J. Phys. Chem. 1992, 96, 5309-5314
5309
Cyclodextrin-Induced Asymmetry of Achiral Nitrogen Heterocycles Jodi M. Schuette, Thilivhali T. Ndou,+ and Isiah M. Warner* Chemistry Department, Emory University, Atlanta, Georgia 30322 (Received: December 13, 1991;
In Final Form: March 24, 1992)
Recent studies investigating the nature of fluorescence quenching effects of 8-cyclodextrin (8-CDx) upon various nitrogen heterocycles suggest the importance of the position and number of nitrogen heteroatoms within the guest molecule. The sensitivity of molecules such as acridine (ACR) and phenazine (PHEN) toward the microenvironment provided by cyclodextrins appears to originate with the heteroatom. Quenching has also been observed for ACR and PHEN in the presence of CYand y-CDx, although it is less extensive than in the P-CDx system. Induced circular dichroism (ICD) measurements are able to provide a more reliable estimate of apparent formation constants for the &CDx systems since complex formation is a prerequisite for guest-induced symmetry in the cases discussed here. Fluorescence measurements were used to assess the association constants of the CY- and y-CDx with PHEN and ACR. The influence of size compatibility between guest and host and the orientation of the guest within the CDx molecule are also evaluated.
Introduction Under certain circumstances, cyclodextrins (CDx) may accelerate mechanisms of deactivation for some nitrogen-containing dyes as well as a few heterocyclic compounds.I4 Limited solubility and the acute microenvironmental sensitivity of both coumarin and acridine derivatives contribute to the low fluorescence quantum yields of these compounds in aqueous systems. Normally, such characteristics would be improved by incorporation of the dye into a CDx assembly. Cyclodextrins are oligosaccharides formed by the cyclic conformation of individual glucopyranose units.5 These individual units are connected through oxygen bridges linking the C( 1) and C(4) positions of each ring. Cyclodextrins have the ability to incorporate a guest molecule on the basis of both dimensional considerations and hydrophobic interactions. Through complete or partial inclusion of the guest within or as a result of association with the CDx, the guest molecule is often afforded some protection from external quenchers. Such external quenchers may take the form of dissolved oxygen or, in certain cases, H20molecules.2 In other instances, the CDx can actually induce quenching. Bergmark et ala2concluded from emission yields and lifetimes for coumarin laser dyes that H 2 0molecules, which are coincluded with the dye inside the CDx structure, exert a quenching phenomenon on coumarin excited states. Furthermore, the inclusion of an organic modifier as a third component resulted in the exclusion of the water molecules and a corresponding increase in fluorescence intensity for coumarin. Similar reductions in quenching have been observed for acridine (ACR) upon addition of 1% (v:v) of a selected alcohol? Numerous studies have reported the overall enhancement in fluorescence resulting from modification of the CDx:pyrene system7v8 with alcohol components. As chiral entities, cyclodextrins are able to induce chirality in otherwise symmetrical molecules as a result of complexation with these guest molecules. The circular dichroism which arises from direct hydrophobic interaction of the guest molecule with the CDx cavity is termed, in these instances, induced circular dichroism (ICD). Information derived from ICD can be used to gain a better understanding of guest molecular orientations and conformat i o n ~polarization , ~ ~ ~ directions of electronic transitions,18estimates of binding c o n ~ t a n t s , ~the J ~ -extent ~ ~ of association of the guest with various substituted CDx ligands encountered by the guest?bz Opallo et have used circular dichroism measurements to examine the effect of various cosolvents on the stoichiometry of the complex formed between 2,3-anthracenedicarboxylateand 8or r-CDx. Changes in the relative intensities of circular dichroism bands reflect the asymmetric interactions of the hostguest complex. ~~
~
Author to whom correspondence should be addressed. ' Present address: Gillette Research Institute, Gaithersburg, M D 20879.
According to Kirkwood-Tina oscillator theory,3O a positive enhancement in a guest long-axis transition indicates a parallel polarization of the electronic transition with respect to the molecular axis of the CDx host. Conversely, a perpendicular transition should result in a negative ICD. This information is used to assign the orientation of the N heterocycles, examined in the present study, within the CDx molecule. Phenazine (PHEN), a derivative of ACR which contains two nitrogen heteroatoms within its central ring, exhibits properties similar to ACR. However, PHEN, like its anthracene counterpart, lacks the symmetry-intempting properties of the ACR nitrogen heteroatom. Much of the susceptibility of ACR to radiational deactivation has been attributed to the symmetry-disrupting nitrogen heteroatom. Consequently, PHEN and ACR are compared in order to ascertain the importance of the number and position of nitrogen heteroatoms within the tripleringed system in addition to their interaction with the CDx molecule. Apparent association constants are calculated using a modification of a Benesi-Hildebrand equation previously employed by Ishizuka et al.9 Apparent formation constants for ACR and PHEN with 8-CDx have been previously reported as 87 and 86 M-I, respectively.'* However, it seems unlikely that ICD would be observed with such low binding constants. Furthermore, it is not clear which bands were used to make those binding constant calculations. Consequently, association constants are reexamined for these two systems. ICD provides a means for assessing the proportion of complexes formed in solution since only those molecules which are actually complexed within the CDx molecule will experience the induced asymmetry. Therefore, this technique is utilized to confirm estimates of the extent of host:guest association determined from fluorescence measurements.
