Letter Cite This: Org. Lett. 2018, 20, 4178−4182
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7‑Hydroxy‑N‑Methylquinolinium Chromophore: A Photolabile Protecting Group for Blue-Light Uncaging Tetsuo Narumi,*,†,‡ Koichi Miyata,† Akitaka Nii,† Kohei Sato,† Nobuyuki Mase,†,§ and Toshiaki Furuta∥
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†
Course of Applied Chemistry and Biochemical Engineering, Department of Engineering, Graduate School of Integrated Science and Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan ‡ Green Chemistry Research Division, Research Institute of Green Science and Technology, Shizuoka University, Hamamatsu, Shizuoka 432-8561, Japan § Green Energy Research Division, Research Institute of Green Science and Technology, Shizuoka University, Hamamatsu, Shizuoka 432-8561, Japan ∥ Department of Biomolecular Science, Toho University, Funabashi, Chiba 274-8510, Japan S Supporting Information *
ABSTRACT: The development of the N-methyl-7-hydroxymethylquinolinium (N-Me-7-HQm) caging chromophore as a novel visible-light-sensitive photolabile protecting group is described. N-Me-7-HQm-caged compounds can be photoactivated by blue-light-emitting diode (LED) light (458 nm) with high photolytic efficiency, supporting applications to caging chemistry, and they also have sufficient water solubility and high resistance to spontaneous hydrolysis.
T
>450 nm having high photolytic efficiency remains highly challenging. As part of our research programs aimed at developing new photoactivatable tools, we now disclose the N-methyl-7hydroxyquinolinium (N-Me-7-HQm) caging chromophore as a novel PPG, sensitive to visible light. key to the success of the development of the N-Me-7-HQm photocage was Nmethylation of the 7-hydroxyquinoline chromophore. This modification allows access to visible-light absorbance and facile photoactivation by blue-light-emitting-diode (LED) light (458 nm) with high photolytic efficiency, excellent water solubility, and high resistance to spontaneous hydrolysis. At the onset of our studies, we set three criteria for the design of the new chromophore. First, the absorption maxima wavelength of the chromophore should be in the visible-light region (>400 nm) for activation. Second, the molar absorptivity at the absorption maxima should be considerable, ideally >10 000 M−1 cm−1, which is a great advantage for the photolytic efficiency that is quantitatively evaluated by the product of the photolysis quantum yield of a photolysis reaction and molar absorptivity. Third, we wanted the chromophore to have the potential for late-stage variation of the structure by elaboration such as functionalization of visiblelight sensitivity after the conjugation of targeted molecules. Consequently, we focused on the potential of 7-hydroxyquino-
he development of novel photolabile protecting groups (PPGs)1 with high levels of photolytic efficiency and hydrophilicity can provide attractive photoactivatable tools such as caged compounds to enable the spatiotemporal dynamic analysis of various chemicals, including biologically active compounds,2 gases,3 and inorganic species4 in living cells. With the goal of developing new caged compounds, we recently identified 8-azacoumarin-type PPGs5 that show clearly enhanced hydrophilicity and significant photolytic efficiency. Despite the promising character of 8-azacoumarin-type PPGs for caging chemistry, one of the drawbacks is the requirement for one-photon activation of ultraviolet light (250−400 nm), which is harmful to living cells and has low tissue penetration power. This is not only the problem for our 8-azacoumarin-type PPGs, but it is a long-standing problem of most reported photolabile protecting groups, resulting in the limited use of most caged compounds in vitro and not in vivo. An attractive approach to overcome these problems would be the use of longer-wavelength light, providing advantages such as lower phototoxicity and higher tissue penetration power than UV irradiation.6 Several caging chromophores with photoactivation capability by visible light have been reported,7 but only a limited number of PPGs can be photoactivated by wavelengths of >450 nm with high photolytic efficiency. These include ruthenium-bipyridine complexes,8 fluorescein analogues,9 coumarin derivatives,10 and boron dipyrromethene derivatives.11 Thus, the development of the visible lightsensitive PPGs that can be photoactivated with wavelengths © 2018 American Chemical Society
Received: May 12, 2018 Published: June 29, 2018 4178
DOI: 10.1021/acs.orglett.8b01505 Org. Lett. 2018, 20, 4178−4182
Letter
Organic Letters
Scheme 1. Synthesis of N-Methylquinolinium Derivatives 7−10
