Chiral synthetic pseudopeptidic derivatives as triplet excited state quenchers

Article abstract of DOI:10.1016/j.tetlet.2009.06.047

The behavior of 6 pseudopeptidic models, synthesized by connecting different protected amino acids (Trp, Tyr, Phe, and Lys) with various diamino spacers, as quenchers of the triplet excited state of tiaprofenic acid (and its methyl ester), has been invest

Full text of DOI:10.1016/j.tetlet.2009.06.047

Tetrahedron Letters 50 (2009) 4859–4862
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chiral synthetic pseudopeptidic derivatives as triplet excited state quenchers
b,
Xavier J. Salom-Roig a, Jean Martínez a, M. Isabel Burguete b, Francisco Galindo b, , Santiago V. Luis
,
*
*
Miguel A. Miranda c, María C. Morant-Miñana d, Julia Pérez-Prieto d,
*
a Institut de Biomolécules Max Mousseron (IBMM), UMR 5247, Synthèses Stéréosélectives, Place Eugène Bataillon, Université Montpellier II, France
b Departamento de Química Inorgánica y Orgánica;Universitat Jaume I, Av. Sos Baynat, s/n, 12071 Castellón, Spain
c Departamento de Química/Instituto de Tecnología Química (UPV-CSIC), Universidad Politécnica de Valencia, Camino de Vera, s/n, 46071 Valencia, Spain
d Departamento de Química Orgánica / ICMOL, Universidad de Valencia, Av. Vicent Andrés Estellés, s/n, 46100 Burjassot, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The behavior of 6 pseudopeptidic models, synthesized by connecting different protected amino acids
(Trp, Tyr, Phe, and Lys) with various diamino spacers, as quenchers of the triplet excited state of tiaprofe-
nic acid (and its methyl ester), has been investigated. A series of quenching constants have been deter-
mined, which depend on the nature of the quencher and on the stereochemistry of the excited drug. A
significant degree of stereodifferentiation has been found for the peptidomimetic synthesized with Phe
and Tyr linked by a piperazine bridge. The obtained results support the utility of laser flash photolysis
(LFP) as a tool to investigate the interactions between photoexcited drugs and simple models of binding
sites in proteins.
Received 12 May 2009
Revised 2 June 2009
Accepted 5 June 2009
Available online 12 June 2009
Keywords:
Laser flash photolysis
Photochemistry
Triplet excited state
Quenching
Ó 2009 Elsevier Ltd. All rights reserved.
Synthetic models for the understanding of complex biological
processes are attracting considerable interest. Among them, pepti-
domimetics are emerging as a prominent type of mimics.1 In this
regard, the interaction of drugs with transport proteins, such as
serum albumins, has been modeled by means of abiotic conjugates
between amino acids and non-steroidal anti-inflammatory drugs
(NSAIDs),2 with the aim of explaining the reported phototoxic
and photoallergic side effects. From such studies, a more precise
view of the primary events taking place when a photoexcited drug
interacts with the binding pockets of human and bovine serum
albumins (HSA and BSA, respectively) is emerging.3 The laser flash
photolysis technique is a valuable tool in such investigations.2,4–6
In order to complete the picture, additional models are introduced
here based on the intermolecular deactivation of excited NSAID
derivatives by means of pseudopeptides. Two families of com-
pounds have been used, bearing either tyrosine (Chart 1) or trypto-
phan (Chart 2), due to the role played by these amino acids in
photochemical processes within proteins.2–4
quenching processes, where an important degree of stereodifferen-
tiation has been found in some cases.
The synthesis of tyrosine derivatives 1 and 2 is carried out in
two steps as depicted in Scheme 1. Thus, reaction between com-
mercial N-Boc-phenylalanine and an excess of the corresponding
diamine in DMF by using benzotriazole-1-yl-oxy-tris-pyrrolidino-
phosphonium hexafluorophosphate and hydroxybenzotriazole
(PyBOP/HOBt methodology)8 in the presence of N,N-diisopropyl-
ethylamine (DIEA) provided the corresponding monoacylated
products 7 and 8. These compounds were subsequently coupled
to N-Boc-tyrosine leading to pseudopeptidic models 1 and 2, which
were characterized by 1H NMR, 13C NMR, and ESI-MS.9
On the other hand, tryptophan derivatives 3–5 (Chart 2)
will be described elsewhere.10 Model 6 was synthesized for this
study analogously to 4, and its chemical characterization is given
herein.9
Laser flash photolysis11 of TPA and its derivatives in deoxygen-
ated dioxane yielded a transient absorption spectrum in the 300–
700 nm range, with maxima at 350 and 600 nm (lifetime ca.
The objective of this work is to evaluate the effect of the chem-
ical structure of the pseudopeptide (nature of the amino acid and
type of linker) on the rate constant for quenching of the triplet ex-
cited state of NSAIDs, using tiaprofenic acid and its methyl ester as
photoexcitable species possessing intrinsic chirality (Chart 3).7
Special attention has been paid to the stereochemistry of the
* Corresponding authors. Tel.: +34 964728239; fax: +34 964728214 (S.V.L.).
E-mail addresses: francisco.galindo@uji.es (F. Galindo), luiss@uji.es (S.V. Luis),
julia.perez@uv.es (J. Pérez-Prieto).
Chart 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.047

