Light-induced hetero-diels-alder cycloaddition: A facile and selective photoclick reaction

Article abstract of DOI:10.1021/ja200356f

2-Napthoquinone-3-methides (oNQMs) generated by efficient photodehydration (φ = 0.2) of 3-(hydroxymethyl)-2-naphthol undergo facile hetero-Diels-Alder addition (k_D-A～ 4 × 10⁴ M^-1 s ^-1) to electron-rich polarized

Full text of DOI:10.1021/ja200356f

ARTICLE
pubs.acs.org/JACS
Light-Induced Hetero-DielsꢀAlder Cycloaddition: A Facile
and Selective Photoclick Reaction
Selvanathan Arumugam and Vladimir V. Popik*
Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
S Supporting Information
b
ABSTRACT: 2-Napthoquinone-3-methides (oNQMs) generated
by efficient photodehydration (Φ = 0.2) of 3-(hydroxy-
methyl)-2-naphthol undergo facile hetero-DielsꢀAlder addition
(kD-A∼ 4 ꢁ 104 Mꢀ1 sꢀ1) to electron-rich polarized olefins in an
aqueous solution. The resulting photostable benzo[g]chromans are
produced in high to quantitative yield. The unreacted oNQM is
rapidly hydrated (kH2O ∼ 145 sꢀ1) to regenerate the starting diol.
This competition between hydration and cycloaddition makes
oNQMs highly selective, since only vinyl ethers and enamines are
reactive enough to form the DielsꢀAlder adduct in an aqueous solution; no cycloaddition was observed with other types of alkenes. To
achieve photolabeling or photoligation of two substrates, one is derivatized with a vinyl ether moiety, while 3-(hydroxymethyl)-2-naphthol is
attached to the other via an appropriate linker. The light-induced DielsꢀAlder “click” strategy permits the formation of either a permanent or
hydrolytically labile linkage. Rapid kinetics of this photoclick reaction (k=4ꢁ 104 Mꢀ1 sꢀ1) is useful for time-resolved applications. The short
lifetime (τ ∼ 7 ms in H2O) of the active form of the photoclick reagent prevents its migration from the site of irradiation, thus, allowing for
spatial control of the ligation or labeling.
’ INTRODUCTION
conditions, and does not produce any byproducts.14 The
DielsꢀAlder click reaction has found applications in materials
chemistry,15,16 derivatization of nanoparticles and surfaces with
various bioactive molecules,17 as well as the labeling of oligonu-
cleotides, proteins, and oligosaccharides.18ꢀ20 However,
DielsꢀAlder cycloaddition reactions are often slow and require
either thermal activation21 or the use of chemical promoters for
the in situ generation of reactive dienes.22
Connection (or ligation in biochemistry) of two or more
substrates or immobilization of various compounds is often achieved
with the help of click chemistry. The term “click chemistry” was
introduced by K. Barry Sharpless to describe a set of bimolecular
reactions that are “modular, wide in scope, high yielding, create only
inoffensive by-products, are stereospecific, simple to perform and
that require benign or easily-removed solvent”.1 Although meeting
all of the above requirements is difficult to achieve, several processes
have been identified as coming very close to the ideal click reaction.
Among them are 1,3-dipolar and DielsꢀAlder cycloadditions,
nucleophilic ring-openings, nonaldol carbonyl chemistry, and addi-
tion to carbonꢀcarbon multiple bonds. Cu(I)-catalyzed versions of
the Huisgen acetylene-azide cycloaddition, also known as azide click
reaction, became the gold standard of click chemistry and have been
applied in fields ranging from materials science2 to chemical
biology3,4 and drug development.5 However, copper ions are
cytotoxic,6 can cause degradation of DNA molecules7 and aggrega-
tion of azide-labeled antibodies,8 as well as induce protein
denaturation.9 In addition, copper complexation hampers functio-
nalization of substrates with chelating agents (e.g., for the introduc-
tion of radiolabels).10 The use of catalysts complicates kinetics of the
immobilization process, requires polar solvents, and can alter surface
properties.11 Catalyst-free 1,3-dipolar cycloaddition of azides to
cyclooctynes12 and dibenzocyclooctynes13 allows for alleviation of
these limitations.