Experimental Section Apparatus. A SPEX Model F2T2 1I Fluorolog-2 spectrofluorometer equipped with a thermostated cell housing was used to acquire fluorescence spectra. Samples were contained in an anaerobic cell where each was purged with dry nitrogen for 25 min prior to analysis. Excitation wavelengths were set at 365 and 390 nm for ACR and PHEN, respectively. Emission spectra were collected using a 3.44-nm emission and excitation bandwidth. Absorption measurements were performed on a CARY 3 UV-vis spectrophotometer. Circular dichroism measurements were acquired using a JASCO 5-600 spectropolarimeter. Samples for the latter measurements were placed in a 0.5- or 1-cm quartz cell depending on the signal level required. The bandwidth was set at 1 nm and the time constant was 4 s. The scan rate for this experiment was 20 nm/min. Spectra were scanned five times and later smoothed using JASCO software. Maintaining these set parameters between samples and sets of solutions appeared to produce the most consistent results, both for spectral intensity and
0022-3654/92/2096-5309$03.00/00 1992 American Chemical Society
5310 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
7 \\
7 94 200 j \
\
I
A
Schuette et al. 110 I
I
- 0.0 mLI
_____
a-CDx
t
s
O-CDx
1.0 M O-CDx
2.5 mM O-CDx 7.5 mM O-CDx
3800
4
_-.
1.0 mM O-CDx
1
2000 0.000
0.005
0.010
0.015
-0 405 4 2 8 453 4 7 7 5 0 1 525
Wavelength (nm)
[CDxl (MI
Influence of CDx size on ACR (5.0
X
M) fluorescence.
final formation constant calculations. Materials. Acridine (99% purity) and phenazine (98% purity) were purchased from Aldrich and used as received. All cyclodextrins were obtained from American Maize Products (Hammond, IN). The @-CDxwas recrystallized twice from deionized H20. Method. Preparation of Fluorescence and Circular Dichroism Samples. Separate 1.0 X or 5.0 X M solutions of aqueous ACR and PHEN were prepared by pipeting 2.5 or 12.5 mL of the respective 1.0 X M ethanol stock solutions into a 250-mL flask. The ethanol was then evaporated under a stream of nitrogen, and the residue was dissolved in deionized H 2 0 (Continental System, Atlanta). The aqueous stock solution was allowed to equilibrate overnight before being transferred to individual 10-mL flasks containing the appropriately weighed dry CDx quantities. Samples for absorbance measurements used H 2 0 blanks containing appropriate CDx concentrations. Samples for CD measurements used a H 2 0 blank for background subtraction since CDx produced no observable background signal. A 2.6 X M aqueous ACR and PHEN solution was required for the 300400-nm ICD region.
Results and Discussion Acridine Fluorescence. Figure 1 represents a plot of maximum fluorescence intensity of ACR as a function of CDx concentration. The quenching is most dramatic (Stern-Volmer quenching constant, Ksv = 197.3 M-I) in the presence of @-CDx. In contrast, minimal quenching is observed for ACR in the presence of a-CDx (Ksv = 12.5 M-I) as well as y-CDx (Ksv = 25.6 M-l). Furthermore, the characteristic hypsochromic shift of the ACR fluorescence spectrum, upon increased @-CDxconcentration, is absent in the corresponding a- and y-CDx systems. CoreyPauling-Kolton molecular models indicate a size incompatibility of ACR with a-CDx. Consequently, the persistent quenching might be explained through shallow or noninclusional interactions of ACR with a-CDx. Such interactions have been reported for anthracene and ~ - C D X . In ~ ' contrast, the larger y-CDx cavity may accommodate both an axial and equatorial orientation of ACR. The absence of a hypsochromic shift in this case may reflect the less hydrophobic or less confining nature of the y-CDx cavity. The smaller quenching constant further demonstrates the weaker interaction of ACR inside the y-CDx cavity. Phenazine Fluorescence. The acute sensitivity of nitrogen heterocycles to microenvironmental and media effects is generally ascribed to the self-induced heavy atom effects of the N heteroatom.32 Phenazine exhibits some unique fluorescence properties in this respect (Figure 2). A consistent quenching is observed for the 426-nm shoulder peak of PHEN upon increasing the b-CDx concentration. However, the band located at 471 nm appears to be. more sensitive to changes in polarity than the 426nm band. As the concentration of 8-CDx approaches approximately 5.0 X M, this band begins to emerge as a separate entity. The decrease in the 426-nm PHEN shoulder peak is not as pronounced as that for the 429-nm fluorescence maximum peak of ACR.] Although PHEN lacks the symmetry disrupting properties of the lone nitrogen located within the central ring of ACR, the
Figure 2. Fluorescence of PHEN (5.0 X lo-' M) in the presence of 0.0, 1.0 X 2.5 X 7.5 X and 0.01 M 8-CDx. Samples were buffered at pH = 10 with a Na2C03/NaHC03buffer solution.
