N- versus C-terminal control over the screw-sense preference of the configurationally achiral, conformationally helical peptide motif Aib 8GlyAib8

Article abstract of DOI:10.1021/ja100662d

Conformational control over the screw sense of a helical 17-mer of achiral amino acids, Aib₈GlyAib₈, from a single chiral residue located at the N-terminus is better than that from a single amino acid located at the C-terminus. X-ray crystallography indicates that Aib ₈GlyAib₈ forms the longest 3₁₀ helical structure observed crystallographically to date.

Full text of DOI:10.1021/ja100662d

Published on Web 03/16/2010
N- versus C-Terminal Control over the Screw-Sense Preference of the
Configurationally Achiral, Conformationally Helical Peptide Motif Aib8GlyAib8
Jordi Sola`, Madeleine Helliwell, and Jonathan Clayden*
School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K.
Received January 25, 2010; E-mail: clayden@man.ac.uk
Helical conformations are commonly adopted by both biological
and synthetic polymers, and the chirality of the monomers from
which they are built generally determines the screw sense of the
helix.1 The amino acid aminoisobutyric acid (Aib), while strongly
promoting the adoption of helical peptide conformations,2 is achiral,
and otherwise achiral oligomers containing Aib can be induced to
prefer one of the two screw senses by ligation with a single terminal
amino acid.3,4 In this work, we have used a simple NMR method
to show that screw-sense control is more effective when applied
from the N-terminus rather than from the C-terminus. We also report
the crystal structure of nearly six full turns of a peptide 310 helix,
the longest 310 helix ever observed in the crystalline state.
The degree of screw-sense bias at any site in a configurationally
achiral but conformationally fluxional helix (a class which includes
oligomers of Aib5) can be probed using NMR spectroscopy by
incorporating into the helix a pair of potentially diastereotopic
reporter groups, such as a CH2 unit.4-6 At slow exchange, the local
chiral environment would render the CHAHB signals anisochronous
regardless of the relative populations of the two helical screw-sense
conformers. In a rapidly inverting, configurationally achiral helix
(i.e., one built entirely of achiral monomers) the two “reporter”
nuclei HA and HB can become isochronous (Figure 1a), but only if
Figure 1. Detection of a helical preference by NMR spectroscopy in the
the population of the two helical screw-sense conformers is equal.
When a remote chiral influence X* or Y* succeeds in preferentially
inducing one screw sense in the helical oligomer (i.e., Keq * 1),
the symmetry of the local environment of the reporter is broken.
Provided that the chiral influence is located sufficiently far away
to avoid direct interaction with the reporter, the anisochronicity of
the HA and HB signals results from a weighted average of two
pseudoenantiomeric environments and is directly proportional to
the induced level of screw-sense control (Figure 1b).4,5 Measuring
the anisochronicity as a chemical shift difference ∆δ (in ppb) allows
the relative effectiveness of the screw-sense control supplied by a
series of controllers X* or Y* to be quantified.
fast-exchange regime.
Scheme 1. Synthesis of the 17-19-mersa
We previously used such a method to show that the helicity of
hybrid Aib-Gly peptides containing up to 20 residues can be
controlled from a single N-terminal residue.4 Here we show that
placing a reporter group in the middle of a peptide chain allows a
direct comparison to be made between the screw-sense control
achievable from the N- and C-termini of a configurationally achiral
helical peptide motif. We chose as a reporter the methylene group
of a single Gly residue flanked on either side by an Aib8 motif. A
series of XaaAib8GlyAib8Yaa 17-19-mer peptides were con-
structed, as shown in Scheme 1: the azido-Aib octamer 14,7 was
reduced to give 3 and coupled with either CbzPhe (giving 4) or
CbzGly (giving 6). C-terminal deprotection of the latter gave 7,
which was coupled with a choice of capping C-terminal amino ester
or amide Yaa to provide C-terminal-moiety decamers 8. Ligation
of the peptide 9- and 10-mers 6 and 8 with 2 or 5 via their
azlactones7 gave the Aib8GlyAib8-containing 17-19-mers 9a-e.
a Conditions: (a) CF3CO2H; (b) Ac2O, 120 °C; (c) deprotection of
coupling partner (conditions d) then MeCN, ∆; (d) H2Pd/C, MeOH; (e)
CbzLPheOH, PyBOP, iPr2NEt, CH2Cl2, DMF; (f) CbzGlyOH, EDC, HOBt,
CH2Cl2, DMF; (g) coupling partner given, PyBOP, i-Pr2NEt, CH2Cl2, DMF.
In each case, the chemical shift difference within the diaste-
reotopic Gly CH2 group was determined by 1H NMR spectroscopy
at 500 MHz in CD3OD at 23 °C, and the results are shown in Table
1. In the 9-mers 6 and 7, the N-terminal Gly residue displays a 2H
singlet, as expected, since the M and P conformers of helical Aib
oligomers interconvert rapidly on the NMR time scale at ambient
temperature.6 Incorporation of an N-terminal Cbz-L-Phe residue (in
9c), however, causes the Gly CH2 to appear as a clear AB system,4
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4548 J. AM. CHEM. SOC. 2010, 132, 4548–4549
10.1021/ja100662d  2010 American Chemical Society