4860
X. J. Salom-Roig et al. / Tetrahedron Letters 50 (2009) 4859–4862
Chart 2.
of the excited state in the absence (s0) and in the presence (s) of a
quencher (Q) is related to the concentration of quencher ([Q]),
through the bimolecular quenching rate constant (kq).13
s0
=s
¼ 1 þ kq ꢁ s0½Qꢂ
ð1Þ
Noteworthy, triplet deactivation was clearly dependent on the
nature of the protected amino acids used as building blocks, the
type of spacer, and the stereochemistry of TPA or TPA-Me. Figure
2 shows two typical plots which illustrates the influence of the
type of linker on the reactivity of the excited triplet state.
Table 1 contains a summary of all quenching constants and ste-
reodifferentiation factors. From these data some conclusions can
be drawn:
Chart 3.
Rigid derivative 2 is more reactive than its flexible counterpart
1, especially in the case of TPA-Me. Here, stereodifferentiation fac-
tor reaches 1.36, a fairly high value for the intermolecular quench-
ing of a NSAID triplet state.5 This can be probably attributed to the
rigid nature of the linker, which allows for a better access of the
quencher to the excited chromophore and enhances chiral discrim-
ination between the two faces.
When comparing 1 with 3 as quenchers,5,12 it becomes evident
that the higher reactivity of the latter should be related to the low-
er oxidation potential of Trp as compared to Tyr,12b in accordance
with the proton/electron transfer nature of the quenching
process.5,12
As regards the comparisons within the series 3–6 a key factor
appears to be the possible formation of a salt bridge between the
carboxylic acid of TPA and the free amino group of the lysine res-
idue. The fact that TPA-Me is not quenched to such extent seems to
support this hypothesis.
This preferential binding for basic pseudopeptides is relevant in
the context of transport proteins HSA and BSA, where drugs like
tiaprofenic acid are known to bind polar regions in the binding
sites. In fact, the role of basic polar residues at the entrance of bind-
ing pockets in HSA and BSA is well established, as anchors of acidic
drugs through the formation of salt bridges.14 A similar effect has
been described for cyclooxygenases 1 and 2 (COX-1 and COX-
2).15 In this regard, compounds 4 and 6 could be seen as minimal-
istic models of binding sites in serum albumins and COX proteins,
potentially useful in future bioorganic studies.1
Scheme 1. Reagents and conditions: (a) PyBOP (1 equiv), HOBt (2 equiv), DIEA
(2 equiv) in anhydrous DMF, 24 h.
4 ls), which matched that previously reported for the benzoylthi-
ophene triplet.12 A typical example is shown in Figure 1A for
S(ꢀ)TPA-Me. Upon addition of 1–6, the triplet–triplet absorption
band decayed faster. In addition, a diminished transient absorption
was observed beyond 625 nm, the maximum in the visible region
shifted to 590 nm, and the ratio
DA350/DA600 became higher
(Fig. 1B and C). These changes are ascribed to the formation of ketyl
and phenoxy or indolyl radicals, concomitantly with decay of the
triplet excited state.12
The kinetics of the intermolecular interaction was analyzed by
means of the Stern–Volmer relationship (Eq. 1), where the lifetime
In summary, quenching of the triplet excited state of tiaprofenic
acid and its methyl ester by the six pseudopeptidic models 1–6
strongly depends on the quencher chemical structure (amino acid
composition and nature of the linker) and (to some extent) on the