The utility of click reaction-based strategies can be further
extended by employing photochemically triggered click reac-
tions, as this approach allows for the spatial control of the
process. Several photoclick methods are currently under devel-
opment, including cycloaddition of alkenes to photochemically
generated nitrile imines,23 as well as photoinitiated thiolꢀene24
and thiolꢀyne25 reactions.
Here, we report a novel photoclick platform based on a facile
and efficient light-induced hetero-DielsꢀAlder click reaction.
Photochemical dehydration of 3-hydroxy-2-naphthalenemetha-
nol derivatives
2 produces o-naphthoquinone methides
(oNQMs) 1. The latter undergo facile cycloaddition to vinyl
ethers 3 or enamines 4 to produce photostable derivatives of
benzochromans 5 and 6 (Scheme 1). Unreacted oNQM 1 is
rapidly hydrated to regenerate starting material 2. This method
allows for efficient ligation under ambient conditions in aqueous
solution and does not require catalysts or promoters. In addition,
Click methods based on a DielsꢀAlder cycloaddition are
gaining popularity due to the fact that this reaction does not
require catalysts, can proceed in high yield under physiological
Received: January 13, 2011
Published: March 21, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 2
the fast rate of this reaction permits high spatial and temporal
resolution of the ligation.
Table 1. Formation of Adducts 5aꢀd in Photolysis of 2a in
the Presence of Vinyl Ethersa
’ RESULTS AND DISCUSSION
olefin
yield 5 (% conversion)
The 3-hydroxy-2-naphthalenemethanol chromophore (2) has
two major absorption bands in UV region above 210 nm: at
λmax = 275 nm (log ε = 4.06) and at λmax = 324 nm (log ε = 3.70,
Figure 1). Introduction of an alkoxy substituent at the 5 position
of the chromophore (e.g., 2d: R0 = (OCH2CH2)3OCH3)
results in a ca. 10 nm bathochromic shift of both bands.
o-Naphthoquinone methide precursors 2 also show significant
fluorescence with a short lifetime (ΦFl = 0.230 ( 0.002 with
τFL ∼ 7 ns for 2a26). The emission spectrum of 2a in aqueous
solutions contains two major bands at 360 and 423 nm.
3a
3b
3c
3d
96 ( 2 (>99)
94 ( 2 (>99)
95 ( 3 (>99)
97 ( 2 (>99)
a Concentration of 2a = 1 mM in 50% MeCN and 0.01 N aqueous biphosphate
buffer (BR = 1); [vinyl ether] = 1.5 mM; irradiation wavelength = 300 nm.
Table 2. Observed Rates of oNQM 1aa Decay of in the
Presence of 0.05 M Vinyl Ethers 3aꢀd
olefin
observed rate (sꢀ1
)
3a
3b
3c
3d
3100 ( 121
3050 ( 75
3080 ( 101
3090 ( 66
a Generated by 266 nm laser pulse from 0.0001 M solution of 2a in
CH3CN: 0.01 N biphosphate buffer (BR = 1).
not react with 3-hydroxy-2-naphthalenemethanol 2a in aqueous
solution, even at elevated temperatures.
In a typical experiment, a solution of 1 mM of 2a and 1.5 mM of
olefin 3 in a 50% acetonitrileꢀ0.01 N aqueous biphosphate buffer
(BR = 1) mixture was irradiated using 300 nm 4 W fluorescent
tubes. The yield of adduct 5 was measured using HPLC analysis
using pure product as a reference.27 As illustrated by the data
presented in Table 1, the presence of various substituents at the
oxygen, R-carbon, and β-carbon atoms of vinyl ether 3 has very
minimal effect on the yield of the adduct 5. It is also important to
note that nearly quantitative yields of DielsꢀAlder products 5aꢀd
were obtained using only a 50% molar excess of vinyl ether. Both
300 and 350 nm light-induced cycloadditions are very clean
reactions; no photoproducts other than benzochromans 5 were
detected in the reaction mixtures by HPLC. Photolysis using
350 nm light requires longer irradiation time to achieve the same
conversion as in 300 nm experiments, apparently due to a lower
extinction coefficient of chromophore 2 at this wavelength.
Interestingly, the reactivity of oNQM 1a in the cycloaddition
reaction with vinyl ethers is also insensitive to the substitution
Figure 1. UV-spectra of ca. 10ꢀ4 M solutions of 3-hydroxy-2-naphthalene-
methanol (2a, green line) and adduct 5a (red line) in (1:1) acetonitrileꢀwater.
