C. Peggion E. Mossel F. Formaggio M. Crisma B. Kaptein Q.B. Broxterman J. Kamphuis C. Toniolo
(aMe)Aun: a highly lipophilic, chiral, Ca-tetrasubstituted a-amino acid. Incorporation into model peptides and preferred conformation
Authors' af®liations:
Key words: a-aminoisobutyric acid; 310-helix; hydrophobicity;
C. Peggion, E. Mossel, F. Formaggio, M. Crisma
lipophilic side chain; Ca-methyl, Ca-n-nonylglycine; peptide
and C. Toniolo, Biopolymer Research Center, CNR, Department of Organic Chemistry,
conformation; peptide synthesis; spectroscopy
University of Padova, 35131 Padova, Italy.
Abstract: Using a chemo-enzymatic approach we prepared the B. Kaptein and Q.B. Broxterman, DSM Research, Organic Chemistry and Biotechnology Section, PO Box 18, 6160 MD Geleen, The Netherlands.
highly lipophilic, chiral, Ca-methylated a-amino acid (aMe)Aun. Two series of terminally protected model peptides containing either D-(aMe)Aun in combination with Aib or L-(aMe)Aun in
J. Kamphuis, DSM Specialty Intermediates,
combination with Gly were synthesized using solution methods
PO Box 5489, 6130 PL Sittard, The Netherlands.
and fully characterized. A detailed solution conformational analysis, based on FT-IR absorption, 1H NMR and CD techniques, allowed us to determine the preferred conformation of this
Correspondence to:
Claudio Toniolo Department of Organic Chemistry
amino acid and the relationship between chirality at its a-carbon atom and screw sense of the helix that is formed. The results
University of Padova
obtained strongly support the view that D-(aMe)Aun favors the
Via Marzolo 1
formation of the left-handed 310-helical conformation.
35131 Padova Italy Tel.: 39-049-827-5247
Abbreviations: Ac, acetyl; Aib, a-aminoisobutyric acid or Ca,a-
Fax: 39-049-827-5239
dimethylglycine; (aMe)Aun, Ca-methyl, Ca-n-nonylglycine or
E-mail:
[email protected]
2-methyl-2-amino-undecanoic acid; DMSO, dimethylsulfoxide; EDC, N-ethyl, N'-[3-(dimethylamino)propyl]carbodiimide; HOAt, 1-hydroxy-7-azabenzotriazole; MeCN, acetonitrile; MeOH, methanol; OMe, methoxy; OtBu, tert-butoxy; NMM, N-methylmorpholine; ROESY, rotating-frame nuclear Overhauser
Dates:
Received 25 June 1999 Revised 7 September 1999 Accepted 4 October 1999
enhancement spectroscopy; TEMPO, 2,2,6,6-tetramethylpiperidinyl-1-oxy; TFA, tri¯uoroacetic acid; TFE, 2,2,2-tri¯uoroethanol; Z, benzyloxycarbonyl.
To cite this article:
Peggion, C., Mossel, E., Formaggio, F., Crisma, M., Kaptein, B., Broxterman, Q.B., Kamphuis, J. & Toniolo, C. (aMe)Aun: a highly lipophilic, chiral,
Non-proteinogenic, non-natural a-amino acids have increas-
Ca-tetrasubstituted a-amino acid. Incorporation into
ingly attracted the attention of chemists interested in the
model peptides and preferred conformation. J. Peptide Res., 2000, 55, 262±269.
engineering and synthesis of potential constituents of
Copyright Munksgaard International Publishers Ltd, 2000
pharmaceuticals and bioactive molecules. Ca-Tetrasubsti-
ISSN 1397-002X
tuted a-amino acids, in particular, have been taken into
262
Peggion et al . (aMe)Aun peptides
consideration for their well-known ability to signi®cantly
synthesis, characterization and conformational analysis
reduce peptide backbone conformational freedom (1±13).
(using FT-IR absorption, NMR and CD techniques) of a
a
More speci®cally, the helical rigidity conferred by C -
number of model peptides containing (aMe)Aun in combi-
methylated a-amino acids to a peptide sequence has been
nation with Aib or Gly.
studied extensively (4±11). The geometric and spatial constraints induced by these hindered amino acids might contribute to the improvement in the af®nity and selectivity
Experimental Procedures
of a peptide for its receptor, thereby helping in the design of `bio-interesting' peptide-based drugs.