line (HQ)-type photocages such as 8-bromo-7-hydroxyquinoline-2-ylmethyl (BHQ)12 and 8-cyano-7-hydroxyquinoline-2ylmethyl (CyHQ),13 which enable one-photon activation with high quantum efficiency, adequate water solubility, and an sp2 nitrogen in the pyridine ring that is poised for further elaboration. Inspired by the utility of N-alkyl-7-hydroxyquinolinium derivatives as optical pH sensor14 and green fluorescent dyes,15 we envisioned that N-alkylation of the HQ cage could lead to a red shift of the absorption maximum from the UV region to the visible-light region and increase the molar absorptivity while maintaining the high quantum efficiency of the photolysis reaction and excellent water solubility. Our hypothesis was tested by N-alkylation of a simple substrate, such as 2-methylquinolin-7-ol (1),15 to evaluate the photophysical properties of several N-alkyl-quinolinium compounds 2−5 (see Table 1). We were pleased to find that
10) were synthesized from the corresponding alcohols using similar procedures.17 The photosensitivity of N-Me-7-HQm-OAc (7) was evaluated under photolysis in 10 μM PBS at pH 7.2 at 458 nm. Blue LED light at 458 nm (SugarCUBE LED Illuminatorblue with a fiber-optic light guide, Edmund Optics) was used as the light source. As expected, photolysis of N-Me-7-HQmOAc (7) gave the corresponding alcohol, N-Me-7-HQm−OH (11) (Figure 1). HPLC analysis of the photolysis reaction of 7
Table 1. N-Alkyl Effect on Photophysical Properties
compound
R
X
λmaxa (nm)
εmaxb (M−1 cm−1)
1 2 3 4 5
CH3 CH2Ph CH2CO2Et CH2CO2H
I Br Br Br
330 425 410 412 410
3200 12 300 19 100 11 000 12 200
a
Long wavelength of absorption maxima in PBS (0.1% DMSO). Molar absorptivity at absorption maxima.
b
simple modification of 1 resulted in the red shift of the absorption maxima from the UV region to the visible-light region. These structures absorb between 400 nm and 425 nm. Notably, N-methylation of 1 was the most effective for access to the longer wavelength: the absorption maximum of the Nmethyl derivative (2) shifted from 330 nm for 1 to 425 nm for 2. N-methylation was also effective in increasing the molar absorptivity: the molar absorptivity using blue light (λ = 458 nm) is given as ε458 = 12 300 M−1 cm−1 for 2, which is ∼4-fold higher than that of original compound 1. The N-alkylation effect on the improvement of photophysical properties is not limited to N-methylation: N-benzylation, N-(ethoxycarbonyl)methylation, and N-(carboxy)methylation are effective not only in delivering absorbance at the longer wavelength but also for the increase in the molar absorptivity. These results indicate that N-alkylation of 7-hydroxyquinoline would be a favorable modification for a visible-light-sensitive caging chromophore. Taking into account the absorption maxima at 425 nm and the high molar absorptivity of 2, the N-methylated 7-hydroxyquinolinium (N-Me-7-HQm) structure was selected for further development. Next, N-Me-7-HQm with a 2-acetoxymethyl functionality (N-Me-7-HQm-OAc, 7) was prepared from the acetate 612 in two steps (see Scheme 1). After N-methylation of the N atom of 6, removal of the TBDPS group by H2O at 80 °C, followed by preparative high-performance liquid chromatography (HPLC), gave N-Me-7-HQm-OAc (7). In order to evaluate the substituent effect on the N-methylated quinolinium structure, the 7-O-methyl derivative, N-Me-7-MQm-OAc (8), and 7-dimethylamino derivatives16 (N-Me-7-DMAQm) (9 and
Figure 1. HPLC analyses for the photolysis reaction at 458 nm of 7 in PBS buffer (pH 7.4, containing 0.1% DMSO).
showed that the peak corresponding to 7 disappeared with increasing irradiation time and the peak of 11 was generated at the same time. This result clearly indicates that the N-Me-7HQm cage can be used as a visible-light-sensitive photolabile protecting group. Figure 2 shows the time course of photolysis reactions of compounds 7−10, in terms of the consumption of the starting materials, as determined by HPLC analysis. The photolysis reaction of N-Me-7-HQm-OAc (7) follows a single-exponen-
Figure 2. Time courses of the photolysis reaction of 7−10. 4179
DOI: 10.1021/acs.orglett.8b01505 Org. Lett. 2018, 20, 4178−4182
Letter
Organic Letters Table 2. Selected Photophysical and Photochemical Properties of Compounds 7−10 compound
wavelength of absorption maxima, λmaxa (nm)
7 8 9 10
418 357 449 441
molar absorptivity at 458 nm, ε458 (M−1 cm−1)
time required to reach 90% conversion, t90 (s)
5200
42
7400 10 400
1151 334
quantum yield,b Φchem 0.045 0.0012 0.0028
photolytic efficiency, ε458•Φchem 234 8.9 29
a
In PBS (0.1% DMSO). bAs determined for photoreaction irradiated at 458 nm.
