Effect of Sequence on Peptide Geometry
A R T I C L E S
enhance the cis-isomer population, several approaches have been
tried to stabilize this geometry by the means of conformational
constraint using structural links and steric interactions.10-18
Alternatively, double bond isosteres have been employed to
mimic the spatial orientation presented by the cis conformer.19
These approaches have achieved effective replication of the
backbone geometry of the type VI â-turn as well as analogues
exhibiting inhibitory activity against PPIases.20 Moreover,
stabilization of a hairpin conformation has been achieved in a
model linear tetrapeptide possessing an indolizidinone amino
acid mimic of the central residues of type VIa â-turn as
demonstrated by NMR in DMSO and IR spectroscopy in
dichloromethane.14d These approaches have succeeded in rep-
licating cis conformer geometry; however, an important aspect
of prolyl amides that many such examples by design fail to
mimic has been the conformational equilibrium exhibited by
prolyl peptides.
contingent upon the stereochemistry of the N-terminal residue.22
Dipeptides possessing Ala and Leu residues adopted type VIa
and VIb â-turn conformations when the N-terminal amino acid
possessed respectively L- and D-configuration as shown by NMR
and CD spectroscopy as well as X-ray analysis.22 Furthermore,
the presence of phenylalanine at the N-terminal of 5-tBuPro
caused a remarkable increase in cis-isomer population (Ac-
Phe-5-tBuPro-NHMe exhibited >90% prolyl amide cis isomer
in water).22
As well as its power to augment the cis-isomer population,
the 5-tert-butyl substituent influences the barrier for amide
isomerization.21b In the case of the (2S,5R)-diastereomer, the
sterically bulky tert-butyl group interacts with the N-terminal
residue such as to twist the amide bond away from planarity.22
In N-acetylproline N′-methylamides, twisting of the prolyl amide
was among factors that caused a reduction in the barrier for
isomerization of 3.7 kcal/mol in the (2S,5R)-5-tBuPro analogue
relative to its proline counterpart.21b The influences of sequence
and stereochemistry on the amide equilibrium become thus more
apparent in (2S,5R)-5-tBuPro peptides, because of the combina-
tion of the reduced isomerization barrier and the enhanced cis-
isomer population.
We have synthesized and used (2S,5R)-5-tert-butylproline (5-
tBuPro) to explore both prolyl amide cis-isomer geometry as
well as the amide isomer equilibrium N-terminal to proline in
various peptides.21-23 In Ac-Xaa-5-tBuPro-NHMe, the 5-tBuPro
residue stabilized type VIa and VIb â-turn conformations
We have synthesized a diverse array of 5-tBuPro peptides
by employing N,N′-bis(2-oxo-3-oxazolidinyl)phosphonic chlo-
ride (BOP-Cl)24 as a coupling reagent to attach different amino
acid electrophiles onto the sterically hindered prolyl residue.
This synthesis achievement has allowed us to explore the
influence of sequence on the equilibrium N-terminal to the prolyl
residues. Study of the effect of sequence on isomer equilibrium
in natural prolyl peptides has previously shown that aromatic
residues adjacent to proline caused an augmentation in the cis-
isomer population.8,25-27a Although aromatic residues N-terminal
to proline have been shown to cause a 10-fold reduction in the
cis to trans isomerization rate,3a to the best of our knowledge,
little has been reported about the factors by which aromatic
residues augment the cis-isomer population and increase the
isomerization energy barrier. Amino acid residues possessing
side chains with hydrogen-bond acceptor and donor moieties
have been shown to stabilize turn conformations when adjacent
to proline.1a,27b We report now the influence of hydrogen-
bonding residues on the prolyl amide equilibrium and the cis-
isomer population.
Examining the influence of sequence on turn geometry, we
have introduced 5-tBuPro into a series of dipeptide and tetra-
peptide analogues possessing aromatic and hydrogen-donor and
acceptor residues. By studying the conformations of these ana-
logues using NMR and CD spectroscopy as well as X-ray
diffraction, we have itemized factors that control the prolyl
amide equilibrium and stabilize type VI â-turn geometry. The
high cis-isomer populations and preponderance of type VIa
â-turn conformation brought about by aromatic amino acid
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