Peptide synthesis
Unfortunately, the therapeutic use of many potentially bioactive peptides is limited by their insuf®cient bioavail-
Melting points were determined using a Leitz model
ability, which is related mainly to their poor membrane
Laborlux 12 apparatus and are not corrected. Optical
solubility and easy accessibility to enzymatic attack.
rotations were measured using a Perkin±Elmer model 241
Classically, to overcome this dif®culty, modi®cations
polarimeter equipped with a Haake model D thermostat.
suitable for increasing the membrane-like character of
Thin-layer chromatography was performed on Merck
peptides have been introduced. In this respect particular
Kieselgel 60/F254 precoated plates. The chromatograms
attention has been focused on lipoamino acids (amino acids
were developed by quenching of UV ¯uorescence, chlor-
with long alkyl chains) and their corresponding oligomers
ine±starch±potassium iodide or ninhydrin chromatic reac-
(lipopeptides) (14±18). Alternatively, lipoconjugates pre-
tion as appropriate.
pared by introducing a lipophilic moiety at some point of the peptide sequence (after the peptide synthesis) have been
FT-IR absorption
demonstrated to ef®ciently increase the overall hydrophobicity of the peptides (19, 20). Interestingly, all these studies
The solid-state infrared absorption spectra (KBr disk
have demonstrated that the long alkyl side chains have the
technique) were recorded using a Perkin±Elmer model
additional effect of protecting a labile parent drug from
580 B spectrophotometer equipped with a Perkin±Elmer
enzymatic degradation.
model 3600 IR data station. The solution IR absorption
In this context, an interesting approach in the preparation
spectra were recorded using a Perkin±Elmer model 1720X
of peptide drugs would be a combination of the structural
FT-IR spectrophotometer, nitrogen-¯ushed, equipped with a
a
features of a C -methylated a-amino acid with those of a
sample-shuttle device, at 2 cm±1 nominal resolution, aver-
lipidic chain. Indeed, juxtaposition of lipophilic character
aging 100 scans. Solvent (baseline) spectra were obtained
with conformational rigidity could result in a structurally
under the same conditions. Cells with path lengths of 0.1,
well-de®ned drug that may interact easily with membranes,
1.0 and 10 mm (with CaF2 windows) were used. Spectro-
thereby becoming a potentially useful therapeutic agent. In
grade deuterochloroform (99.8% d) was purchased from
our view the ®rst step in this strategy is represented by the
Fluka.
a
synthesis of a highly lipophilic, C -methylated a-amino acid such as (aMe)Aun.
Nuclear magnetic resonance CH3
CH3 NH
CO CH3
Aib
(CH2)8 NH
CO CH3
(aMe)Aun
The 1H NMR spectra were recorded with a Bruker model AM 400 spectrometer. Measurements were carried out in deuterochloroform (99.96% d; Aldrich) and deuterated dimethylsulfoxide (99.96% d6; Acros Organics) with tetramethylsilane as the internal standard. The free radical TEMPO was purchased from Sigma.
Recently, we have incorporated this atypical, chiral, lipoamino acid into an analog of the membrane-active
Circular dichroism
lipopeptaibol antibiotic trichogin GA IV, where it successfully replaced the naturally occurring N-terminal n-octanoyl
The CD spectrum was obtained on a Jasco model J-715
chain essential for activity (21). To better understand the
dichrograph. Cylindrical fused quartz cells of 1.0- and
conformational propensity of (aMe)Aun, here we present the
0.2-mm path lengths were used. The values are expressed J. Peptide Res. 55, 2000 / 262±269
| 263
Peggion et al . (aMe)Aun peptides
in terms of [h]T, the total molar ellipticity (deg.cm2/dmol).
acids were prepared from the corresponding tert-butyl
Spectrograde tri¯uoroethanol (TFE; Acros Organics) was
esters by treatment with diluted TFA. All products were
used as a solvent.
puri®ed by ¯ash chromatography. The physical and analytical properties of the (aMe)Aun derivatives and peptides are
Results and Discussion
listed in Table 1. All compounds were also characterized by 1
H NMR (data not reported).