TABLE I Comparison of log Apparent K for ACR and PHEN in the Presence of Various CDx's Usinn NLR of Fluorescence Data PHEN CDx WCDX @CDX
ACR 1.05
a
a
2.72
2.02
Y-CDX
1.52
2.32( 2.15d
2.34 2.36b 2.7@
I426
I47 I
1.85d
OInconsistent peak maximum. bBuffered at pH = 10 (Na2C03/ NaHCO,). 'Buffered at pH = 10 (Na2B4O7.H2O/NaOH). dBuffered at pH = 10 (Na2C03/NaOH).
unique spectral characteristics of the CDx:PHEN systems, as compared to those for ACR, suggest that the interaction of the nitrogen heteroatoms of PHEN with the CDx cavity is still an important factor in the characterization of the internal and external microenvironment provided by the CDx. A polarity study with PHEN demonstrates the sensitivity of the 471-nm peak to decreasing polarity. For example, a plot of I42,/I4,, ratio vs polarity (p') shows a decreasing trend (Z426/Z471 = 0.89-0.21) as polarity is decreased from ca. 10.20 in H20 to ca. 3.90 in 1-butanol. However, toluene (p' = 2.40) and cyclohexane (p' = -0.20) cause the peak ratio to increase above that in water. The Z426/Z471ratio for PHEN decreases from 0.89 in the absence of @-CDxto 0.60 in the presence of 15 mM p-CDx, suggesting that PHEN experiences a microenvironment of increasing nonpolarity as the BCDx concentration is increased. This, in turn, provides strong evidence that PHEN is transferred from the bulk aqueous environment into the nonpolar O-CDx cavity. Such an effect is also observed for PHEN in the presence of aand y-CDx. The internal polarity of the cyclodextrin cavity approximates that of methanol (p' = 5.10) or 70% aqueous dioxane (p' = 4.80).2 Polarity studies indicate that the microenvironment experienced by PHEN in the presence of O-CDx does, in fact, have a polarity in this range. In contrast, the Z426/Z471ratio for PHEN reaches a value of only 0.71 and 0.80 in the presence of 10 mM a-CDx and 15 mM y-CDx, respectively. This may further suggest that the interaction between PHEN and the a- or y-CDx molecule is not as extensive as with 8-CDx. Although the strongest influence on the PHEN ratio is noted with a-and b-CDx, the smaller cavity size of a-CDx precludes complete complexation with PHEN. Thus, it is possible that this effect might result from an overall polarity change due simply to the addition of CDx to H20. The weaker dimensional constraints and concurrently lower observed hydrophobicity of the y-CDx cavity would explain the less dramatic PHEN ratio changes as compared to those with b-CDx. Calculation of Apparent Formation Constants. The apparent formation constant of ACR with B-CDx (524.8 M-I) has been determined using a nonlinear regression analysis of steady-state fluorescence measurements. Application of this same methodl to the fluorescence information provided by the PHEN 426-nm shoulder band results in an apparent binding constant of 104.7 M-I for the 8-CDxIPHEN complex. A comparison of binding
The Journal of Physical Chemistry, Vol. 96, No. 13, I992 5311
Asymmetry of Achiral Nitrogen Heterocycles
55000
0.800
I
I
0.407
e
5.400
-2
'
c
;
15000
n 4
35000
5.000
'
\ 242
248
-
10.0 mM
5'0
254
-5000 200
220
260
240
2so
Wavelength (nm) Wavelength (nm)
Figure 3. Absorbance spectrum of P-CDxIPHEN (5.0 X
M). The 248-nm band is enlarged to show detail. (Inset) The 320-340-nm region.