Irradiation of aqueous solutions of 3-hydroxy-2-naphthalene-
methanol derivatives 2 using low pressure Hg lamps (254 nm), as
well as 300 or 350 nm fluorescent tubes, results in the efficient
dehydration (Φ300 = 0.17 ( 0.02 for 2a)26 of the substrate and
the formation of o-naphthoquinone methides (oNQM) 1
(Scheme 1). oNQMs 1 rapidly react with water (k2a = 145 sꢀ1
or 2.61 Mꢀ1 ꢀ1) to quantitatively regenerate the starting
s
material. In the presence of vinyl ethers 3, however, oNQMs 1
undergo facile DielsꢀAlder cycloaddition to produce substituted
2-alkoxy-3,4-dihydro-2H-naphtho[2,3-b]pyrans 5 in high or
quantitative yield (Scheme 2, Table 1). The bimolecular rate of
the addition of oNQM 1a to various vinyl ethers in aqueous
solutions is (4ꢀ6) ꢁ 104 Mꢀ1 sꢀ1 (Table 2).26 Vinyl ethers 3 do
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pattern of the latter (Table 2). oNQM 1a was generated by laser
flash photolysis of a 0.001 M solution of 3-hydroxy-2-naphtha-
lenemethanol (2a) in a 50% acetonitrileꢀ0.01N biphosphate
buffer (BR = 1) mixture with 7 ns 266 nm pulses using a Nd:YAG
laser. The decay of 1a in the presence of 0.05 M of vinyl ethers
3aꢀd was followed using time-resolved UV spectroscopy.27 The
pseudo-first-order rates of the reaction of oNQM 1a with vinyl
ethers 3aꢀd are identical within experimental error.
was generated by 300 nm photolysis of 1 mM solutions of
3-(ethoxymethyl)-2-naphthol (2-Et) in 50% aqueous acetoni-
trile in the presence of 0.1 M of the following alkenes: ethyl vinyl
ether (3a), 2,5-dihydrofuran (8), dimethyl maleate (9),
1-methylcyclohexene (10), methyl acrylate (11), and phenyl
vinyl ether (12). Only in the presence of 3a, the quantitative
formation of the DielsꢀAlder adduct 5a was observed, both at
low and high conversion (Scheme 4, Table 3). In all other cases,
only the product of rehydration, for example, 3-hydroxy-2-
naphthalenemethanol (2a), was produced. No adducts of 1a to
alkenes 8ꢀ12 were detected in 50% aqueous acetonitrile
(Scheme 4). At low conversion photolysis, 2a is formed quanti-
tatively, while at higher conversions, the yield of hydration
product is somewhat reduced. Since no new photoproducts were
detected by HPLC, we believe that the secondary photoproducts
are most likely oNQM oligomers,29 which were trapped in the
2-Alkoxybenzochromans 5aꢀd are photochemically stable
and show no decomposition, even after prolonged irradiation
at 254, 300, or 350 nm. The hydrolytic stability of 5a was
evaluated by incubating this compound in an aqueous bipho-
sphate buffer (pH = 7.4), a 0.1 M sodium hydroxide solution, as
well as a 0.1 M perchloric acid, for 24 h. HPLC analysis of the
reaction mixtures indicated no products of hydrolysis and
showed no changes in 5a concentration in aqueous solution at
neutral pH and in the presence of 0.1 N sodium hydroxide. At pH =
1, however, 5a undergoes slow hydrolysis (lifetime τ ∼ 9.5 h) to
produce benzochroman-2-ol (5e, Scheme 3). In other words, light-
directed DielsꢀAlder cycloaddition-based ligation can produce
hydrolytically labile (connected via the vinyl ether oxygen atom)
or hydrolytically stable linkers (via R- or β-carbon atoms of vinyl
ether moiety, Scheme 3).