Peptide synthesis Conformational analysis
For the large-scale production of the enantiomerically pure l-(aMe)Aun and d-(aMe)Aun we exploited an economically
A detailed analysis of the solution preferred conformation of
attractive and generally applicable chemo-enzymatic synth-
selected (aMe)Aun peptides was carried out using FT-IR
esis developed a few years ago by DSM Research (22±24). It
absorption, CD and 1H NMR techniques.
involved a combination of partial Strecker synthesis for the
FT-IR absorption analysis was performed in a solvent of
preparation of the racemic a-amino acid amide followed by
low polarity (CDCl3) as a function of peptide main-chain
the use of a broadly speci®c amino acid amidase from
length and concentration. Figure 1 shows the FT-IR absorp-
Mycobacterium neoaurum to achieve optical resolution,
tion spectra of the Z-protected (aMe)Aun/Aib peptide series
eventually affording the free l-amino acid and the d-amino
to the octamer level in the 3500±3200 cm±1 (N±H stretching)
acid amide. Acid hydrolysis of the latter gave the corre-
region. The curves of the higher homologs are characterized
sponding free d-amino acid. Synthesis of racemic (aMe)Aun
by two bands at < 3430 cm±1, assigned to the free (solvated)
and the gas-chromatographic separation of selected diaster-
NH groups, and at 3380±3330 cm±1, assigned to H-bonded
eomeric esters at the analytical level have been published
NH groups (29). The intensity of the low-frequency band,
elsewhere (25).
relative to the high-frequency band, increases linearly as
Benzyloxycarbonyl (Z)-protected derivatives were pre-
main-chain length increases. Concomitantly, the absorption
pared by reacting the free amino acids with N-(benzylox-
maximum shifts markedly towards lower wavenumbers.
ycarbonyl)-succinimide in MeCN in the presence of the
Figure 2 shows the FT-IR absorption spectra in the same
lipophilic base tetramethylammonium hydroxide (26) to
region for the Z-protected (aMe)Aun/Gly series, from trimer
solubilize the otherwise sparingly soluble zwitterionic
through to hexamer. Albeit less pronounced, a trend similar
amino acid. Peptide synthesis was performed step-by-step
to that exhibited by the (aMe)Aun/Aib series is observed.
in solution, beginning from the C-terminal tert-butyl or
Furthermore, from a comparison of the two (aMe)Aun/Gly
methyl ester. Aib and Gly tert-butyl esters were obtained by
tripeptides with the Ca-tetrasubstituted amino acid either in
esteri®cation of the corresponding Z-protected amino acid with isobutylene in the presence of a catalytic amount of sulfuric acid (27). Peptide bond formation was achieved of NMM. Using this approach, formation of the sterically hindered (aMe)Aun-Aib and Aib-(aMe)Aun peptide bonds occurred in moderately good yields (45±75%). The Naprotected octapeptide Z-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-
Absorbance
using the EDC/HOAt method (28) in CH2Cl2 in the presence
Aib-Aib-d-(aMe)Aun-Aib-OtBu was obtained using the 5(4H)-oxazolone from Z-d-(aMe)Aun-Aib-OH. The Na-protected dipeptide 5(4H)-oxazolone was prepared from Z(aMe)Aun-Aib-OH and EDC in MeCN. The Na-blocked octapeptide Ac-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-Aib-Aibd-(aMe)Aun-Aib-OtBu was synthesized by treatment of H-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-Aib-Aib-d-(aMe)AunAib-OtBu with an excess of acetic anhydride in CH2Cl2. Removal of the Z Na-protecting group was performed by catalytic hydrogenation. The Na-protected, peptide free 264 |
J. Peptide Res. 55, 2000 / 262±269
3500
3400 3300 Wavenumber (cm_1)
3200
Figure 1. FT-IR absorption spectra in the 3500±3200 cm±1 region of Z-d-(aMe)Aun-Aib-OtBu (2), Z-Aib-d-(aMe)Aun-Aib-OtBu (3), Z-AibAib-d-(aMe)Aun-Aib-OtBu (4), Z-d-(aMe)Aun-Aib-Aib-d-(aMe)AunAib-OtBu (5), Z-Aib-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-Aib-OtBu (6), Z-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-Aib-OtBu (8) in CDCl3 solution. Peptide concentration: 1.0 mm.
