou, and I. M. Warner, J. Coll.

Inter. Sc. 156, 359 (1993). 10. P. R. Bevington, Data Reduction and Error Analysis for the Physical

Sciences (McGraw-Hill, New York, 1969). 11. A. Mufioz de la Pefia, T. T. Ndou, J. B. Zung, and I. M. Warner,

J. Phys. Chem. 95, 3331 (1991). 12. G. Patonay, K. Fowler, G. Nelson, and I. M. Warner, Anal. Chim.

Acta 207, 251 (1988). 13. G. Nelson, G. Patonay, and I. M. Warner, Anal. Chem. 60, 274

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fluorescence characterization of the cyclodextrin/pyrene complex interaction with chiral alcohols...

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