Table 3. Yields of Diels-Alder Adducts and the Hydration
Product (2a) Formed in Photolyses of 3-(Ethoxymethyl)-
2-naphthol 2-Et in the Presence of Various Olefins 8ꢀ12a
yield of 2a
yield of the DielsꢀAlder
adduct (% conversion)
alkene
(% conversion)
o-Quinone methides are known to react with various alkenes
resulting in the formation of DielsꢀAlder products.28 However,
when o-quinine methides are generated in the presence of water
(or an alcohol or thiol), the cycloaddition reaction competes with
rehydration that yields back the starting material. This competi-
tion can be employed to enhance the selectivity of o-quinone
methides. To assess the selectivity of oNQM addition to alkenes,
two set of experiments were carried out. First, oNQM 1a
3a
0 (15)
97 ( 2 (15)
96 ( 2 (>99)
97 ( 2 (>99)
0 (14)
0 (>99)
3a þ 8ꢀ12
8
0 (>99)
97 ( 2 (14)
71 ( 3 (89)
93 ( 2 (14)
69 ( 3 (87)
94 ( 2 (14)
73 ( 2 (90)
96 ( 2 (14)
71 ( 2 (92)
95 ( 2 (14)
71 ( 2 (85)
0 (89)
9
0 (14)
0 (87)
10
11
12
0 (14)
0 (89)
Scheme 3
0 (14)
0 (89)
0 (14)
0 (89)
a [Olefin] = 0.1 M; [2-Et] = 1 mM. Irradiation wavelength = 300 nm.
Scheme 4
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Table 4. Rate of Decay of NQM 1a in the Presence of Alkenes
Scheme 5
3a, and 8ꢀ11a
alkene
observed rate (sꢀ1
)
No alkene
119 ( 6
2703 ( 55
125 ( 9
118 ( 4
123 ( 7
121 ( 8
3a
8
9
10
11
a Solution in 50% MeCNꢀ0.01 N aqueous phosphate buffer mixture.
[alkene] = 0.04 M.
HPLC column. These experiments indicate that while aliphatic
vinyl ethers efficiently add to oNQMs in an aqueous solution,
hydration of 1a to 2a outcompetes cycloaddition to other
alkenes. It is interesting to note that aromatic vinyl ethers, such
as 12, show substantially lower reactivity in this hetero-
DielsꢀAlder reaction than aliphatic vinyl ethers.
Scheme 6
Next, we employed a competitive reaction to directly compare
the reactivity of oNQMs toward various dienophiles. A 1 mM
solution of 3-(ethoxymethyl)-2-naphthol (2-Et) in 50% aqueous
acetonitrile was irradiated in the presence 0.1 M of ethyl vinyl
ether (3a) and equal amounts (0.1 M each) of olefins 8ꢀ12.
HPLC analysis of the reaction mixture showed the clean forma-
tion of only one product, vinyl ether adduct 5a, which was
produced in excellent yield (Scheme 4, Table 3). While oNQMs
only form DielsꢀAlder adduct with electron rich polarized
alkenes in aqueous solutions, the hypothetical possibility exists
that oNQM adducts of alkenes 8ꢀ12 are rapidly formed but then
undergo facile photochemical or dark decomposition to 1a or
hydrolysis to 2a. To explore the feasibility of such a process, we
have studied the quenching of transient 1a with various alkenes.
oNQM 1a was generated by laser flash photolysis of 3-
(hydroxymethyl)-2-naphthol 2a in 50% aqueous acetonitrile in
the presence of 0.04 M of olefins 3a, and 8ꢀ11 [Laser flash
photolytic experiments using phenyl vinyl ether (12) were not
performed due to significant extinction coefficient of this com-
pound at the excitation wavelength (266 nm).] The disappear-
ance of 1a was followed using kinetic spectrometry.27 In the
presence of 3a, the rate of 1a decay was significantly enhanced
(Table 4). In all other cases, the rate of oNQM decay was equal to
the rate determined in neat water. This observation clearly
indicates that only cycloaddition of oNQMs to vinyl ethers is
fast enough to outcompete hydration. In the case of nonpolarized
olefins and electron poor polarized olefins, the hydration reac-
tion is the only reaction observed.
Scheme 7
rate of the reaction of oNQM 1a with thioethanolamine (kSH
=
2.2 ꢁ 105 Mꢀ1 sꢀ1) is about 5 times faster than the cycloaddi-
tion reaction (k = 4 ꢁ 104 Mꢀ1 sꢀ1) and the azide ion (kN3 = 2.0
ꢁ 104 Mꢀ1 sꢀ1) reactivity with oNQM is slightly slower. While
the azide ion is uncommon in biological systems, concentration
of endogenous thiols (e.g., glutathione) can achieve millimolar
concentrations. Therefore, thiol quenching of the oNQMs 1 can
potentially reduce the efficiency of photo-DielsꢀAlder-based
click ligation in biological systems. To explore the properties of
oNQMꢀthiol adducts, we prepared thioether 7 by irradiation of
2a in the presence of sodium salt of methylthiol (Scheme 7).