179±181
54 57
Z-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib-OtBu
Z-Aib-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib-OtBu
oil 61±62
51 66 61 61 45
Z-L-(aMe)Aun-Gly2-OtBu
Z-L-(aMe)Aun-Gly2-OMe
Z-Gly-L-(aMe)Aun-Gly2-OMe
Z-Gly2-L-(aMe)Aun-Gly2-OMe
Z-L-(aMe)Aun-Gly2-L-(aMe)Aun-Gly2-OMe
oil
CH2Cl2/Et2O/PE
CH2Cl2/Et2O/PE
CH2Cl2/Et2O/PE
±
EtOAc/PE
EtOAc/PE
±
CH2Cl2/PE
EtOAc/PE
±
CHCl3/PE
±
CHCl3/PE
Et2O/PE
±
±
Et2O/PE
EtOAc/PE
±
solventa
Crystallization
e
±7.7
±0.7
±3.8
±5.2
3.3
±1.6
± 7.3
1.2
2.7
9.4
11.8
5.5
30.0
e
0.50
0.40
0.55
0.65
0.95
0.65
0.80
0.55
0.80
0.85
0.90
0.50
0.80
0.60
0.90
6.4e 1.0
0.45 0.95
1.1e
0.90
±0.8
0.60
±2.4e
I
0.85
0.75
0.80
0.85
0.70
0.95
0.95
0.95
0.95
0.95
0.95
0.90
0.95
0.95
0.95
±
0.95
0.95
0.95
II
0.00
0.20
0.25
0.35
0.50
0.50
0.45
0.20
0.35
0.40
0.30
0.20
0.30
0.25
0.40
0.60
0.20
0.80
0.30
III
TLC RF valuesc
±9.6e
(8)b
[a]D
20
3308, 1746, 1658, 1535
3305, 1744, 1656, 1538
3307, 1747, 1704, 1652, 1538
3314, 1748, 1699, 1660, 1534
3312, 1734, 1714, 1678, 1535
3390, 1749, 1704, 1672, 1651, 1550
3342, 1728, 1659, 1520
3303, 1727, 1656, 1535
3306, 1723, 1698, 1658, 1534
3315, 1730, 1700, 1661, 1531
3416, 3323, 1726, 1697, 1664, 1534
3303, 1738, 1700, 1658, 1528
3427, 3319, 1730, 1685, 1667, 1537
3312, 1736, 1696, 1659, 1527
3433, 3351, 1731, 1703, 1667, 1525
3401, 3342, 1817, 1722, 1692, 1660, 1538
3422, 3367, 3289, 1722, 1690, 1661
3309, 3294, 1720, 1536
3413, 3332, 1708, 1586
IR (cm±1)d
a. EtOAc, ethyl acetate; PE, petroleum ether; Et2O, diethyl ether. b. C 0.5, MeOH. c. Solvent systems: I, CHCl3/EtOH (9:1); II, 1-BuOH/AcOH/H2O (3:1:1); III, toluene/EtOH (7:1). d. The IR absorption spectra were obtained in KBr pellets (only bands in the 3500±3200 and 1800±1500 cm±1 regions are reported). e. [a]20 436.
121±122
128±129
84±85
62±63
25 40
Z-Gly-L-(aMe)Aun-Gly-OtBu
210±212
129±131
93±95
oil
Z-L-(aMe)Aun-Gly-OtBu
45
60
80
73
Z-Aib2-D-(aMe)Aun-Aib-OtBu
Z-Aib2-D-(aMe)Aun -Aib-OH
Z-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib-OtBu
153±155 oil
41
Z-Aib-D-(aMe)Aun-Aib-OH
Ac-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib2-D-(aMe)Aun-Aib-OtBu
oil
70
Z-Aib-D-(aMe)Aun-Aib-OtBu
122±123 oil
96
101±102
98
75
Z-D-(aMe)Aun-Aib-OtBu
oil
5(4H)-Oxazolone from Z-D-(aMe)Aun-Aib-OH
85
Z-D-(aMe)Aun-OH
(8C)
Mp
Z-D-(aMe)Aun-Aib-OH
(%)
Compound
Yield
Table 1. Physical and analytical properties for the (aMe)Aun peptides
Peggion et al . (aMe)Aun peptides
J. Peptide Res. 55, 2000 / 262±269
| 265
Peggion et al . (aMe)Aun peptides
(A)
(B) 100
80
7.0
60 2
Dn1 (Hz)
d (p.p.m.)
