1264
Chemistry Letters Vol.35, No.11 (2006)
Introduction of F3-Threonine to the Hexapeptide, DSLET:
Investigation of the Effect of Fluorine Atoms toward Peptidic Conformations
Takamasa Kitamoto, Shunsuke Marubayashi, and Takashi Yamazakiꢀ
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588
(Received August 3, 2006; CL-060885; E-mail: tyamazak@cc.tuat.ac.jp)
WSC HCl,
HOBt H O
Introduction of trifluoromethyl-containing threonine to the
Boc−Phe
2
target hexapeptide, Tyr–D-Ser–Gly–Phe–Leu–Thr (DSLET),
led to the apparent conformational alteration due to the
electron-withdrawing effect of the CF3 group when compared
with the original DSLET on the basis of their NOESY spectra.
Boc−Phe−Leu−OBn
Et N, CH Cl
2
3
2
5
Leu−OBn TsOH
(97%)
CF CO H
WSC HCl, HOBt H O
3
2
2
Boc−Gly−Phe−Leu−OBn
CH Cl , 0 °C
Et N, CH Cl (96%)
2
2
3
2
2
6
Accomplishment of the human genomic base sequence
decoding in 2003 allowed us to obtain information about
proteins responsible for some specific diseases,1 which will lead
to clarification of the three-dimensional shapes of their active
sites with the aid of computational technique.2 This complete
analysis enables us to rationally design more effective drugs
by installing appropriate elements so as to construct firm interac-
tion with their target active sites.
H , Pd/C
2
WSC HCl, HOBt H O
2
Boc−Gly−Phe−Leu−Thr−OBn
MeOH
Et N, CH Cl (76%)
3 2 2
7
Boc−Tyr
CDMT, NMM
AcOEt (85%)
Boc−Tyr−D-Ser−OBn
8
D−Ser−OBn TsOH
Incorporation of a few fluorine atoms to an organic molecule
possibly brings about enhancement of the original biological ac-
tivity and improvement of the functional selectivity because of
the similar atomic size of fluorine to hydrogen,3 but such substi-
tution would affect formation of additional hydrogen bonds and
cause unfavorable electronic repulsion with other electronega-
tive groups. Conformational change is then expected based on
these phenomena, which is considered as one of the most impor-
tant determinants whether donor molecules fit target active sites,
but such issue seems not to have been drawing significant atten-
tion thus far.4 These facts prompted us to start our investigation
to clarify how much a fluorine atom or a fluorinated substituent
in amino acids5 affects original peptidic conformations. For this
purpose, we have selected the hexapeptide, Tyr–D-Ser–Gly–
Phe–Leu–Thr 1 (DSLET6) known as the enkephalin-related
peptide selectively bound to the ꢀ opioid receptor.
CF CO H
3
2
7
8
CH Cl , 0 °C
2
2
WSC HCl, HOBt H O
2
3
Et N, DMF (79%)
3
H , Pd/C
2
MeOH
Scheme 1. Synthesis of protected F0-hexapeptide 3.
OH
Ph
O
HO
CH3
OBn
O
O
O
H
H
N
N
N
N
N
H
H
H
HN
O
O
HO
Boc
In this study, most of the peptide syntheses were performed
7
Weak
Strong
Medium
.
by the solution-phase method using WSC HCl in the presence
of HOBt H2O as the representative condensing reagent partner
8
.
Figure 1. NOESY spectrum of 3.
which usually attained good to excellent yields. However, Boc–
Tyr–D-Ser–OBn 8 was the exception furnished only in ca. 50%
yield and this problem was solved by CDMT9 (2-chloro-4,6-
dimethoxy-1,3,5-triazine) in the presence of 2.2 equiv. of
NMM (N-methylmorpholine).10 As a result, this step effectively
proceeded under the usual atmosphere in an AcOEt solvent to
attain 85% isolated yield of the product 8. Combination of 7
and 8 in a usual manner produced the desired compound 3 in
79% yield (Scheme 1).
Tyr–OH proton and Thr–OH oxygen worked as the hydrogen-
bond donor and acceptor, respectively, when their acidity was
considered.11 Thus, it was strongly anticipated that substitution
of a Thr–CH3 group for a CF3 moiety should effectively lower
the electron density of F3-Thr–OH which would apparently
result in weakening the hydrogen bond between Tyr and Thr.
This is qualitatively supported by the acidity difference between
CH3CH2OH and CF3CH2OH of ꢀpKa = 3.5.
The NOESY spectrum of 3 observed in DMSO-d6 (Figure 1)
indicated that its conformation possessed two characteristic
long-range interactions between two amino acids apart from five
and six residues, which led to estimation of its helix- or spheri-
cal-type structure where N-terminal Tyr should be observed
around the region of C-terminal Thr. The cross peak between
Tyr phenol and Thr hydroxy protons led to speculation that
For this purpose, we have prepared (2S,3S)-4,4,4-trifluoro-
threonine 2 (F3-Thr) following to the synthetic method reported
by Soloshonok et al.12 which was readily converted to the
desired protected hexapeptide 4 (Scheme 2). As shown in
Figure 2, as our expectation, introduction of three fluorine
atoms indeed altered the original three-dimensional shape and
the long-range hydrogen bonding between Tyr and Thr was
Copyright Ó 2006 The Chemical Society of Japan
Kitamoto, Takamasa