The quantum efficiency of this reaction (Φ = 0.2) is similar to
that of the photo-DielsꢀAlder cycloaddition, and 7 is produced
quantitatively. While thioether 7 is hydrolytically stable in both
acidic and basic media, irradiation at 300 or 350 nm regenerates
oNQM 1a with Φ = 0.1 (Scheme 7). In other words, while thiols
are efficient quenchers of oNQMs, the resulting thioethers also
serve as photoprecursors to reactive quinone methides. To test
the ability of oNQM to form adduct with vinyl ether in the
presence of thiols, we compared results of irradiation of 0.1 mM
aqueous solution of 2a containing 0.15 mM of vinyl ether 3a
with or without the presence of methyl thiol (0.15 mM). In the
absence of thiol, 2a was quantitatively converted into the
DielsꢀAlder adduct 5a after 20 min of irradiation at 300 nm,
whereas in the presence thiol, the DielsꢀAlder adduct 5a and
thioether 7 were formed in the 3:1 ratio of after 20 min.
However, after additional 10 min of irradiation, only 5a was
observed in the photolysate. HPLC analysis confirms quantita-
tive formation of the DielsꢀAlder product in the latter case.
These results indicate that presence of thiols does not prevent
the formation of the DielsꢀAlder adduct but require somewhat
longer irradiation.
To explore the reactivity of oNQMs toward other important
families of electron-rich polarized alkenes, that is, enamines, we
irradiated 3-(hydroxymethyl)-2-naphthol (2a) in the presence of
N-(1-isobutenyl)morpholine (13). Photochemically generated
oNQM 1a rapidly adds to enamine 13, resulting in quantitative
formation of 2-morpholinobenzochroman 14 (Scheme 5).
In aqueous solutions, adduct 14 undergoes rapid hydrolytic
cleavage of the amino substituent to produce 2-hydroxybenzochro-
man (5f, Scheme 6). The lifetime of 14 in aqueous solution at pH
∼ 7 is τ = 21.2 ( 0.6 min. Thus, enamines can be also employed in
the photoinduced DielsꢀAlder click ligation if the substrate or label
is attached to the R- or β-carbon atoms of the enamine.
Reactive oNQMs 1 can be efficiently intercepted by good
nucleophiles, such as the azide ion and thiols. The bimolecular
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Scheme 8
Scheme 9
Another concern for the application of DielsꢀAlder photoclick
labeling in live organisms is the potential cytotoxicity of oNQMs 1
and/or their precursors 2. In fact, many o-quinone methide-
producing compounds are known to be cytotoxic, mostly due to
the ability of o-quinone methides to alkylate DNA bases.30
3-Hydroxy-2-naphthalenemethanol derivatives (2), on the other
hand, only form oNQMs photochemically because of a poor
leaving group in the benzylic position. The cytotoxicity of 2d
was assessed by incubating Jurkat cells in the presence of this
compound in the dark and under 350 nm irradiation. MTT assay
showed the same viability of treated and control cells.27
To demonstrate the utility of the photoinduced hetero-Diels-
alder reaction in the ligation experiments, we have prepared
tyrosine-vinyl ethers conjugates 3e and 3f.27 In the first one (3e),
the payload is attached to vinyl ether moiety via the oxygen atom
(Scheme 8). Irradiation of oNQM precursor 2a with 300 nm light
in the presence of 3e resulted in the rapid formation of adduct 5g
in 94% isolated yield (Table 5).