Absorbance
7.5
40
6.5 20 6.0 3500
3400 3300 Wavenumber (cm_1)
3200
Figure 2. FT-IR absorption spectra in the 3500±3200 cm±1 region of Z-l-(aMe)Aun-Gly-Gly-OMe (3), Z-Gly-l-(aMe)Aun-Gly-Gly-OMe (4), Z-Gly-Gly-l-(aMe)Aun-Gly-Gly-OMe (5), Z-l-(aMe)Aun-Gly-Gly-l(aMe)Aun-Gly-Gly-OMe (6) in CDCl3 solution. Peptide concentration: 1.0 mm.
0
0 0.0
4 8 % DMSO in CDCl3
0.2 0.4 % TEMPO in CDCl3
Figure 4. 1H NMR titrations of Z-d-(aMe)Aun-Aib-Aib-d-(aMe)AunAib-Aib-d-(aMe)Aun-Aib-OtBu. (A) Plot of NH chemical shifts as a function of increasing percentages of DMSO added to the CDCl3 solution (v/v). (B) Plot of bandwidth of the NH signals as a function of increasing percentages of TEMPO (w/v) in CDCl3. Peptide concentration: 1.0 mm.
position 1 or 2 (Fig. 3), it is clear that (aMe)Aun residue is able to induce peptide folding more effectively if it is located
was carried out at 1.0 mm concentration where self-
at the N-terminus of the peptide chain than if it is
association is absent. The behavior of NH resonances
incorporated in an internal position. Upon changing the
upon addition of perturbing agents was studied in order to
concentration (in the 1.0±0.1 mm range) all of the model
delineate inaccessible NH groups. In particular, we exam-
peptides investigated display only minor variations in the
ined: (i) the solvent dependence of NH chemical shifts, by
spectra (results not shown).
adding increasing amounts of the strong H-bonding acceptor
The present FT-IR absorption investigation provided convincing evidence that the (aMe)Aun/Aib peptides are highly folded in an intramolecularly H-bonded helical conformation. In contrast, a somewhat lower helical character seems to be typical of the (aMe)Aun/Gly peptides. A
1
H NMR study allowed us to get more detailed
information
on
the
preferred
conformation
of
solvent dimethylsulfoxide (DMSO) (30, 31) to the CDCl3 solution; and (ii) the line broadening of NH resonances induced by adding the free radical TEMPO (32). As a representative example, Fig. 4 illustrates the behavior of the NH resonances of the (aMe)Aun/Aib octapeptide
the
(A)
(aMe)Aun-rich peptides in CDCl3 solution. The analysis
100
(B)
8
60 2
7
40
6 20
5
3500
3400 3300 Wavenumber (cm_1)
3200
Figure 3. FT-IR absorption spectra in the 3500±3200 cm±1 region of Z-Gly-l-(aMe)Aun-Gly-OtBu (A) and Z-l-(aMe)Aun-Gly-Gly-OtBu (B) in CDCl3 solution. Peptide concentration: 1.0 mm.
266 |
Dn1 (Hz)
Absorbance
d (p.p.m.)
80
J. Peptide Res. 55, 2000 / 262±269
0
4 8 % DMSO in CDCl3
0 0.0
0.2 0.4 % TEMPO in CDCl3
Figure 5. 1H NMR titrations of Z-l-(aMe)Aun-Gly-Gly-l-(aMe)AunGly-Gly-OMe. (A) Plot of NH chemical shifts as a function of increasing percentages of DMSO added to the CDCl3 solution (v/v). (B) Plot of bandwidth of the NH signals as a function of increasing percentages of TEMPO (w/v) in CDCl3. Peptide concentration: 1.0 mm.