constants for PHEN and ACR with each of the respective cyclodextrins is presented in Table I. In general, increasing the pH results in stronger binding constants. However, careful attention must be given to the interpretation of such a result particularly when describing a quenched system. At pH = 10, it is anticipated that the majority of ACR or PHEN molecules in solution will be unprotonated. Therefore, in this unprotected state the N heteroatom is expected to interact more strongly with the glycosidic oxygens lining the internal wall of the CDx or the hydroxyl groups crowning the rim of the cavity. In such a case, enhanced quenching is not only predicted, but actually substantiated based on Stem-Volmer quenching constants calculated for ACR/@-CDxin acidic and basic solutions.' Nevertheless, stronger quenching is not necessarily indicative of a stronger binding constant. Furthermore, the number of nitrogen heteroatoms is not necessarily an accurate predictor of the extent of quenching for the N heterocycle system. Rather, the strongest contribution to quenching may be attributed more to the symmetry disruption or corresponding instability of the a-system due to the location of the N heteroatom. Additionally, Table I compares the effect of the buffer composition for the solutions at pH = 10 on the formation constants. Buffer composition had esseatially no significant effect on the calculated formation constants. Absorbance of /3-CDxIPHEN. The absorbance spectrum for ACR in the presence of P-CDx has been described in an earlier paper.' The absorbance spectra for PHEN in the presence of O-CDx (Figure 3) exhibit some interesting differences if compared to that of @-CD/ACR. Upon increasing the BCDx concentration, a decrease in absorbance of the band located at ca. 206 nm (not shown) is observed. Additionally, a distinct red shift is observed for this set of bands. A second, stronger band centered at ca. 248 nm experiences a similar decrease in absorbance. However, the shift of the band maximum is less consistent. It is interesting to note that, in the case of ACR, an isosbestic point is observed under similar conditions.' In contrast, upon addition of increasing amounts of &CDx, the individual spectral bands for PHEN merge, forming a strong overlap beyond ca. 252 nm. The analysis of the 300-400-nm region involves a set of weakly defined bands centered at about 365 nm. In the absence of O-CDx, two separate peaks can be discerned in this region centered at 362 and 369 nm. The addition of O-CDx results in a consistent decrease in intensity for both the 362- and 369-nm bands. However, the 360-nm band decreases at a faster rate, becoming essentially indistinguishable from the 362-nm band at the higher P-CDx concentrations. Although there appears to be a slight blue shift in the 369-nm band, the 362-nm peak appears to be unaltered. ICD of Acridine and Phenazine. Both ACR and PHEN are symmetrical molecules. Due to the lack of a chiral center, therefore, neither molecule is expected to exhibit optical activity alone in solution. However, upon interaction with @-CDx,ACR displays a positive enhancement in the long-axis-polarized transition located within the 200-260-nm region (Figure 4a). A closer examination of the 250-nm-band region of ACR in the presence of 1.0 X M O-CDx reveals a second polarization at ca. 244 nm. Interestingly, this band decreases in intensity relative to the
-333
1
.
..... ..
1
-000 310
337
363
390
Wavelength (nm)
Figure 4. (a) ICD spectrum of ACR (1.0 X lo5 M) for the 200-260-nm 2.5 X wavelength region. [P-CDx] = 0.0 (-), 1.0 X 5.0 X (- (bold)), and 0.01 M (bold)). Cell width = 1.0 cm. (b) ICD spectrum of ACR (2.6 X lo4 M) for the 310-390-nm wavelength 5X region. [P-CDx] = 0.0 (-), 5.0 X lo4 1.0 X (- (bold)), and 0.01 M (bold)). Cell width = 1.0 cm. (e-),
(-a-),
(-a
(-e),
(-e-),
(a-
250-nm band even though both bands increase with increasing &CDx concentration. Furthermore, both bands experience a hypsochromic shift with increasing @-CDxconcentration. Beyond 300 nm, a less intense spectral region consists of a parallel (ca. 320-360 nm) and a perpendicular (ca. 360-390 nm) transition with respect to the molecular axis of the CDx (Figure 4b). PHEN demonstrates a similar trend to that of ACR in the 200-260-nm region with a maximum polarization located at ca. 249 nm, although the spectral characteristics for PHEN are not as welldefined as for ACR (Figure sa). As a consequence of the spectral variations, the assessment of spectral shifts is difficult. Nevertheless, it appears that, upon increasing the P-CDx concentration, the two polarization bands for PHEN located at ca. 240 and 249 nm begin to coalesce in the presence of 1.0 X M p-CDx, forming a single, broad band which encounters a general bathochromic shift. Figure 5b displays the broad polarizations of the 3 10-390-nm region for PHEN. A strong and consistent, positive enhancement is observed in the 359- and 363-nm bands, followed by a weaker, negative set of bands centered at ca. 388 nm. A small positive enhancement of the 249-nm band region has also been observed for ACR and PHEN in the presence of 0.01 M y-CDx (Figure 6, a and b). Unfortunately, the signals are not strong or consistent enough in this region to determine the binding constants. The presence of 0.01 M a-CDx produced a slightly negative ICD for ACR and PHEN, suggesting that a-CDx induces little asymmetry in ACR or PHEN. This is probably the result of shallow insertion or external association of ACR with a-CDx. Kodaka and F ~ k a y arecently ~ ~ reported a similarly unexpected reversal of ICD signs for the bipyridinium moiety of the aCDx/ heptyl viologen complex. They concluded that a chromophore whose transition moment is parallel to the molecular axis of the CDx yet external to the cavity will be negative in sign.34 Thus, they determined that the bipyridinium moiety of the heptyl viologen was outside the a-CDx cavity. In contrast to the aCDx/ACR association, the ICD spectra for ACR and PHEN in the presence of 8- or y-CDx in this region display no negative bands at all. Due to the length of ACR and PHEN, one expects
5312 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
Schuette et al.
