Spatially close porphyrin pair linked by the cyclic peptide Gramicidin S

Article abstract of DOI:10.1039/a902774b

Two porphyrins were attached to the cyclic decapeptide Gramicidin S and its analogs via the side chain amide bonds and the solvent-dependent molecular structure was characterized by various spectroscopic methods.

Full text of DOI:10.1039/a902774b

Spatially close porphyrin pair linked by the cyclic peptide Gramicidin S
Toru Arai,*a Naoki Maruo,a Yuko Sumida,a Chie Korosuea and Norikazu Nishino*b
a Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu,
Japan, 804-8550. E-mail: arai@che.kyutech.ac.jp
b Institute for Fundamental Research of Organic Chemistry, Kyushu University, Higashi, Fukuoka, Japan, 812-8581
Received (in Cambridge, UK) 7th April 1999, Accepted 23rd June 1999
Two porphyrins were attached to the cyclic decapeptide
Gramicidin S and its analogs via the side chain amide bonds
and the solvent-dependent molecular structure was charac-
terized by various spectroscopic methods.
d 8.06/7.61 (4H each) and d 7.79/7.23 (8H each) [Fig. 2(a)]. The
methyl signals of the tolyl groups also appeared as two sets, d
2.66 (6H) and 2.39 (12H). Compared with the monomeric
porphyrin [Fig. 2(b)], apparently four tolyl groups among six in
2b were high field shifted probably because of the ring current
effect of the other porphyrin. The tolyl group near the other
porphyrin (10-position, the inner tolyl groups in Fig. 3) may be
high field shifted, but those at the 15- and 20- positions may be
not. The tolyl signals at d 7.79 and 7.23 were broad at 303 K
[Fig. 2(a)] and were sharp at 333 K (data not shown), which
implied restricted rotation of the porphyrin ring along the
porphyrin–peptide axis. The signals of the 10- and 20-positions
might be averaged and showed a broadened signal in the higher
field. These facts suggest that the two porphyrins are spatially
close. Fig. 3 shows the molecular structure of 2b (bis-zinc
complex) built by CAChe® MM2, adopting the crystal structure
for the peptide moiety.4a,5 This model also suggests that the two
porphyrins are spatially close, and moreover, the left-twisted
chiral orientation of the porphyrins (see below).
In CH2Cl2 1b, 2b and 3b (1.6 mM concentration, determined
by the amino acid analysis) showed strong CD signals (data not
shown), for instance 2b, [q]min and [q]max at 424 and 416 nm
([q]M = 2168 000 and 97 000 deg cm2 dmol21, respectively).
These CD signals were assigned to the split Cotton effects via
the exciton coupling of the two porphyrins.6 The (2+) sign of
the couplets indicated the left-twisted orientation of the
porphyrins, which was also suggested by the CAChe® calcula-
tion (Fig. 3). The intensity of the CD signals were 1b < 2b >
3b. The drastic difference between 2b (Orn) and 3b (Lys)
suggested that the flexible Lys side chains caused the loose
structure of 3b.
Since the elucidation of a natural system with a number of
tetrapyrroles,1 such porphyrin assemblies have attracted much
attention. To understand the function of the multi-porphyrin
systems, many porphyrin arrays with different linker archi-
tecture have been studied as models.2 So far, the porphyrin units
have often been connected by tight linkers which determine
their distance and orientation.2b–d However, a peptide-linked
porphyrin array may be interesting because of its likeness to the
natural system with a flexible peptide moiety.3 The flexible
linker may afford a porphyrin array wherein structural changes
reflect its surroundings such as solvents.
Gramicidin S [cyclo(-Val-Orn-Leu-d-Phe-Pro-)2, GS, Fig. 1]
is a natural cyclic decapeptide with a pair of antiparallel b-
sheets and two b-turns.4 GS has often been modified without
changing its rigid cyclic conformation in various solvents.4b,c
The incorporation of porphyrins to the Orn residues facing each
other may afford a defined porphyrin dimer. Here we report the
syntheses and the spectroscopic characterisation of the por-
phyrin array attached on this rigid cyclic decapeptide and its
analogs.
The cyclic decapeptides with side chain Z-protection (1a, 2a
and 3a) were synthesized via the cyclisation–cleavage method
on the oxime resin in 74–84% yield.4c After Z-deprotection with
H2–Pd/C,
5-(p-carboxyphenyl)-10,15,20-tritolylporphyrin
(2 equiv.) was coupled with PyBOP–HOBt in 70–84% yield.
The products with two porphyrins (1b, 2b and 3b) were
1
characterized with FAB-HIMS and H NMR (1D and 2D)
The Cotton effect for the porphyrins on GS was solvent-
dependent (Fig. 4). The CD spectra of 2b(Zn2) in toluene (1.6
mM) showed a split Cotton effect at 435, 427 and 418 nm ([q]M
= 2951 000, 1275 000 and 2234 000 deg cm2 dmol21). The
CD signals were weaker in other solvents, for instance in
spectra.
In the 1H NMR spectrum of 2b in DMSO-d6, the a-amide NH
signals appeared at d 8.93 (d-Phe), 8.80 (Leu), 8.50 (Orn) and
7.30 (Val) at 303 K (data not shown). Both the chemical shifts
and their temperature dependency (28.3, 22.6, 24.5 and 22.0
ppb K21, respectively) were close to those of GS.4c These facts
suggested the intramolecular hydrogen bonding between the
NH and CO groups of Leu and Val residues (see Fig. 1), i.e. the
conformational similarity of 3b to GS. The attachment of
porphyrins on the Orn positions did not change the peptide
conformation. Such phenomena were also confirmed in the 1H
NMR study in CDCl3.
MeOH, [q]430
=
2803 000 and [q]422
=
594 000
deg cm2 dmol21. The intensity of the Cotton effect was in the
order toluene > MeOH > trimethyl phosphate > TFE >
CH2Cl2 > pyridine. These facts may indicate that the two
The striking feature of the 1H NMR spectrum of 2b was that
the tolyl groups of porphyrins appeared as two sets of doublets,
Fig. 2 1H NMR spectra of (a) 2b and (b) 5-(p-methoxycarbonylphenyl)-
10,15,20-tritolylporphyrin in DMSO-d6 (303 K).
Fig. 1 Structure of Gramicidin S (GS) and its analogs.
Chem. Commun., 1999, 1503–1504
1503