Peggion et al . (aMe)Aun peptides
upon addition of DMSO and TEMPO. All NH proton
20
resonances were assigned by means of two-dimensional rotating-frame nuclear Overhauser ehancement spectroscopy (ROESY) experiments. In this peptide the N(1)H and addition of DMSO and their resonances broaden signi®cantly upon addition of the paramagnetic perturbing agent TEMPO. All the other protons display a behavior characteristic of shielded protons, as their chemical shifts appear relatively insensitive to solvent composition and their
15 [h]T·103 (deg·cm2·dmol_1)
N(2)H proton chemical shifts are remarkably sensitive to the
10
5
linewidths are not in¯uenced by the addition of TEMPO. These 1H NMR results allowed us to conclude that in CDCl3 solution the N(3)H to N(8)H protons are almost
0
inaccessible to perturbing agents and are therefore, most 200
probably, intramolecularly H-bonded. Therefore, it is reasonable that the most populated conformation adopted in CDCl3 solution by the terminally protected (aMe)Aun/ Aib-rich octapeptide would be the 310-helix, where only the two N-terminal NH protons do not participate in the intramolecular H-bonding scheme. Figure 5 shows the behavior of the (aMe)Aun/Gly
210 220 230 240 Wavelength (nm)
250
Figure 6. CD spectrum of Ac-d-(aMe)Aun-Aib-Aib-d-(aMe)Aun-AibAib-d-(aMe)Aun-Aib-OtBu in TFE solution. Peptide concentration: 0.5 mm.
the relationship between a-carbon chirality and screw sense of the helix that is formed.
hexapeptide under analogous experimental conditions. Interestingly, in this case addition of DMSO and TEMPO seems to affect not only N(1)H and N(2)H protons, as already observed for the (aMe)Aun/Aib peptide, but (although less signi®cantly) also the N(4)H proton, thus con®rming the higher ¯exibility of Gly-containing peptides observed in the FT-IR absorption study. The octapeptide Ac-d-(aMe)Aun-Aib-Aib-d-(aMe)AunAib-Aib-d-(aMe)Aun-Aib-OtBu, lacking any potentially disturbing chromophoric group at the N-terminus, was synthesized with the aim of determining by CD the relationship between chirality at the a-carbon atom of this novel a-amino acid and the screw sense of the helix that is
Conclusions The experimental results reported here have helped to elucidate the conformational preferences of the highly lipophilic, chiral, Ca-methylated a-amino acid (aMe)Aun. Our FT-IR absorption, CD and NMR analyses agree with a highly populated 310-helical motif for the longer model peptides in solution. Moreover, we have been able to show that d-(aMe)Aun induces the formation of a left-handed helical structure. It is worth noting that in the octapeptide the three (aMe)Aun residues are positioned one on top of the other, each after one complete turn of the ternary helical
formed. The CD spectrum of this (d,d,d)-peptide in TFE
structure. This superstructural motif confers to the mole-
solution (Fig. 6), displaying a positive maximum centered at
cule some amphipathic character, as one face of the helix is
203 nm and a weak, positive shoulder at < 220 nm, strongly
extremely hydrophobic. Alternating incorporation of hydro-
resembles the dichroic pattern canonical for a 310-helix (33).
philic amino acids-(aMe)Aun residues will generate highly
Moreover, the observed pattern is typical of a left-handed
amphiphilic, helical structures instrumental to an optimal
helix. Therefore, the Ca-methylated a-amino acid (aMe)Aun
design of membrane active peptides. Synthetic efforts along
a
behaves as a C -trisubstituted a-amino acid with respect to
these lines are currently in progress in our laboratories.
J. Peptide Res. 55, 2000 / 262±269
| 267
Peggion et al . (aMe)Aun peptides
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12. Hruby, V.J., Li, G., Haskell-Luevano, C. &
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3. Hruby, V.J., Al-Obeidi, F. & Kazmierski, W. (1990) Emerging approaches in the molecular design of receptor-selective peptide ligands: conformational, topographical and dynamic considerations. Biochem. J. 268, 249±262. 4. Karle, I.L. & Balaram, P. (1990) Structural
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characteristics of a-helical peptide molecules
synthesis of lipidic a-amino acids, peptides
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