16000
1600
8667
867
-
n
n-mX
E
-.-.-
1333
133
'
-6000 200
I 220
240
260
-600 310
Wavelength (nm)
2800
1700
1
.
3000 .....
m
-500 310
337
363
390
(e-),
(-e-),
(-e-).
(-e-),
(-0
35000
(a)
. .
.. .. ..
.. .
20000
.. ... .. ....
n Y
.
n-mx
... . .. ... .
-y-CDX a-COX
5000
i i
- 10000 200
233
267
300
Wavelength (nml
50000
31667 n
E
13333
-5000
/oI
1 I
I
337
II -
200
n-CDy
a-COX
9 LFJ
233
267
390
1
-600 310
I
337
363
390
Figure 7. (a) ICD of ACR comparing the effect of 0.1 M a-,8-, and y-CDx in the 310-390-nm region. (b) ICD of PHEN comparing the effect of 0.01 M a-,@-, and y-CDx in the 310-390-nm region.
portance of a tight, size-compatible fit for chiral recognition by the CDx molecule. Furthermore, a shoulder located at ca. 256 nm begins to emerge in the presence of y-CDx, perhaps signaling a skewed or partially equatorial interaction of PHEN and ACR with y-CDx. Figure 7a displays positive (ca. 355 nm) and negative (ca.378 nm) bands of ACR; and, in general, more definition is observed in the presence of @-CDxthan with a-or y-CDx. At 0.01 M, a-CDx induced a very slight negative chirality. Information gathered from the y-CDx system in the 310-390-nm region was hindered by the turbidity of the 2.6 X lo4 M y-CDxIACR solution. In contrast, PHEN caused no observable precipitation under the same conditions. The effect of y-CDx on PHEN in the 310-390-nm region is presented in Figure 7b. Similar to a-CDx/ACR, only a slight negative inducement of chirality is observed for PHEN and a-CDx in this region. Additionally, the emergence of a negative peak at ca. 380 nm for ACR is characteristic of the @-CDxsystem and to a lesser degree of the y-CDx system. Interestingly, the absorbance spectra of both PHEN and ACR in the presence of a- and yCDx are slightly blue-shifted with respect to the corresponding &CDx systems. Red-shifting of the absorbance spectrum is an indication that the probe experiences a hydrophobic environment, and therefore, suggests the formation of an inclusive complex between @-CDxand ACR or PHEN. Following Kirkwood-Tinoco oscillator theory, the positive enhancements in the 250- and 355-nm bands correspond to axial inclusion of ACR. The 378-nm band is most likely attributed to a short-axis transition. The negative enhancement of this series of bands suggest a perpendicular transition of the short-axis band with respect to the molecular axis of the CDx molecule. This further corroborates an axial inclusion of ACR with @-CDx. Kobayashi et a1.18 and references cited therein have assigned the 359- and 363-nm polarizations of PHEN to a long- and short-axis transition, respectively. The long-axis transition oscillator strength (f= 0.2097) is stronger than that of the short-axis transition (f = 0.1846). Consequently, the positive enhancement of this set of bands suggests a parallel transition of the long axis of PHEN with respect to the molecular axis of @-CDx. The 388-nm region is ascribed to a short-axis transition, the negative enhancement
:
4
363
Wavelength (nm)
Wavelength (nm)
Figure 5. (a) ICD spectrum of PHEN (5.0 X M) for the 200260-nm wavelength region. [O-CDx] = 0.0 (-), 5.0 X lo-" 1.0 X 2.5 X (- (bold)), 5.0 X (-- (bold)), and 0.01 M Cell width = 0.5 cm. (b) ICD spectrum for PHEN (saturated) for the 310-390-nm wavelength region. [@-CDx]= 0.0 (-), 1.0 X 2.5 X 5.0 X (- (bold)), and 0.01 M (bold)). Cell width = 1.0 cm. (-e),
a-CDx
Wavelength (nm)
...........:::...
