Syntheses of fluorescence-labeled artificial leaf-opening substances, fluorescent probe compounds useful for bioorganic studies of nyctinasty

Article abstract of DOI:10.1016/S0040-4039(00)00394-4

In a previous paper, the syntheses of potassium galactolespedezate (1) and potassium galactoisolespedezate (2), artificial leaf-opening substances useful for bioorganic studies of nyctinasty were reported. The fluorescent probe compounds, fluorescence-labeled galactoisolespedezates (3, 10, 13), designed on the basis of 1 and 2, were prepared. In particular, compound 3 was bioactive at 5 x 10^-5 M, one-fiftieth as strong as the natural leaf- opening substance. This fluorescent probe would be useful for bioorganic studies of nyctinasty. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00394-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3433–3436
Syntheses of fluorescence-labeled artificial leaf-opening
substances, fluorescent probe compounds useful for bioorganic
studies of nyctinasty
Minoru Ueda, Yoshiyuki Sawai and Shosuke Yamamura ∗
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Yokohama 223-8522, Japan
Received 9 February 2000; revised 28 February 2000; accepted 3 March 2000
Abstract
In a previous paper, the syntheses of potassium galactolespedezate (1) and potassium galactoisolespedezate
(2), artificial leaf-opening substances useful for bioorganic studies of nyctinasty were reported. The fluorescent
probe compounds, fluorescence-labeled galactoisolespedezates (3, 10, 13), designed on the basis of 1 and 2, were
prepared. In particular, compound 3 was bioactive at 5×10−5 M, one-fiftieth as strong as the natural leaf-opening
substance. This fluorescent probe would be useful for bioorganic studies of nyctinasty. © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: plants; natural products; biologically active compounds; fluorescent probe.
Most leguminosae plants close their leaves in the evening, as if to sleep, and open them in the morning.1
This is called nyctinasty, and such a circadian rhythmic movement has been known to be controlled by
their biological clocks.2 Recently, we have identified several bioactive substances that regulate this leaf-
movement.3–16 These bioactive substances can be used for probe compounds which would be highly
useful for the purification of their receptors to enable bioorganic studies of nyctinasty.
Investigation of the site where bioactive substances are perceived at the cellular level is an essential
step towards the isolation and identification of the receptor molecule. A fluorescence-labeled bioactive
substance is most suitable for the study of this problem, because the fluorescence-labeled compounds
are widely used for the identification of receptor molecules.17–19 In this paper, we report the syntheses
of fluorescence-labeled leaf-movement factors designed on the leaf-opening substance of Lespedeza
cuneata G. Don, potassium lespedezate (4) and potassium galactoisolespedezate (5).4,5
∗
Corresponding author. Fax: 81 45 563 5967; e-mail: yamamura@chem.keio.ac.jp (S. Yamamura)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00394-4
tetl 16674

3434
The most important problem involved in the design of fluorescence-labeled leaf-movement factors
is the unstability of 4 and 5 in the plant body, which are easily hydrolyzed by β-glucosidase. Thus,
we designed a probe compound based on the structure of an artificial leaf-opening substance, potassium
galactolespedezate (1) and potassium galactoisolespedezate (2), which could not be hydrolyzed in a plant
body.20,21
The introduction of a large fluorescent functional group such as FITC (fluorescein-5-isothiocyanate) to
artificial leaf-opening substances requires careful consideration of the structure–activity relationship in 4
Scheme 1. Synthetic route of the fluorescence-labeled potassium galactoisolespedezate (13)