C O M M U N I C A T I O N S
with a peak separation ∆δ ) 74 ppb. We interpret this collapse of
local symmetry at the Gly residue as evidence of a thermodynamic
bias toward one helical sense, in this case P (right-handed).8 This
bias must arise from a local organizational effect of the N-terminal
Cbz-L-Phe residue, which, because of the powerful helicogenic
properties of the Aib residues, is relayed through the 27 bonds
separating it from the Gly residue.
Gly residue. Values for ψ remain close to the 30° characteristic of
a 310 helix11 over most of the peptide but drift up to 40° at residues
7-10, leading to the slight kink evident in Figure 2. However, close
H-bonded contacts exist for every CdO and the i + 3 N-H except
for the three C-terminal carbonyl groups. One unexpected feature
of the crystal structure is the fact that the helix is left- and not
right-handed;8 this is presumably a crystal packing effect.3a,b
Long-range conformational control is a promising biomimetic
strategy for the communication of information on the nanometer
scale,12,13 and screw-sense switching is a plausible method of
communication over such distances.13 We conclude that the potential
to use the screw sense of an oligo-Aib helix as a means of
communcation is greater if the “input” of information is incorporated
at the N-terminus rather than the C-terminus of the helix.
Table 1. Anisochronicity in the Central Gly CH2 of 9a-e and in the
N-Terminal Gly CH2 of 8a and 8b
entry
compound
N-terminal residue Xaa
C-terminal residue Yaa
∆δ (ppb)
b
1
2
3
4
4
5
6
9c
9a
9b
9d
9e
8a
8b
Cbz-L-Phe
-
-
-
74
0
a
L-PheOMe
L-PheNH2
L-PheOMe
D-PheOMe
L-PheOMe
L-PheNH2
47
74
67
35
100
Cbz-L-Phe
Cbz-L-Phe
Acknowledgment. We are grateful to the EPSRC and the
Leverhulme Trust for support and the Departament d’Innovacio´
Universitats i Empresa de la Generalitat de Catalunya for a
fellowship to J.S.
c
-
-
c
a N-terminal N3Aib unchanged from 1. b C-terminal AibOt-Bu
unchanged from 6. c N-terminus is reporter CbzGly.
Supporting Information Available: Experimental details and
characterization of new compounds, CD spectra, and crystallographic
data (CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
Incorporating L-PheOMe at the C terminus (in 9a) led to no
detectable anisochronicity in the signals arising from the Gly CH2
group. Since the local environments at the Gly reporters in 9a and
9c are essentially identical, this result indicates that an N-terminal
chiral amino acid exhibits a much greater propensity to control the
screw sense of the helix than the same residue placed at the
C-terminus. The similarity of the ∆δ values for 9d and 9e, which
contain potentially matched and mismatched pairs of Phe residues,
confirms the relative weakness of screw-sense control from the C
terminus: in both cases, the anisochronicity of the central Gly CH2
group was hardly changed from that observed in 9c. However,
replacement of the C-terminal ester of 9a with the C-terminal amide
of 9b (which can in contrast participate in hydrogen bonding
through its NH bond) improved the conformational control. The
∆δ value within the Gly CH2 group of 9b increased to 47 ppm,
though still falling short of that seen with the N-terminal controller.
Comparison of the anisochronicities ∆δ in the diastereotopic
N-terminal Gly CH2 groups of 8a and 8b also indicates that an
amide-capped C-terminal residue provides a greater screw-sense
bias than an ester-capped C-terminal residue.
References
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Figure 2. X-ray crystal structure of 9c: five turns of a 310 helix.
Crystals of 9c were obtained by slow precipitation from
acetonitrile.9 The conformation of 9c in the crystalline state is shown
in Figure 2. It consists of nearly six turns of a continuous 310 helix
(the longest ever observed crystallographically7c) stretching over
17 of the 18 residues of the peptide. Only the C-terminal AibOt-
Bu residue is excluded from the helix, forming a characteristic
“Schellmann motif”.10 The lack of control exerted by a C-terminal
ester residue is most likely due to the tendency of C-terminal esters
to form such conformationally flexible features.
Tabulation of hydrogen-bond distances and bond angles (see the
Supporting Information) indicated that the 310 helix (with H bonds
between residues i and i + 3) is distorted toward an R helix (with
H bonds between residues i and i + 4) in the vicinity of the central
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507) 9c in solution was shown by NMR and CD spectroscopy to adopt a
thermally stable 310 conformation with P helicity (see the Supporting
Information).
(9) The X-ray data have been deposited with the Cambridge Crystallographic
Data Centre (deposition reference 762200).
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