..
y-CDX
300
Wavelength (nm)
Figure 6. (a) ICD of ACR comparing the effect of 0.01 M a-,@-, and y-CDx in the 200-300-nm region. (b) ICD of PHEN comparing the effect of 0.01 M a-,8- and y-CDx in the 200-300-nm region.
the orientation of ACR or PHEN within these two cyclodextrins to be axial for the most part varying only in the degree of mobility within the cavity. The weaker effect of y-CDx upon ACR and PHEN as compared to the effect of @-CDxindicates the im-
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5313
Asymmetry of Achiral Nitrogen Heterocycles
0.60
of which suggests a perpendicular polarization. Thus, PHEN is assigned an axial orientation within b-CDx as well. BindingcaItsmtDeterrm*rrptionfromIcDMeasuremeaQ. ICD can provide beneficial information conerning the binding of a guest molecule to CDx. This, in combination with the polarization direction measurements and CPK modeling, allows one to discern the extent of binding where, for example, fluorescence measurements tend to have shortcomings, such as in nonfluorescent or fluorescence-quenched systems. At equilibrium, a compatible guest molecule (G) will associate with CDx (B) to form a complex (B:G) according to the following equation: B+G
B:G
(1)
(C,
(3) = [ecl(Cc - [B:GI) + [e~:cl[B:Gl Here, [e,] and [e,,,] represent the molar ellipticity of ACR or PHEN, and the complex, respectively. Since ACR and PHEN do not exhibit optical activity, however, eq 3 can be reduced to ~ S O L N= [e~:cil [B:GI (4) Rearranging eq 2, we obtain
+ C, - [B:G])
We can then substitute eq 4 for [B:G] in eq 5 and dividing both sides of the resulting equation by POB:. we amve at the following q u a t ion : l + c, + c, - [B:GI - ~,~,[e,:,l (6) KA%:G AeBG A~B:G%OLN The parameters POB:, and AeSOLNare defined in eq 7 and eq 8, respectively. Since [e,] = 0 for ACR and PHEN, eq 7 and
P,:,l - [e,] = [e,:,]
(7)
eq 8 are reduced accordingly. Making appropriate substitutions of eq 7 and eq 8 for [e,,,] and C,, respectively, we derive the final equation: C, CB + C, - [B:G] 1 -= (9) A~SOLN AeB:G KAeBG Using 5.0 X M ACR or PHEN means the highest concentration of complex attainable, [B:G], is 5.0 X M. This is negligible with respect to the sum of C, + C,. Plotting C,/ A&,LN vs C, + C, should give a linear relationship with a slope of l/AeBa. The formation constant, K, can be initially estimated by dividing the slope by the intercept. The amount of complex actually formed, (B:G], at any given CDx concentration is then estimated using the value of K in eq 2. A second plot of C,/ A&L,N vs CB C, + [B:G] is performed and a new value of K is estimated. This procedure is reiterated several times until convergent values of K are arrived upon. A plot of CB/AeSOLNvs c, + c, is given in Figure 8 for both ACR and PHEN. The apparent binding constants calculated through this modified Benesi-Hildeband method using data in the 200-300-nm region are 417 and 284 M-l, respectively. While these values differ from the values calculated through NLR of fluorescence data for each system, they confirm the stronger binding of ACR to 8-CDx. In general, Benesi-Hildebrand methods tend to give higher binding constant estimates than NLR
+-
+
(W)
TABLE II: Comparison of Association Constants Calculated through NLR of ICD Data for ACR and PHEN in the Presence _ of &CDx . region 200-300 nm 310-390 nm
%OLN
A%:, =
+ CJ
relation coefficients for ACR and PHEN are 0.9806 and 0.9958, respectively. (Inset) NLR of ACR and PHEN ICD data.
- [B:G])
Here, C, and C , represent the analytical concentrations of the CDx and the guest, respectively. Under conditions where both the guest and the complex possess optical activity, the ellipticity of the solution (eSOLN) is given by
- = - -cBcc (C, K [B:G]
0.01 1
0.007
0.004
Figure 8. Modified Benesi-Hildebrand plot of ICD data for ACR ( 1 .O X M) and PHEN (1.0 X lo-' M) in the presence of @-CDx. Cor-
where [B:G] K= (C,- [B:G])(CG
0.000
-m
a
K w a , M-'
41 3 a
KPUFN,M-' 270 210
Large associated error made value unreliable.