3435
Table 1
Bioactivities of fluorescent probe compounds compared with 1, 2, 4 and 5
and 5. Previous studies showed that the substituents at the phenolic hydroxyl group of 4 and 5 as well as
the carboxyl group led to a decrease of activity to 10−3 or 10−4 M.5 However, considering the bioactivity
of 1 and 2, the substitution of the sugar moiety in 4 and 5, from glucose to galactose, did not affect the
bioactivity at all. From these results, it is expected that the introduction of a large fluorescent functional
group in the hydroxyl group at the 60-position of the galactose moiety would not weaken the bioactivity
of 1 and 2 to any extent. Moreover, because of the resistance to the esterase in a plant body, an amide
bond would be better than an ester bond to connect the FITC group with 1 and 2. Thus, to introduce an
FITC group at the 60-position of 1 (or 2), we should convert the hydroxy group on the 60-position of 1
(or 2) into an amino group.
Compound 6 was deprotected with sodium methoxide, converted into its tosylate 7, and then acetylated
to give 8 (Scheme 1). The resulting 8 was treated with sodium azide, and then reduced by catalytic hydro-
genation, and the resulting amine 9 was coupled with FITC. After deprotection, the fluorescence-labeled
potassium galactoisolespezate (10) was derived with a trace amount of potassium galactolespedezate
(11). However, 10 showed leaf-opening activity at 5×10−4 M, which is one-five hundredth as strong as 1
and 2 (Table 1). The decrease of bioactivity could be due to the steric hindrance of the large fluorescent
functional group; thus, the insertion of some linker such as glycine between the FITC group and the
artificial leaf-opening substance would be effective for the improvement of bioactivity.
Scheme 1 shows the synthetic route of fluorescence-labeled potassium galactoisolespedezate (13).
Coupling of amine 9 with Z-Gly was carried out using WSC in DMF to give 12. Compound 12 was
deprotected with Pearlmann’s catalyst, coupled with FITC reagent, and treated with KOH to give 13.
The bioactivity of 13 was improved up to five times the strength of 10, thus providing the effectiveness
of the spacer (Table 1).
Scheme 2. Synthetic route of the fluorescence-labeled potassium galactoisolespedezate with triglycine-linker 3

3436
Finally, we synthesized the third fluorescent probe with a linker of glycylglycylglycine. According to
the same procedure as that of 13, using Z-Gly-Gly-Gly instead of Z-Gly, 3 could not be obtained because
of the reduction of the enol double bond in catalytic hydrogenation. Thus, we changed the protective
group of amine from a benzyloxycarbonyl to a tert-butoxycarbonyl group. Glycylglycylglycine (14) was
coupled with (Boc)2O and the resulting protected tripeptide was coupled with 9 to give 15 (Scheme 2).
Compound 15 was further coupled with FITC, then deprotected to give 3.22 The leaf-opening activity
of fluorescent probe compound 3 was improved up to 5×10−5 M. The bioactivity of 3 was one-fiftieth
as strong as that of the natural product; thus, 3 would be useful as a molecular probe for the bioorganic
studies of nyctinasty.
Acknowledgements
We are indebted to the Ministry of Education, Science, Sports and Culture (Japan) for a Grant-in-
Aid for Scientific Research on Special Promotion Research No. 09101001, the Asahi Glass Foundation,
and a Pioneering Research Project in Biotechnology given by the Ministry of Agriculture, Forestry and
Fisheries for financial support.
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22. Fluorescence-labeled galactoisolespedezate (3): 1H NMR (400 MHz, CD3OD, 30°C) δ 8.15 (1H, d, J=2.0 Hz), 7.75 (1H,
dd, J=8.3, 2.0 Hz), 7.63 (2H, d, J=8.8 Hz), 7.02 (1H, d, J=8.3 Hz), 6.81 (1H, s), 6.77–6.70 (6H, m), 6.61 (2H, dt, J=8.7, 2.0
Hz), 4.85 (1H, d, J=8.8 Hz), 4.85 (1H, d, J=17.1 Hz), 3.86 (1H, d, J=17.1 Hz), 4.30 (1H, d, J=17.1 Hz), 3.92 (1H, d, J=17.1
Hz), 3.79–3.74 (4H, m), 3.59–3.55 (2H, m), 3.40 (1H, d, J=13.7, 5.4 Hz), 3.24 (1H, dd, J=13.7, 8.3 Hz) ppm; 13C NMR
(400 MHz, CD3OD, 30°C)δ 184.9, 173.9, 173.3, 172.8, 172.1, 162.6, 159.6, 155.1, 150.2, 143.2, 133.9, 132.8, 131.2,
130.0, 127.7, 126.5, 123.4, 121.2, 116.8, 114.7, 112.4, 104.9, 104.4, 75.9, 75.5, 73.8, 71.2, 44.7, 44.2, 41.7, 33.8, 31.5,
24.5, 21.8, 15.2, 10.0; HR FAB MS (negative): [M−2K+H]− Found m/z 900.2019, C42H38O16N5S requires m/z: 900.2034;
20
IR (film) ν: 3294, 1736, 1661, 1639, 1606, 1586, 1546, 1513 cm−1; [α]D +58.0 (c 1.0, MeOH).