Scheme 1
peroxidase and is prone to interference by electron donors,
such as urea or bilirubin. In contrast, redox indicators that
were inspired by the trimethyl lock effect as a means to reveal
the cloaked fluorescence, which had been demonstrated by
Raines’s group. The rapid formation of 4,4,5,7-tetrameth-
8
12c
directly accept an electron from an oxidizing enzyme instead
9
of oxygen are preferred. The resazurin, transition metal Os,
ylhydrocoumarin from the corresponding o-hydroxycinnamic
10
13
and Ru complexes are the oxygen-independent fluorescent
indicators for glucose. In the case of transition metals, their
fluorescence efficiency varies with the oxygen content of
the sample. On the other hand, the nonfluorescent resazurin
is converted into fluorescence-developing resorufin by DTD,
acid is a result of the trimethyl lock effect. The unfavorable
steric interactions between three methyl groups enhance the
nucleophilicity of the phenolic oxygen and lead to a rapid
lactone formation, and this is an example of the use of strain
to enhance the reactivity. Our strategy for the optical
detection of the DTD activity also relies on the trimethyl
lock effect (Scheme 1). The chemistry of the trimethyl lock
effect of quinone as a masked phenol has been thoroughly
+
â-nicotinamide adenine dinucleotide (NAD ), and glucoses
9
dehydrogenase (GDH) in the presence of glucose. How-
ever, the resorufin can be further reduced by DTD to yield
a nonfluorescent product,11 and the emission bands of
resorufin formed by the redox reaction strongly overlap the
absorption bands of the nonreacted resazurin, which sig-
nificantly reduces the sensitivity of the glucose determina-
tion.
14
15
studied and utilized in the pro-drug strategies; however,
there have been no reports of this chemistry for designing
latent fluorophores. We envisioned that the trimethyl lock
effect would be suitable for the design of a novel latent
fluorophore for DTD. We selected rhodamine 110 (2) as the
fluorophore to be masked because of its high quantum yield,
long emission, excitation wavelength, and popularity for
Latent fluorophores are stable probes that unmask their
intense fluorescence only by a user-designated chemical
reaction, and they are especially useful tools for basic
research in the biological sciences. Recently, there are great
interests in developing latent fluorophores due to their unique
selectivity and minimal interference from the probe concen-
16
basic research in the biological sciences. We reasoned that
the installation of quinone acid 1 at the 3′ and 6′ positions
of a xanthenone scaffold would allow this platform to adopt
a closed and nonfluorescent lactone form. Upon selective
reduction of the quinone moiety in 3, the highly reactive
phenol 4 was generated followed by rapid lactone formation
with the concomitant release of the open and fluorescent
rhodamine 110 (2).
1
2
tration, excitation intensity, and emission sensitivity. We
(
8) Haughland, R. Handbook of Fluorescent Probes and Research
Chemicals, 6th ed.; Molecular Probes, USA 1996.
9) Matsu-Ura, S.; Yamauchi, Y.; Ohmori, H.; Maeda, H. Bunseki Kagaku
002, 51, 111-115.
10) (a) Ryabov, A. D.; Firsova, Y. N.; Ershov, A. Y.; Dementiev, I. A.
(
2
(
J. Biol. Inorg. Chem. 1999, 4, 175-182. (b) Woltman, S. J.; Even, W. R.;
(13) (a) Milstien, S.; Cohen, L. A. J. Am. Chem. Soc. 1972, 94, 9158-
9165. (b) Karle, J. M.; Karle, I. L. J. Am. Chem. Soc. 1972, 94, 9182-
9189.
Webber, S. G. Anal. Chem. 1999, 71, 1504-1512.
(11) Nims, R. W.; Prough, R. A.; Lubet, R. A. Arch. Biochem. Biophys.
1
984, 229, 459-465.
12) (a) Zlokarnik, G.; Negulescu, P. A.; Knapp, T. E.; Mere, L.; Burres,
N.; Feng, L.; Whitney, M.; Roemer, K.; Tsien, R. Y. Science 1998, 279,
(14) Borchardt, R.; Cohen, L. A. J. Am. Chem. Soc. 1972, 94, 9175-
(
9182.
(15) Gharat, L.; Taneja, R.; Weerapreeyakul, N.; Rege, B.; Polli, J.;
Chikhale, P. J. Int. J. Pharm. 2001, 219, 1-10.
(16) Leytus, S. P.; Melhado, L. L.; Mangel, W. F. Biochem. J. 1983,
209, 299-307.
8
4-88. (b) Chang, M. C. Y.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am.
Chem. Soc. 2004, 126, 15392-15393. (c) Chandran, S.; Dickson, K. A.;
Raines, R. T. J. Am. Chem. Soc. 2005, 127, 1652-1653.
266
Org. Lett., Vol. 8, No. 2, 2006