methods. Barra et al.19 have used a nonlinear form of the Benesi-Hildebrand equation to obtain binding constant information for xanthone with a-,j3-, and y-CDx. The inset of Figure 8 depicts such a plot for the ICD information of ACR and PHEN. The formation constants calculated with the SAS statistical program' are tabulated in Table 11. The binding of ACR to j3-CDx is still approximately twice that of PHEN. In the study reported by Barra and co-workers, it was apparently necessary to work in the concentration range of 2.5 X to 0.1 1 M for a-CDx and 5.6 X lo4 to 0.05 M range for y-CDx in order to elucidate the extent of the binding with xanthone. The study described in this paper, however, has only examined concentrations up to 10 mM for a-CDx and y-CDx solutions. Kobayashi et al.IB reported formation constants of 87 and 86 M-I for the binding of O-CDx to ACR and PHEN, respectively. Although the reported results were calculated using a Benesi-Hildebrand type of equation, it was unclear which ICD signals were used to generate these values. Therefore, our calculations accounted for maximum ICD signals in both the 200-300- and 310-390-nm regions. As a consequence of the inconsistent effect of j3-CD upon the ICD of ACR at 355 nm, it was necessary to examine the negative 378-nm bands for estimating the binding constant in this region. It should be noted that this gives an estimate of the binding along the short axis rather than along the long axis of ACR. The positive set of peaks centered at 359 nm were examined for PHEN. The modified Benesi-Hildebrand equations gave formation constants of 700 and 247 M-' for ACR and PHEN, respectively. These values are still on the order of those calculated in the 200-300-nm region and are of a more reasonable magnitude for further assuming an ICD signal which results from complex formation. Nonlinear regression analysis of the 310-390-nm region gives a binding constant of 210 M-' for PHEN (Table 11). However, the formation constant determined for the corresponding ACR system using NLR was associated with significant standard error.
Conclusion While fluorescence measurements offer some insight into the mechanism of quenching, ICD information clarifies the axial orientation of ACR and PHEN inside as well as the extent of association with the CDx cavity. The advantage of ICD is that specific chiral effects of the CDx molecule may be delineated without regarding the quenching mechanism of the system. Since optical activity is induced only for those molecules which are complexed with CDx, a more accurate assessment of binding can be determined. Interestingly, it appears that ICD occurs even for heterocycles with much lower binding constants than their
5314
J . Phys. Chem. 1992, 96, 5314-5319
respective polynuclear aromatic hydrocarbon counterparts. This phenomenon remains to be further investigated. Finally, the sensitive nature of chiral recognition by CDx’s may be used to provide further insight into the binding or interaction of third components in CDx/heterocyclic systems. Acknowledgment. Support for this study was made possible through a grant from the National Institutes of Health (GM39844). We express our gratitude to Dr. Arsenio Mufioz de la Pefia for insightful discussions as well as to G. A. Reed of American Maize Products for providing the cyclodextrins used in this study. References and Notes (1) Schuette, J . M.; Ndou, T. T.; Mufioz de la Pefia, A.; Greene, K. L.; Williamson, C. K.; Warner, I. M. J . Phys. Chem. 1991, 95, 4897. (2) Bergmark, W. R.; Davis, A.; York, C.; Macintosh, A.; Jones, G. 11. J. Phys. Chem. 1990, 94, 5020. (3) Lin, S.-F. Ph.D. Dissertation, University of Wisconsin in Madison, 1981. (4) Matsui, Y.; Mochida, K. Bull. Chem. SOC.Jpn. 1979, 52, 2808. (5) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (6) Schuette, J. M.; Ndou, T. T.; Warner, I. M. J. Am. Chem. SOC.,in press. (7) Mufioz de la Pefia, A.; Ndou, T. T.; Zung, J. B.; Greene, K. L.; Live, D. H.; Warner, I. M. J . Am. Chem. SOC.1991, 113, 1572. (8) Zung, J. B.; Ndou, T. T.; Mufioz de la Pefia, A.; Warner, I. M. J. Phys. Chem. 1991, 95, 6701. (9) Ishizuka, Y.; Nagawa, Y.; Nakanishi, H.; Akira, K. J . Inclusion Phenom. Mol. Recognit. Chem. 1990, 9, 2 19. (10) Kobayashi, N.; Opallo, M. J . Chem. SOC.Commun. 1990, 477. (11) Ata, M.; Kubozono, Y.; Suzuki, Y.; Aoyagi, M.; Gondo, Y. Bull. Chem. SOC.Jpn. 1989,62, 3706.
