Electrostatic versus steric effects in peptidomimicry: Synthesis and secondary structure analysis of gramicidin S analogues with (E)-alkene peptide isosteres

Article abstract of DOI:10.1021/ja051002s

A concise synthetic strategy was used for the first preparation of GS analogues with trisubstituted (E)-alkene peptide bond replacements. Solution and solid state conformational analysis demonstrated that the bistrifluoromethylated analogue was a superior mimic of the natural product, whereas the incorporation of methyl groups into the alkene peptide isostere led to a far greater perturbation of the secondary structure features of GS. The difference between CF₃- and CH₃-substitution can be explained by the superior electrostatic carbonyl group mimicry of the former function. Copyright

Full text of DOI:10.1021/ja051002s

Published on Web 04/02/2005
Electrostatic versus Steric Effects in Peptidomimicry: Synthesis and
Secondary Structure Analysis of Gramicidin S Analogues with (E)-Alkene
Peptide Isosteres
Jingbo Xiao,§ Bernard Weisblum,# and Peter Wipf*,§
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and
Pharmacology Department, UniVersity of Wisconsin Medical School, 1300 UniVersity AVenue,
Madison, Wisconsin 53706
Received February 16, 2005; E-mail: pwipf@pitt.edu
Transition state mimicry of the tetrahedral intermediate of peptide
bond hydrolysis in serine, cysteine, and aspartic protease inhibitors
has been very successful,1 but effective ground state isosteric and
isoelectronic analogues of the amide bond have yet to be devel-
oped.2 Rigid (E)-alkenes provide a suitable fit for the Ci(R)-Ci+1(R)
distance (3.8 Å) in the peptide linkage,3 and we have developed
stereospecific syntheses of trisubstituted (E)-alkene dipeptide iso-
steres (TEADIs) that are conformationally preorganized into
â-turns.4 Furthermore, we have postulated that a CF3-substituted
(E)-alkene (µ ) 2.3 D) allows for an improved mimicry of the
electrostatic potential surface of the parent amide bond as well as
its large dipole moment (µ ) 3.6 D).5 We now describe the
synthesis and conformational evaluation of cyclo[(-Val-Orn-Leu-
ψ[(E)-C(R)dCH]-DPhe-Pro-)2], with RdCF3 and CH3, which
represent the first alkene peptide analogues of the cyclodecapeptide
antibiotic Gramicidin S (GS). The location of the amide bond
replacements at a critical hydrogen bond acceptor position in the
â-hairpin motif was selected in order to probe the effects of polar
versus steric peptide bond mimicry (Figure 1).
Figure 1. Gramicidin S and two (E)-alkene isostere analogues.
Scheme 1. Synthesis of TEADI Building Blocks 6a and 6ba
GS is a broadly utilized scaffold for the study of the effects of
turn inducers and peptide mimetics.6 The rigid, amphipathic
antiparallel â-pleated sheet is held in place by two type II′ â-turns
(at both DPhe-Pro positions) and four intramolecular hydrogen bonds
between the valine and leucine residues.7 GS displays antibiotic
activity against a wide spectrum of both Gram-negative and Gram-
positive bacteria, as well as against several pathogenic fungi.8
The preparation of TEADIs 6a and 6b was facilitated by a new
transition metal based methodology for synthesis of allylic amides
(Scheme 1).9 Hydrozirconation10 of 2a11 with Cp2ZrHCl followed
by transmetalation to Me2Zn and addition of N-Boc-isovaleraldimine
afforded the allylic amide 3a as a 1:1.5 mixture of diastereomers
(desired favored), which were separated after desilylation and
acetylation.12 Deprotection of 4a and oxidation provided the
methylated TEADI 6a. The trifluoromethylated 6b was prepared
using an indirect route. Hydrozirconation-iodination of stannyl
alkyne 2b11 afforded the vinyl iodide which was subjected to
lithium-halogen exchange and addition to imine to provide 3b as
a 1:1.5 mixture of diastereomers (desired favored). Iododestannyl-
ation of 3b, separation of the diastereomers, and Cu-mediated CF3-
coupling13 followed by deacetylation and two-step oxidation14 gave
the CF3-substituted TEADI 6b.
a (a) 3a: Cp2ZrHCl, Me2Zn, N-Boc-isovaleraldimine, 70%; 3b: (i)
Cp2ZrHCl, I2, 80%; (ii) BuLi, N-Boc-isovaleraldimine, 70%. (b) 4a: (i)
TBAF, 70%; (ii) Ac2O, TEA, DMAP, 92%; 4b: (i) NIS, 80%; (ii) TBAF,
84%; (iii) Ac2O, TEA, DMAP, 99%; (iv) FSO2CF2CO2Me, CuI, 92%. (c)