The label or second component for ligation has to be attached
to the naphthalene ring of the oNQM precursor 2 via an
appropriate linker (Scheme 1). We have demonstrated that
various functional groups can be attached via a triethyleneglycol
(TEG)-linker to 3,8-dihydroxy-2-naphthalenemethanol (2bꢀg,
Scheme 9).27 Introduction of a thriethylene glycol substituent in
8-position of a oNQM precursor does not affect its photochem-
istry. Thus, irradiation of a solution of the triethylene glycol ether
of 3,8-dihydroxy-2-naphthalenemethanol (2d) in 50% aqueous
acetonitrile in the presence of ethyl vinyl ether (3a) quantita-
tively produces adduct 5i (Table 5). Product 5i was isolated in
96% yield and fully characterized. Since azide groups are photo-
reactive at shorter wavelengths, we have investigated the photo-
stabilty of the oNQM precursor 2e that contains azido
functionality at 300 nm. HPLC analysis clearly indicates that
there is no change in the concentration of starting material after
30 min of irradiation at 300 nm in aqueous solution. This control
experiment clearly demonstrates that the azide group remains
intact at the DielsꢀAlder photoclick conditions. Such bifunc-
tional linker molecules allow for combining light-directed liga-
tion with conventional azide-acetylene click chemistry.
Table 5. Yields of Cycloaddition Products upon Photolysis of
oNQM Precursors 2a and 2d in the Presence of Various Vinyl
Ethersa
alkene
oNQM precursor
benzochroman
yield (% conversion)
3e
3f
2a
2a
2b
5g
5h
5i
94 ( 4 (>99)
96 ( 2 (>99)
96 ( 2 (>99)
3a
a In 50% MeCNꢀ0.01 N phosphate buffer mixture. [alkene] = 1.5 mM;
[oNQM precursor] = 1 mM. Irradiation at 300 nm.
The link between the payload and benzochroman moiety in 5g is
hydrolytically labile and can be cleaved by incubating the adduct (5g)
in an acidic solution. For the applications that require permanent
ligation, the payload can be attached to a vinyl ether moiety through
either the R-orβ-carbon atoms. Thus, we prepared methyl vinyl ether
derivative 3f, where a tyrosine unit is attached to β-carbon of the vinyl
ether through a short linker (Scheme 8). Irradiation of 50% aqueous
acetonitrile solution of 3-(hydroxymethyl)-2-naphthol (2a) at
300 nm in the presence of 1.5 equiv of vinyl ether 3f resulted in
the quantitative formation of ligation product 5h (Table 5). The
resulting linkage between the benzochroman and tyrosine is stable in
both acidic and basic solutions. Cycloadducts 5g and 5h are stable
under 300 or 350 nm irradiation.
’ CONCLUSION
The photochemical DielsꢀAlder cycloaddition described in
this report offers a new platform for light-induced ligation, which
has potential to serve as orthogonal click system to the widely
employed acetylene-azide click chemistry. Irradiation of 3-
(hydroxymethyl)-2-naphthols produces 2-napthoquinone-3-
methides (oNQMs), which react rapidly with vinyl ethers to
produce 2-alkoxybenzochromans. oNQM precursors and vinyl
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’ AUTHOR INFORMATION
Corresponding Author
vpopik@chem.uga.edu
(16) Delgado, J. L.; de la Cruz, P.; Langa, F.; Urbina, A.; Casado, J.;
Lopez Navarrete, J. T. Chem. Commun. 2004, 1734.
’ ACKNOWLEDGMENT
(17) (a) Latham-Timmons, H. A.; Wolter, A.; Roach, J. S.; Giare, R.;
Leuck, M. Nucleosides, Nucleotides Nucleic Acids 2003, 22, 1495. (b) Sun,
X.-L.; Stabler, C. L.; Cazalis, C. S.; Chaikof, E. L. Bioconjugate Chem.
2006, 17, 52. (c) Proupin-Perez, M.; Cosstick, R.; Liz-Marzan, L. M.;
Salgueirino-Maceira, V.; Brust, M. Nucleosides, Nucleotides Nucleic Acids
2005, 24, 1075. (d) Yousaf, M. N.; Houseman, B. T.; Mrksich, M. Proc.
Natl. Acad. Sci. U.S.A. 2001, 98, 5992. (e) Dillmore, W. S.; Yousaf, M. N.;
Mrksich, M. Langmuir 2004, 20, 7223. (f) Rusmini, F.; Zhong, Z.; Feijen,
J. Biomacromolecules 2007, 3, 1775.
Authors thank Professor Geert-Jan Boons and Mbua Eric
Ngalle for the useful discussions and the analysis of phototoxicity
of o-napthoquinone precursors; NSF (CHE-0842590) and
Georgia Cancer Coalition for the support of this project.
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