(12) Kobayashi, N.; Zao, X.; Osa, T.; Kato, K.; Hanabusa, K.; Imoto, T.; Shirai, H. J . Chem. Soc., Dalton Trans. 1987, 1801. (13) Le Bas, G.; de Rango, C.; Rysanek, N.; Tsoucaris, G. J . Inclusion Phenom. 1984, 2, 861. (14) Yamaguchi, H. J. Inclusion Phenom. 1984, 2, 747. (15) Kobayashi, N.; Hino, Y.; Uneo, A.; Osa, T. Bull. Chem. SOC.Jpn. 1983,56, 1849. (16) Kobayashi, N.; Saito, R.; Hino, H.; Hino, Y.; Uneo, A.; Osa, T. J . Chem. SOC.,Perkin Trans. 2 1983, 1031. (17) Kano, K.; Tatsumi, M.; Hashimoto, S. J . Org. Chem. 1991,56, 6579. (18) Kobayashi, N.; Minato, S.; Osa, T. Makromol. Chem. 1983, 184, 2123. (19) Barra, M.; Bohne, C.; Scaiano, J. C. J. Am. Chem. SOC.1990,112, 8075. (20) Patonay, G.; Warner, I. M. J . Inclusion Phenom. Mol. Recognit. Chem. 1991, 11, 313. (21) Yorozu,T.; Hoshino, M.; Imamura, M.; Shizuka, H. J. Phys. Chem. 1982,86, 4422. (22) Ueno, A.; Suzuki, I.; Osa, T. J . Am. Chem. SOC.1989, 1 1 1 , 6391. (23) Ueno, A.; Moriwaki, F.; Osa, T.; Hamada, F.; Murai, K. J. Am. Chem. SOC.1988, 110, 4323. (24) Harata, K.; Tsuda, K.; Uekama, K.; Otagiri, M.; Hirayama, F. J . Inclusion Phenom. 1988, 6, 135. (25) Moriwaki, F.; Kaneko, H.; Ueno, 4.; Osa, T.; Hamada, F.; Murai, K. Bull. Chem. SOC.Jpn. 1987,60, 3619. (26) Yamaguchi, H.; Higashi, M. J . Inclusion Phenom. 1987, 5, 725. (27) Kobayashi, N.; Osa, T. Chem. Lett. 1986, 421. (28) Hira, H.; Toshima, N.; Uenoyama, S.Bull. Chem. SOC.Jpn. 1985, 58, 1156. (29) Opallo, M.; Kobayashi, N.; Osa, T. J. Inclusion Phenom. Mol. Recognit. Chem. 1989, 6, 413. (30) Tinoco, I. Jr. Adu. Chem. Phys. 1962, 4, 113. (31) Blyshak, L. A.; Patonay, G.; Warner, I. M. Anal. Chim. Acra 1990, 232, 239. (32) Woods, R.; Love, L. J. C. Spectrochim. Acra 1984, 40A(7), 643. (33) Kcdata, M. and Fukaya, T. Bull. Chem. SOC.Jpn. 1989, 62, 1154. (34) Kcdata, M. J . Phys. Chem. 1991, 95, 2110.
Gas-Phase Ion-Molecule Charge-Exchange Reactions of Fe+ with Fe(CO),: Observation of Higher Lying Metastable Electronic States J. V. B. Oriedo and D. H.Russell* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: December 16, 1991; In Final Form: February 24, 1992)
Charge-exchange ion-molecule reactions of Fe’ with Fe(C0)5 formed by 20-70-eV electron impact of Fe(C0)5 are reported. The charge-exchange reaction chemistry data indicate that electronic states with energies up to about 4.0 eV are produced and that 40% of the excited-state ions are in the first excited state 4F(3d7)and the remaining 60% are in the 4D(4s13d6) to *G(4s’3d6)or higher energy states. Of the Fe+ produced by 50-70-eV ionizing energy 65% are formed as excited states and about 35% as ground state 6D(4s’3d6). The results from charge-exchange ion-molecule reactions are compared to the excited states determined by other methods.
Introduction Studies of gas-phase ion-molecule reaction chemistry of transition-metal ions have fluorished during the past decade’,*and an entire field of gas-phase organometallic chemistry has emerged3 Gas-phase ion chemistry studies provide fundamental insights into reactions that are important to many areas of chemistry and thermochemical properties of metal-containing species. For example, ion-molecule reactions involving C-H and C-C bond insertion (activation) provide details important to homogeneous and heterogeneous catalysis and the general area of organometallic chemistry. Eller and Schwarz have compiled an amazingly comprehensive review of this entire field.’ Interest in the chemical and physical properties of transition-metal species has resulted in significant growth in the gas-phase ion-molecule reaction chemistry of transition atomic metal ions and ionic cluster fragments. The focus of much of this work is on the ion-molecule reactions of metal ions with small molecules and functionalized organic molecule^.^^^ Early studies of metal ion chemistry em-
phasized the type of reactions, product ion distribution, and reaction mechanism; however, the most important contribution of much of this work is to the understanding of reaction energetics and bond energies to metal centers.6 More recently, considerable attention has been given to the specific electronic state@) of the reacting metal ion (M’) and how the reactivity of M+ might change if different excited states are formed by the ionizing process.’ The attention to state-specific reactivity of transition-metal ions has increased the demand for sensitive, reliable methods for measuring the distribution of excited states formed by various ionization methods. The most commonly used method for forming reactant metal ions for gas-phase ion-molecule chemistry studies is electron impact (EI) ionization of volatile transition-metal We now know that a si@icant fraction of the metal ions produced by E1 of transition-metal compounds are formed as long-lived (seconds) metastable states.I4 For example, Ridge and -workers estimated that approximately 75% of Cr+ ions formed by E1 (70
0022-365419212096-53 14%03.00/0 0 1992 American Chemical Society