10,15,20-tritolylporphyrin was coupled using PyBOP–HOBt
(quantitative). Then, Fmoc was removed with piperidine and
zinc 5-(p-carboxyphenyl)-10,15,20-tritolylporphyrinate was
coupled. After HPLC purification, 2b(FbZn) was isolated in
60% yield (2 steps), and characterized with FAB-HIMS, HPLC
1
and H-NMR. The visible spectrum of 2b(FbZn) (data not
shown) was close to the average of 2b (bis free-base) and
2b(Zn2), suggesting no interaction of the two porphyrins in
2b(FbZn) in the Q-state. However, the Soret band was
somewhat broadened, suggesting a weak interaction in the B-
state.2a
Upon exciting 2b(FbZn) in toluene at 552 nm, the emission
appeared at 608, 655 and 723 nm with an intensity ratio of
14+58+28. The ratio of e552 of 2b (bis free-base) and 2b(Zn2)
was 40+60, i.e. 40% of the excitation light at 552 nm was
absorbed by the free-base component of 2b(FbZn) and 60% by
the zinc porphyrin. From the comparison with the fluorescence
spectra of 2b (lem = 655, 719 nm, 74+26) and 2b(Zn2) (lem
=
608, 652 nm, 46+54), intramolecular quenching of the excited
state zinc porphyrin by the free-base porphyrin was suggested.
The energy transfer from excited Zn porphyrin to free-base
smoothly occurred intramolecularly, because these two por-
phyrins were in close proximity. The quenching efficiency was
calculated to be ca. 82%. The same experiment performed in
CH2Cl2 showed that 65% of the energy of the excited zinc
porphyrin in 2b(FbZn) was quenched by the free-base
porphyrin. This semi-quantitative fluorescence experiment
might suggest that the orientation of porphyrins differs in
solvents, as suggested by CD and UV spectroscopy. However,
the different energy transfer efficiencies in toluene and in
CH2Cl2 could be due to a single solvent effect, as one of the
reviewers pointed out.
Fig. 3 CAChe® MM2 generated structure of 2b(Zn2).
We thank Professor Hideo Akisada at Kyushu Kyoritsu
University for CD measurements. The molecular modeling
study was partially done by Dr Masako Fujiwara at JOEL,
which is gratefully acknowledged.
Notes and references
1 G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-
Lawless, M. Z. Papiz, R. J. Cogdell and N. W. Isaacs, Nature, 1995, 374,
517.
2 (a) R. L. Brookfield, H. Ellul, A. Harriman and G. Porter, J. Chem. Soc.,
Faraday Trans. 2, 1986, 82, 219; (b) H. Kurreck and M. Huber, Angew.
Chem., Int. Ed. Engl., 1995, 34, 849; (c) A. Osuka, N. Mataga and T.
Okada, Pure Appl. Chem., 1997, 69, 797; (d) S. I. Yang, R. K. Lammi, J.
Seth, J. A. Riggs, T. Arai, D. Kim, D. F. Bocian, D. Holten and J. S.
Lindsey, J. Phys. Chem. B, 1998, 102, 9426 and references cited
therein.
3 (a) H. Tamiaki, K. Nomura and K. Maruyama, Bull. Chem. Soc. Jpn.,
1993, 66, 3062; (b) T. Arai, K. Takei, N. Nishino and T. Fujimoto, Chem.
