Conformational modulation of peptide secondary structures using β-aminobenzenesulfonic acid

Article abstract of DOI:10.1039/c3cc48850k

This communication describes the influence of β-aminobenzenesulfonic acid (^SAnt) on the conformational preferences of hetero foldamers. The designed (Aib-^SAnt-Aib)_n and (Aib-^SAnt-Pro) _n oligomers display a well-defined folded conformation featuring intramolecular mixed hydrogen bonding (7/11) and intra-residual (6/5) H-bonding interactions, respectively.

Full text of DOI:10.1039/c3cc48850k

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Conformational modulation of peptide secondary
structures using b-aminobenzenesulfonic acid†‡
Cite this: Chem. Commun., 2014,
50, 2886
Sangram S. Kale,a Shrikant M. Kunjir,b Rupesh L. Gawade,c Vedavati G. Puranik,c
P. R. Rajamohananb and Gangadhar J. Sanjayan*a
Received 20th November 2013,
Accepted 24th January 2014
DOI: 10.1039/c3cc48850k
www.rsc.org/chemcomm
This communication describes the influence of b-aminobenzenesulfonic modulating peptide secondary structures. Our primary objective was
acid (SAnt) on the conformational preferences of hetero foldamers. to investigate the effect of the torsional rigidity of SAnt on the
The designed (Aib-SAnt-Aib)n and (Aib-SAnt-Pro)n oligomers display a conformational features of well studied peptide sequences. In this
well-defined folded conformation featuring intramolecular mixed context, we designed two sets of oligomers having tripeptide building
hydrogen bonding (7/11) and intra-residual (6/5) H-bonding interac- blocks Aib-SAnt-Aib and Aib-SAnt-Pro (Fig. 1) to investigate the extent
tions, respectively.
of conformational modulation in peptides enforced by SAnt.
Extensive efforts of crystallization trials culminated in the
Modulation and control of the secondary structure of peptides and formation of crystals of 2a and 5b (Fig. 2). Analysis of the crystal
proteins are vital to the de novo design of synthetic proteins with data of hexapeptide 2a revealed that (Aib-SAnt-Aib)n oligomers dis-
intriguing structural architecture and function. Introduction of play three different types of intramolecular hydrogen-bonding
functionalized homologated amino acid residues (b, g and d) in
the native peptide leads to conformationally diverse secondary
structural architectures.1 In the past decade, an enormous amount
of effort has been expended to diversify the conformational space of
peptides using this approach.1–3 The extent of knowledge generated
so far clearly aids the de novo design of novel peptide architectures –
mimicking any type of secondary structure. Furthermore, unnatural
amino acids offer considerable advantage over their natural counter-
parts, primarily owing to their proteolytic stability.4
Among the diverse class of unnatural amino acids, a-amino-
isobutyric acid (Aib) has attracted major attention of researchers for
modulating secondary structures of polypeptides.5 The homo-
oligomer of Aib [(-Aib)n] has been shown to display a 310-helical
architecture,6 thougha sequentiallyinserted Pro analogue [(Aib-Pro)n]
exhibits a b-bend ribbon conformation.7 This conformational
disparity clearly suggests that the secondary structural modulation
greatly depends on the dihedral angle constraints of amino acids.
Herein we demonstrate the utility of b-aminobenzenesulfonic acid in
a Division of Organic Chemistry, National Chemical Laboratory,
Homi Bhabha Road, Pune 411 008, India. E-mail: gj.sanjayan@ncl.res.in;
Web: http://nclwebapps.ncl.res.in/gjsanjayan/index.html
b Central NMR Facility, National Chemical Laboratory, India
c Center for Materials Characterization, National Chemical Laboratory, India
† Dedicated to Prof. C. N. R. Rao on the occasion of his 80th birthday.
‡ Electronic supplementary information (ESI) available: General experimental
procedures, 1H, 13C, DEPT-135 NMR spectra and ESI mass spectra of all new
compounds. CCDC 931177 and 931178. For the ESI and crystallographic data in
Fig. 1 Molecular structures of oligomers described in this work.
CIF or other electronic format see DOI: 10.1039/c3cc48850k
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Fig. 2 PyMOL-rendered crystal structures of 2a (a) and 5b (b). Hydrogens, other than the polar amide ones, have been removed for clarity.
The H-bonding interactions are indicated by double-headed arrows in the molecular structures.
S
respectively. The torsional angles c and o of the Ant rings are
S
about 661 and 821, respectively. The Ant displays an angle f of
about 1421 and the y value is close to zero.
The single crystal X-ray diffraction data of hexapeptide 5b revealed
that (Aib-SAnt-Pro)n oligomers display intra-residual hydrogen-bonded
fully extended conformation, which is extremely rare for Aib resi-
dues5c (Fig. 3b). Molecule 5b shows six-membered hydrogen bonding
(C6 H-bonding) formed within SAnt itself and five-membered hydro-
gen bonding (C5 H-bonding)8 formed within the Aib residues. The
H-bonding distances of C6 H-bonding and C5 H-bonding are about
1.98 Å and 2.10 Å, respectively. The inner Aib residue flanked between
Pro and SAnt residues shows torsional angles f and c of about 1801
(176.81 and À177.71), suggesting planarity of the Aib residue, which
leads to intra-residual C5 H-bonding. The SAnt rings display torsional
angles c and o of about 921 and 691, respectively. The torsional angle
y of the SAnt is close to zero (7.01 and 9.91). The Pro residues show a
torsional angle f of about 901. It is noteworthy that the oligomer 5b is