X. J. Salom-Roig et al. / Tetrahedron Letters 50 (2009) 4859–4862
4861
Figure 2. Stern–Volmer plots for quenching of (+)-TPA-Me triplet, generated after
355 nm laser flash photolysis in dioxane, by pseudopeptide 1 or 2.
Table 1
Kinetic parameters for the quenching of photoexcited tiaprofenic acid (TPA) and its
methyl ester (TPA-Me) by pseudopeptidic compounds 1–6 in dioxane
Stereodifferentiationb
a
Quencher
Irradiated compound
kq
R(+)
S(ꢀ)
1
2
3
4
5
6
TPA
TPA-Me
2.0
1.1
1.6
1.1
1.25
1.00
TPA
TPA-Me
3.9
7.5
4.5
5.5
0.87
1.36
TPA
TPA-Me
3.9
4.3
4.1
3.6
0.94
1.19
TPA
TPA-Me
6.4
3.2
6.7
3.6
0.95
0.89
TPA
TPA-Me
3.1
3.7
3.8
4.3
0.79
0.87
TPA
TPA-Me
6.4
3.7
5.3
3.2
1.21
1.16
a
Units: 108 Mꢀ1 sꢀ1
Ratio = kqR(+)/kqS(ꢀ).
.
b
from MICINN (Ramón y Cajal Program). Technical support by
SCIC/UJI is also acknowledged.
References and notes
Figure 1. Representative example of laser flash photolysis measurements. Spectra
recorded 0.5 (top trace), 1.0, 2.0, and 4.0 (bottom trace) s after the laser pulse
(355 nm). (A) Transient absorption spectra in deaerated dioxane of S-(ꢀ)-TPA-Me
(0.7 mM); (B) idem, in the presence of compound 1 (2.4 mM); (C) idem, in the
presence of compound 2 (0.5 mM). Note the analogous quenching by 1 and 2 but
using markedly different quencher concentrations.
1. (a) Eichler, J. Curr. Opin. Chem. Biol. 2008, 12, 707–713; (b) Robinson, J. A. Acc.
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l
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Org. Chem. 2004, 69, 374–381; (b) Miranda, M. A.; Lahoz, A.; Boscá, A.; Metni,
M. R.; Abdelouahab, F. B.; Castell, J. V.; Pérez-Prieto, J. Chem. Commun. 2000,
2257–2258; (c) Miranda, M. A.; Lahoz, A.; Martínez-Máñez, R.; Boscá, F.;
Castell, J. V.; Pérez-Prieto, J. J. Am. Chem. Soc. 1999, 121, 11569–11570.
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stereochemistry of the excited drug. These results also support la-
ser flash photolysis as a tool to investigate the interactions be-
tween photoexcited drugs and peptidomimetic models.
4. (a) Jiménez, M. C.; Miranda, M. A.; Vayá, I. J. Am. Chem. Soc. 2005, 127, 10134–
10135; (b) Vayá, I.; Bueno, C. J.; Jiménez, M. C.; Miranda, M. A. ChemMedChem
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Acknowledgments
Financial support from the Spanish MICINN (CTQ2006-15672-
C05-02, CTQ2008-02907-E/BQU, CTQ2008-06777-C02-01), GVA
(projects GV/2007/277, ARVIV/2007/079, ARVIV/2007/081, Prome-
teo/2008/090), Fundació Caixa Castelló-UJI (projects P1ꢃ1B2004-
13, P1ꢃ1A2007-05) is acknowledged. F.G. thanks for the support
9. N-(tert-Butoxycarbonyl)-N-(2-{[N-(tert-butoxycarbonyl)-L-phenylalanyl]amino}
ethyl)-
(br, 1H), 7.93 (br, 1H), 7.29–7.21 (m, 5H), 7.04 (d, 2H, J = 9 Hz), 6.90 (d, 1H,
L
-tyrosinamide) (1): 1H NMR (300 MHz; DMSO-d6) d 9.20 (br, 1H), 7.98