Scheme 2. Synthesis of N-Me-7-HQm-caged Amino Acids 14−18
tial decay, which is most rapid in the synthesized compounds: the time to reach 90% conversion (t90) is 42 s. A comparison of the time courses of the photolysis reactions revealed that the photosensitivity of the N-methyl quinolinium chromophore is significantly affected by the substituents. The O-methylated derivative (8) is not photoreactive under these conditions. Replacement of the 7-hydroxyl group of 7 with a 7dimethylamino group resulted in the significant decrease of the photosensitivity. On the other hand, introduction of the 4ethyl group led to the increase of the photosensitivity for 10, which is still lower than that of 7. The photolysis quantum yields (Φchem) of 7, 9, and 10 were calculated from the single decay curves, using the relationship Φchem =
1 I × 103εt 90
where I is the irradiation intensity in ein cm−2 s−1, determined by potassium ferrioxalate actinometry, and ε is the molar absorptivity of the irradiation wavelength in cm2 mol−1.18 Each reported quantum yield is the average of three independent experiments. In addition, the photolytic efficiencies were determined by the products of the photolysis quantum yield and molar absorptivity (ε458), and a value of 100 for Φchemε would be sufficient for caged compounds to have practical utility in biological experiments.19 These data are summarized in Table 2, with selected absorption data. The photolytic efficiency of 7 is 234, which exceeds the practical standard value for caged compounds. On the other hand, the photolytic efficiencies of DMAQm derivatives 9 and 10 are lower than that of 7, albeit with the longer absorption maximum and the larger molar absorptivity of 9 and 10 than that of 7, because of the significantly lower photolysis quantum yields of 9 and 10 (0.0012 for 9 and 0.0028 for 10). These results indicate that the N-Me-7-HQm photocage is a superior chromophore to the N-Me-7-DMAQm photocage and has potential to be useful for caging chemistry. Encouraged by these findings, we attempted to prepare NMe-HQm-caged amino acids using a three-step procedure, including esterification and subsequent N-methylation, followed by cleavage of other protecting groups. Although we had hoped that the N-Me-HQm cage could be applied to the caging group for the α-carboxyl group of glutamic acid (Glu), the N-Me-HQm ester of Glu was difficult to isolate in a pure form. HPLC analysis of the reaction mixture revealed that, while the desired N-Me-HQm ester of Glu was formed, the immediate decomposition of N-Me-HQm ester was observed, possibly due to the undesired hydrolysis.20 We reasoned that isolation of N-Me-HQm-caged compounds could be achieved if the compounds were highly resistant to hydrolysis. In an attempt to benefit from this, we prepared the N-Me-HQm carbamate of Glu (Scheme 2), leading to the successful realization that N-Me-HQm carbamates are suited to their isolation as pure forms by
preparative HPLC. CDI coupling of H-Glu(tBu)-OtBu·HCl with the alcohol (12) in the presence of Et3N gave the carbamate (13), which was alkylated with MeOTf, resulting in the corresponding glutamate with an N-Me-HQm cage. Treatment of the glutamate with TBAF for removal of the TBDPS group and TFA for removal of t-butyl esters gave the desired N-Me-HQm carbamate (14) in 49% isolated yield (three steps from 12) after purification by preparative HPLC. This strategy is also applicable to the preparation of N-MeHQm-caged compounds (14−18) from the corresponding tbutyl esters of Gly, GABA, Ala, and Lys, respectively.21 The photochemical properties of 14−18 were evaluated under photolysis in 10 μM PBS at 458 nm and are summarized in Table 3 with selected absorption data. Substitution of the ester group with the carbamate group for conjugation with amino acids did not result in a significant change in optical properties: the absorption maxima of 14−18 are ∼415 nm and the molar absorptivities at the absorption maxima are 7000− 14 000 M−1 cm−1. HPLC analyses revealed that photolysis reactions of N-Me-HQm-caged carbamates (14−18) proceeded quantitatively without formation of other byproducts with blue light (458 nm), and the values of photolytic efficiency are 96−173. The N-Me-HQm-caged Glu (14) has the largest photolytic efficiency (Φchemε458 = 173), which is lower than that of the commercially available rutheniumbipyridine complex of Glu (Rubi-Glu, 19) but is a sufficiently higher value to support practical use in caging chemistry. NMe-HQm-caged carbamates (15−17) of Gly, GABA, and Ala have similar photolytic efficiencies. In addition to the N-MeHQm photocage of the α-amino group, a photocage of the ε4180
DOI: 10.1021/acs.orglett.8b01505 Org. Lett. 2018, 20, 4178−4182
Letter
Organic Letters Table 3. Selected Photophysical and Photochemical Properties of Compounds 14−18 compound
long wavelength of absorption maxima, λmax (nm)
molar absorptivity at 458 nm, ε458a (M−1 cm−1)
time to reach 90% conversion, t90 (s)
quantum yield, Φchemb
photolytic efficiency, ε458•Φchem
14 15 16 17 18 19
413 415 416 416 416 450
4800 2500 2500 5400 2000 4000
72 85 85 57 100 22
0.036 0.044 0.046 0.025 0.048 0.068
173 110 115 135 96 272
a
In PBS (0.1% DMSO). bAs determined for photoreaction irradiated at 458 nm.