K2CO3, MeOH; 5a, 90%; 5b, quant. (d) (i) 6a: Dess-Martin periodinane;
6b: TEMPO, trichloroisocyanuric acid; (ii) NaClO2, NaH2PO4, 2-methyl-
2-butene.
t
analogues 9a and 9b (Scheme 2). Variable temperature NMR was
used to probe the degree of intramolecular hydrogen bonding in
9a, 9b, and Orn-δ-NHCbz-protected GS (Cbz2GS). The amide NH
protons in leucine and valine residues of all derivatives had
temperature coefficients between -4 and 0 ppb/K, which supported
intramolecular hydrogen bonding at these sites.15 Moreover,
NOESY studies showed interstrand NH(Leu)-NH(Val) and NH-
(Leu)-HR(Orn) nOe in all three compounds.
As expected, CD spectra of 9a and 9b were more highly
perturbed over the natural product (Figure 2). The presence of a
negative band at ∼205-225 nm for 9b and Cbz2GS is consistent
with a combination of type II′ â-turn and â-sheet conformations.16
In contrast, the CD spectra of 9a have a negative band centered at
204 nm followed by a positive band at 220 nm, consistent with a
disordered peptide conformation.16 Overall, spectroscopic analysis
of the solution conformation of 9a and 9b confirmed that the
electronic properties of the trifluoromethyl group promoted a
preference for the native pseudo type II′ â-turns and backbone
â-sheet and resulted in superior mimicry of the conformational
properties of the parent cyclopeptide.
The fragment assembly of TEADIs 6 and H-Pro-Val-Orn(Cbz)-
OMe (10) was accomplished using EDC as a coupling reagent to
provide the pentapeptides 7. Stepwise coupling proceeded smoothly
to afford the linear decapeptides 8. Saponification of 8 followed
by N-Boc removal and macrolactamization afforded the desired GS
In the solid state, 9b indeed adopts the pleated antiparallel â-sheet
conformation with two hydrogen bonds (1.96 and 2.00 Å, respec-
§ University of Pittsburgh.
# University of Wisconsin Medical School.
9
5742
J. AM. CHEM. SOC. 2005, 127, 5742-5743
10.1021/ja051002s CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of GS Analogues 9a and 9ba
peptide backbone, including the remaining two intramolecular
hydrogen bonds, is only minimally perturbed by this movement of
the plane of the trisubstituted alkene. The ornithine side chains
extend in an orthogonal fashion away from the perimeter of the
â-sheet, a feature that is essential for antibiotic activity.6,8 Indeed,
the Orn-δ-amino deprotected 1a and 1b demonstrated functional
mimicry of the natural product with MICs of 5-15 µg/mL,
equivalent to GS, against Bacillus subtilis.18
In conclusion, a concise synthetic strategy was used for the first
preparation of GS analogues with trisubstituted (E)-alkene peptide
bond replacements. Solution and solid state conformational analysis
demonstrated that the bistrifluoromethylated analogue 9b was a
superior mimic of the natural product, whereas the incorporation
of methyl groups into the alkene peptide isostere 9a led to a far
greater perturbation of the secondary structure features of GS. The
difference between CF3- and CH3-substitution can be explained by
the enhanced electrostatic carbonyl group mimicry of the former
function.
a (a) H-Pro-Val-Orn(Cbz)-OMe (10), EDC, HOBt, DMAP; 7a, 94% from
5a; 7b, 77% from 5b; (b) 1.0 N NaOH; (c) 4.0 N HCl in dioxane; (d)
EDC, HOBt, DMAP; 8a, 97% from 7a; 8b, 85% from 7b; (e) (i) 1.0 N
NaOH; (ii) 4.0 N HCl in dioxane; (iii) EDC, HOBt, DMAP; 9a, 42%; 9b,
42% (after reverse phase semi-prep HPLC purification).
Acknowledgment. Support from the NIGMS CMLD program
(P50-GM067082) for this project is gratefully acknowledged. We
thank Prof. A. Rheingold (UCSD) and Dr. S. J. Geib (Pittsburgh)
for the X-ray crystallographic analysis. J.X. gratefully acknowledges
an Andrew W. Mellon Predoctoral Fellowship for support, and Dr.
Corey R. J. Stephenson for helpful discussion.
Table 1. Tabular Display of Amide Proton Temperature Shift
Coefficients (∆δ/∆T) of 9a, 9b, and Cbz2GS in DMSO-d6 [ppb/K]
Cbz2GS
9a
9b
Orn-NH
Leu-NH
Orn-δ-NH
Val-NH
-6.7
-3.8
-8.6
-2.2
-11.8
-7.0
-3.1
-8.4
-0.4
N/A
-4.6
-2.9
N/Aa
-0.01
N/A
Supporting Information Available: Crystal information files (CIF)
for compounds 5a and 9b. Experimental procedures, 1H and 13C spectra
for selected compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
DPhe-NH
1
a Signal obscured in H NMR.
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