4862
X. J. Salom-Roig et al. / Tetrahedron Letters 50 (2009) 4859–4862
J = 9 Hz), 6.79 (d, 1H, J = 9 Hz), 6.68 (d, 2H, J = 9 Hz), 4.14–4.04 (m, 2H), 3.11–
32.8, 29.2, 29.1, 28.7, 28.6, 26.2, 26.0, 23.1; ESIMS m/z 631.4 [M+H]+; FAB(+) m/z
631.6 [M+H]+; HRMS calcd for C33H55N6O6 [M+H]+: 631.4183, found 631.4195.
10. Burguete M. I.; Fawaz, G.; Galindo, F.; Izquierdo, M. A.; Luis, S. V.; Martínez, J.;
Salom-Roig, X. J. Tetrahedron, manuscript submitted for publication.
11. Laser flash photolysis system was based on a pulsed Nd:YAG laser, using
355 nm as exciting wavelength. The single pulses were ca. 10 ns duration and
the energy was ca. 15 mJ/pulse.
2.66 (m, 8H), 1.34, 1.26 (2 ꢁ s, 18H) 13C NMR (300 MHz; DMSO-d6) d 172.8,
172.6, 156.7, 156.1, 139.2, 131.0, 130.1, 129.1, 128.9, 127.1, 115.7, 78.9, 78.8,
57.0, 56.7, 38.6, 38.4, 37.9, 29.0, 28.8; ESIMS m/z 571.3 [M+H]+; FAB(+) m/z
571.5 [M+H]+; HRMS calcd for C30H43N4O7 [M+H]+: 571.3132, found
571.3134; N-(tert-butoxycarbonyl)-N-(2-{[N-(tert-butoxycarbonyl)-
L-phenylalanyl]
amino}piperazin-1-yl)-
L
-tyrosinamide) (2): 1H NMR (300 MHz; DMSO-d6) d 9.24
(br, 1H), 7.28–7.03(m, 9H), 6.71(br,2H), 4.61–4.49(m, 2H), 3.40–3.16(m, 4H), 2.88–
2.70 (m, 4H), 1.37 (s, 9H), 1.35 (s, 9H). 13C NMR (300 MHz; DMSO-d6) d 171.1, 170.9,
156.8, 155.9, 138.5, 131.2, 130.4, 129.0, 128.5, 128.3, 127.3, 115.8, 79.1, 79.0, 52.3,
52.1 45.8, 45.5, 39.7, 37.3, 28.1; ESIMS m/z 597.4 [M+H]+; FAB(+) m/z 597.5 [M+H]+;
HRMS calcd for C32H45N4O7 [M+H]+: 597.3288, found 597.3291; N-(tert-
12. (a) Pérez-Prieto, J.; Stiriba, S.-E.; Boscá, F.; Lahoz, A.; Domingo, L. R.; Mourabit,
F.; Monti, S.; Miranda, M. A. J. Org. Chem. 2004, 69, 8618–8625; (b) Pérez-Prieto,
J.; Boscá, F.; Galian, R. E.; Lahoz, A.; Domingo, L. R.; Miranda, M. A. J. Org. Chem.
2003, 68, 5104–5113.
13. Turro, N. J. Modern Molecular Photochemistry; University Science Books:
Sausalito, CA, 1991.
14. (a) Banerjee, T.; Singh, S. K.; Kishore, N. J. Phys. Chem. B 2006, 110, 24147–
24156; (b) Ghuman, J.; Zunszain, P. A.; Petitpas, I.; Bhattacharya, A. A.; Otagiri,
M.; Curry, S. J. Mol. Biol. 2005, 353, 38–52.
butoxycarbonyl)-N-(6-{[N2-(tert-butoxycarbonyl)-
L-lysyl]amino}hexyl)-L-(tryptopha-
namide) (6): 1H NMR (300 MHz; CDCl3) d 9.61 (br, 1H), 7.63 (d, 1H, J = 6 Hz), 7.33
(d,1H, J = 6 Hz), 7.15–7.03 (m, 3H), 6.29 (br, 1H), 5.55 (br, 2H), 4.45 (br, 1H), 4.12 (br,
1H), 3.28–3.07 (m, 6H), 2.62 (m, 2H), 2.40 (br, 2H), 1.77–1.02 (m, 14H), 1.43 (s, 9H),
1.40 (s, 9H); 13C NMR (300 MHz; CDCl3) d 172.9, 172.3, 156.3, 155.9, 136.8, 127.8,
123.8, 122.1, 119.6, 119.1, 111.8, 110.6, 80.3, 80.2, 77.6, 55.7, 54.9, 41.7, 39.4, 39.2,
15. Blobaum, A. L.; Marnett, L. J. J. Med. Chem. 2007, 50, 1425–1441.