α-Cyclodextrin/Resveratrol Host–Guest Complexes
925
4
3.5
3
2.5
2.3
2.1
1.9
1.7
1.5
trans
cis
2.5
2
0
50
100
Time [min]
1.5
1
Transition
stage
0.5
0
200
250
300
350
400
Wavelength [nm]
Fig. 9. UV-Visible spectra of a UV-irradiated solution of trans-resveratrol/α-CD host–guest IC in
pH 8.0 phosphate-borate-SDS buffer from 0 min (bottom spectrum) to 100 min (top spectrum). Inset:
the change in the trans-resveratrol absorbance peak at 320 nm with UV exposure time.
3
2.5
2
A one-wayANOVA test was performed on this data at a 90% con-
fidenceleveltodeterminethevariation.Theresultant pvaluewas
0.0662, which indicates a significant difference between trans-
resveratrol as an IC with α-CD and without α-CD. It also shows
that the stability of trans-resveratrol is enhanced upon inclusion
into α-CD.The trans-resveratrol solution that contains α-CD has
a higher absorbance at the completion of irradiation; therefore,
it appears that less trans-resveratrol is converted overall.
(a) trans-resveratrol with α-cyclodextrin
1.5
1
0.5
0
(b) trans-resveratrol without α-cyclodextrin
Conclusions
0
20
40
60
80
100
trans-Resveratrol’s stability to UV light was tested in phosphate–
borate buffer solution over a pH range of 2.0–9.0. Results
showed that isomerization to the cis isomer was lowest at pH
8.0 and that the formation of degradation products (at 260 nm)
was lowest at low pH values. A one-way ANOVA test showed
that the formation of a trans-resveratrol/α-CD IC in pH 8.0
phosphate-borate-SDS buffer greatly enhanced the stability of
the resveratrol with the formation of a longer transition state.
This has implications in the separation of the isomers using
capillary electrophoresis where a UV detector source and other
external UV light sources are present.
Time [min]
Fig. 10. Plot of absorbance versus time of the trans-resveratrol peak at
320 nm for (a) trans-resveratrol/α-CD host–guest IC in pH 8.0 phosphate-
borate-SDS buffer; (b) 100 × 10−6 M resveratrol (no α-CD) in pH 8.0
phosphate-borate-SDS buffer.
was irradiated with UV light from 0 to 100 min and analyzed
by UV-Visible spectrophotometry. Fig. 9 shows a plot of
absorbance versus wavelength at different UV irradiation times
(min). Initially, the trans-resveratrol peaks at 305 and 320 nm
both decrease rapidly after ∼10 min of irradiation, after which
there is a transition stage (see Fig. 9) where no change in the
ratio of trans-to-cis isomer is observed. A possible explanation
for the transition stage is the mechanism behind the isomeriza-
tion, which involves an inversion around the double bond.[22]
There are two radical transition states involved in the mecha-
nism (Fig. 7). Both radicals are in their excited state after 10 min
and no longer contain conjugated double bonds. Studies show
that the inversion step is much more rapid for the excited cis con-
formation than the excited trans because of the steric hindrance
associated with the aromatic groups.[24] Based on the instability
of the cis excited state inverting back to the more stable trans
conformation equilibrium is reached, which is also known as
the photo-stationary state.[25] In this case, when the rate of the
trans isomerization equals the rate of the cis being produced the
solution contains a 90:10 cis-to-trans mixture.[13] An important
point to note is the lack of a peak at 260 nm, which is represen-
tative of photo-degradation. This suggests that the addition of
α-CD further restricts the formation of the degradation product.
Fig. 10 shows a plot of absorbance versus time of the trans-
resveratrol peak at 320 nm both with and without α-CD at pH 8.0.
References
[1] M. Careri, C. Corradini, L. Elviri, I. Nicoletti, I. Zagnoni, J. Agric.
Food Chem. 2003, 51, 5226. doi:10.1021/JF034149G
[2] N. Ratola, J. L. Faria, A. Alves, Food Technol. Biotechnol. 2004,
42, 125.
[3] P. Langcake, R. Pryce, J. Physiol. Plant Pathol. 1976, 9, 77.
doi:10.1016/0048-4059(76)90077-1
[4] M. Moreno-Manas, R. Pleixats, An. Quim. 1985, 81, 157.
[5] J. Yu, M. J. Gaunt, J. B. Spencer, J. Org. Chem. 2002, 67, 4627.
doi:10.1021/JO015880U
[6] M. Jang, J. Cai, G. O. Undeani, K. V. Slowing, C. F. Thomas,
C. W. W. Beecher, H. H. S. Fong, N. R. Farnsworth, A. D. Kinghorn,
R. G. Mehta, R. C. Moon, J. M. Pezzuto, Science 1997, 275, 218.
doi:10.1126/SCIENCE.275.5297.218
[7] E. N. Frankel, A. L. Waterhouse, J. E. Kinsella, Lancet 1993, 341,
1103. doi:10.1016/0140-6736(93)92472-6
[8] L. Castro, B. A. Freeman, Nutrition 2001, 161, 163.
[9] L. Fremont, Life Sci. 2000, 66, 663. doi:10.1016/S0024-3205(99)
00410-5
[10] X. Gu, L. Creasy, A. Kester, M. Zeece, J. Agric. Food Chem. 1999,
47, 3223. doi:10.1021/JF981211E
[11] Q. Chu, M. O’Dwyer, M. G. Zeece, J. Agric. Food Chem. 1998, 46,
509. doi:10.1021/JF970669Y