Commun., 1996, 2133; (c) H. Y. Liu, J. W. Huang, X. Tian, X. D. Jiao,
G. T. Luo and L. N. Ji, Chem. Commun., 1997, 1575.
4 (a) G. Némethy and H. A. Sheraga, Biochem. Biophys. Res. Commun.,
1984, 118, 643; (b) N. Nishino, T. Arai, J. Hayashida, H. I. Ogawa, H.
Yamamoto and S. Yoshikawa, Chem. Lett., 1994, 2435; (c) T. Arai, T.
Imachi, T. Kato, H. I. Ogawa, T. Fujimoto and N. Nishino, Bull. Chem.
Soc. Jpn., 1996, 69, 1383.
5 To construct the molecular model of 2b(Zn)2, the structure of GS
[ref. 4(a)] was first taken into MolSkop®, then translated to the CAChe
software.
6 X. Huang, B. H. Rickman, B. Borhan, N. Berova and K. Nakanishi,
J. Am. Chem. Soc., 1998, 120, 6185 and references cited therein. See also
ref. 4(b).
7 C. Hunter, From Simplicity to Complexity in Chemistry–and Beyond, ed.
A. Müller, A. Dress and F Vögtle, Vieweg, 1996, pp. 113–126.
8 M. Kasha, H. R. Rawls and M. A. El-Bayoumi, Pure Appl. Chem., 1965,
11, 371; T. Nagata, A. Osuka and K. Maruyama, J. Am. Chem. Soc., 1990,
112, 3054.
Fig. 4 CD spectra (1.6 mM for peptide) of 2b(Zn2) in (a) toluene, (b)
CH2Cl2, (c) MeOH, (d) TFE, (e) trimethyl phosphate and (f) pyridine.
porphyrins in 2b(Zn2) were in close proximity in toluene, and
the distance differed in the solvents. The different assembling
ability of porphyrins in these solvents might cause different CD
in the solvents, probably because of the different stabilization
effect of the solvent for the p–p interaction. The details of the
p–p interaction of the porphyrins are not clear yet, however, an
electrostatic model may account for the tendency for porphyrin
assembly in nonpolar solvents.7 The UV spectra also supported
the different orientation of the two porphyrins in these solvents.
The lmax of 2b(Zn2) in toluene (424 nm) was a little red-shifted
from that of zinc tetratolylporphyrin (421 nm), which might be
interpreted by the edge-to-edge interaction of the two p-
systems.8 However, such a shift of the lmax was not observed in
the other solvents investigated in the CD study, probably
because of the weak interaction of the porphyrins.
Anyway, the porphyrins on GS showed different CD, i.e.
different orientations in various solvents. This interesting fact
was further confirmed by steady-state fluorescent spectrome-
try.2a,3a For this purpose, 2b(FbZn) with one free-base
porphyrin and one zinc porphyrin was synthesized. GS with
different side-chain protection (Z and Fmoc) was synthesized in
the same way as 2a. After selective Z-deprotection by TFA–
thioanisole–m-cresol (75% yield),9 5-(p-carboxyphenyl)-
9 H. Yajima, Y. Minamitake, S. Funakoshi, Y. Hirai and T. Nakajima,
Chem. Pharm. Bull., 1981, 29, 1752.
Communication 9/02774B
1504
Chem. Commun., 1999, 1503–1504