devoid of any inter-residual H-bonding. Presumably, it is the torsional
constraints of the inner Aib residues that force the molecules to adopt
Fig. 3 Selected NOE extracts from the 2D NOESY data of 2a, 2b and 5a
intra-residual C6 and C5 H-bonding, instead of other inter-residual
H-bondings.
(CDCl3, 40 mM, 500 MHz). Detailed analyses, including 2D NMR plots,
dilution and variable temperature NMR data, are given in the ESI‡ (S31–S39).
X-ray analysis of 2a revealed the absence of H-bonding at the
N-terminus due to the absence of a H-bond acceptor carbonyl
patterns (Fig. 3a): (i) 11-membered ring H-bonding between CQO group. In order to replicate the C11 H-bonding at the N-terminus,
of Aib2 with NH5 [d(CQOÁÁÁH–N) 2.09 Å, D(D–HÁÁÁA) 1621], (ii) we synthesized 2b (Fig. 3b, vide infra) featuring a H-bond acceptor
12-membered ring H-bonding between NH2 and SQO of SAnt2 carbonyl at the N-terminus. The solution-state conformations of both
[d(CQOÁÁÁH–N) 2.42 Å, D(D–HÁÁÁA) 1231], and (iii) 7-membered ring (Aib-SAnt-Aib)n oligomers and (Aib-SAnt-Pro)n oligomers were investi-
H-bonding between NH2 and SQO of SAnt1 [d(CQOÁÁÁH–N) 2.10 Å, gated by NMR studies. The signal assignments of the oligomers were
D(D–HÁÁÁA) 1621]. Compound 2a also shows two intra-residual done using a combination of 2D COSY, NOESY, HMBC and HSQC.
S
6-membered H-bonding, formed within Ant itself. The torsional The 2D NOESY analysis of 2a revealed the existence of long-range inter-
angles (N–HÁÁÁOQC/S) of C11, C12 and C7 H-bonding are residual nOe between NH2 vs. NH5 along with C16H vs. C29H, NH2 vs.
83.621, 114.631 and À74.041, respectively. The H-bonding angles NH5, NH3 vs. NH5 and NH2 vs. NH4 (Fig. 3a), suggesting the
[D(S/CQOÁÁÁN)] of C11, C12 and C7 are 113.41, 153.61 and 97.01, prevalence of folded conformation. Similarly, the analysis of 2D
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Table 1 Chemical shift variation of amide NHs in NMR titration studies
(10 mM, 400 MHz)
SSK and RLG are thankful to CSIR, New Delhi, for a research
fellowship. GJS thanks NCL-IGIB (New Delhi) for financial support.
Dd (ppm)
Notes and references
Comp
NH1
NH2
NH3
NH4
NH5
NH6
NH7
2a
2b
5a
0.16
0.85
0.25
1.26
0.15
0.05
0.04
0.10
0.25
0.32
0.14
—
0.09
0.18
—
—
0.08
—
—
0.07
—
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NMR data of 2b (Fig. 3b) shows an inter-residual nOe between C17H
vs. NH6 and C17H vs. NH7, suggesting a similar conformation as
observed in 2a. It is noteworthy that the inter-residual nOe between
C32H vs. C14H and C32H vs. NH3 observed in 2b confirms the
existence of 11-membered ring H-bonding between the N-terminus
carbonyl group and NH3. The crystal structure analysis of 5b suggested
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support the helical conformation would be the nOes between aromatic
NH and consecutive a and d Pro protons. The helical conformation of
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between NH1 vs. C11H, NH1 vs. C14H, NH3 vs. C26H and NH3 vs.
C29H (Fig. 3c). The conformation of the large oligomers 3 and 6 could
not be ascertained owing to the difficulty in their signal assignments
(due to the presence of several repetitive similar residues).
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was supported by DMSO titration experiments. The amide NHs
involved in intramolecular H-bonding show negligible changes
in the chemical shift (Table 1). However, the amide NH2 of 2a
involved in C12 H-bonding displays a considerable chemical shift
(1.26 ppm), suggesting a weak hydrogen-bonding in the solution-
state. The variable temperature studies also support intramolecular
H-bonding in both peptides 2a and 2b [temperature coefficients for
2a (Dd/DT) o À3 and for 2b (Dd/DT) o À3.3, ESI,‡ S32 and S33].
Both DMSO titration and variable temperature experiments of 2b
support the presence of intramolecular H-bonding. In variable
temperature experiments, NHs of 5a involved in intramolecular
H-bonding display temperature coefficients oÀ1.7 [for NH1
(Dd/DT) o À1.45, for NH2 (Dd/DT) o À1.64 and for NH3
(Dd/DT) o À1.45], suggestive of intramolecular H-bonding.
´
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In conclusion, we have demonstrated the effect of 2-amino-
benzenesulfonic acid (SAnt) on the conformational preferences of
hetero foldamers featuring Aib-SAnt-Aib and Aib-SAnt-Pro tripeptide
building blocks. Whereas (Aib-SAnt-Aib)n oligomers display a folded
screw-sense inversion conformation with mixed hydrogen bonding
networks (C11 and C7), the (Aib-SAnt-Pro)n oligomers display a
periodically repeating intra-residual 6- and 5-membered hydrogen-
bonded fully extended conformation.9 These findings suggest that
the conformational propensities of these oligomers are primarily
dictated by SAnt, whilst the other amino acids played a less prominent
role. The results obtained underscore the importance of learning
about the conformational propensities of newer types of unnatural
amino acids, which may find application in the de novo design of
peptides with intriguing structures and function.
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9 The X-ray diffraction data of 2a and 5b were collected on a SMART
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of 2a and 5b have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 931177 and
931178, respectively. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
2888 | Chem. Commun., 2014, 50, 2886--2888
This journal is ©The Royal Society of Chemistry 2014