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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Young Scientists (A) from The Ministry of Education, Culture, Sports, Science and Technology, Japan.
amino group of Lys was also feasible: Fmoc-Lys(N-MeHQm)−OH 18 undergoes an efficient photolysis reaction without cleavage of the Fmoc group. We anticipate that the synthesized Fmoc-Lys(N-Me-HQm)−OH 18 will be a useful building block for the synthesis and visible-light control of peptides and related molecules. The water solubility of N-Me-HQm-caged compounds was found to be high. The saturated concentration of N-Me-HQmOAc (7) is up to 20 mM in PBS, which is ∼2−4-fold higher than those of the hydrophilic azacoumarin-type PPGs.22 Even N-Me-HQm-caged Glu (14) shows sufficient water solubility for practical use (up to 8.3 mM in pure water). Furthermore, N-Me-HQm-caged Glu (14) is quite stable under the dark conditions. HPLC monitoring of 14 revealed that no hydrolysis was observed for 24 h at room temperature. Even under the solvent-free dark conditions, no decomposition was observed over 2 weeks at room temperature. These features will be a great advantage in the handling of those compounds. In summary, we have developed a novel, visible-lightsensitive N-Me-7-HQm photocage by N-methylation of the 7hydroxyquinoline-type chromophore that offers access to the longer-wavelength absorption (>400 nm) and large molar absorptivity. The newly identified N-Me-7-HQm-caged compounds can be photoactivated by blue-LED light (458 nm), and also have practical photolytic efficiency in biological applications, in addition to sufficient water solubility and hydrolytic stability. The success of the late-stage upgrading of a chromophore in the synthetic sequence suggests that further functionalization of the caging chromophore will be possible and should aid in the rapid generation of structurally diverse libraries of visible-light-sensitive photocages.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01505. Experimental details, synthesis, and characterization data
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REFERENCES
for new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Tetsuo Narumi: 0000-0003-2412-4035 Notes
The authors declare no competing financial interest. 4181
DOI: 10.1021/acs.orglett.8b01505 Org. Lett. 2018, 20, 4178−4182
Letter
Organic Letters (12) Fedoryak, O. D.; Dore, T. M. Org. Lett. 2002, 4, 3419. (13) Davis, M. J.; Kragor, C. H.; Reddie, K. G.; Wilson, H. C.; Zhu, Y.; Dore, T. M. J. Org. Chem. 2009, 74, 1721. (14) Badugu, R.; Kostov, Y.; Rao, G.; Tolosa, L. Biotechnol. Prog. 2008, 24, 1393. (15) (a) Senda, N.; Miwa, Y.; Tanaka, J.; Momotake, A.; Arai, T. Chem. Lett. 2010, 39, 308. (b) Senda, N.; Momotake, A.; Arai, T. Heterocycles 2010, 81, 2343. (16) For a recent example of aminoquinoline-type, see: Tran, C.; Gallavardin, T.; Petit, M.; Slimi, R.; Dhimane, H.; Blanchard-Desce, M.; Acher, F. C.; Ogden, D.; Dalko, P. I. Org. Lett. 2015, 17, 402 and references cited therein. (17) See the Supporting Information for experimental details. (18) Tsien, R. Y.; Zucker, R. S. Biophys. J. 1986, 50, 843. (19) (a) Suzuki, A. Z.; Watanabe, T.; Kawamoto, M.; Nishiyama, K.; Yamashita, H.; Ishii, M.; Iwamura, M.; Furuta, T. Org. Lett. 2003, 5, 4867. (b) Quann, E. J.; Merino, E.; Furuta, T.; Huse, M. Nat. Immunol. 2009, 10, 627. (20) See the Supporting Information for the details. (21) See the Supporting Information for experimental details. (22) See the Supporting Information for the details.
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DOI: 10.1021/acs.orglett.8b01505 Org. Lett. 2018, 20, 4178−4182
