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Crystal structure of Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBu, a hexapeptide fragment of trichotoxin
Vol.
136,
May
14,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
1986
COMMUNICATIONS Pages
CRYSTAL STRUCTURE A HEXAPEPTIDE Michael Kokkinidis,
870-875
OF Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBn, FRAGMENT OF TRJCHOTOXIN Demetrius Tsernoglou and Hans Briickner *
EMBL, Meyerhofstrasse 1, Postfach 10.2209, D-6900 Heidelberg, FRG * Institute of Food Technology, University of Hohenheim, D-7000 Stuttgart 70, FRG Received
March
13,
1986
SUMMARY. The crystal structure of Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBu, an end-protected hexapeptide with a sequencecorresponding to residues7-12 of several trichotoxin A-50 sequenceanalogueshas been determined by X-ray crystallography. The hexapeptide adopts a right-handed &o-helical conformation consistingof four consecutivep-turns of type III. The helix is stabilized by four intramolecular hydrogen bonds. In the crystal the moleculesare connected head to tail with intermolecular hydrogen bonding interactions among translationally related moleculesthus forming infinitely long helical columns. The column-column interactions in the crystal are hydrophobic and occur predominantly between antiparallel directed columns. 0 1986 Academic Press, Inc.
The sterically hindered amino acid a-aminoisobutyric acid (Aib), a constituent of the peptide antibiotic alamethicin and related microbial peptides (peptaboils) has provoked a great deal of interest into the conformational parameten of these molecules.From the biological point of view an insight into these conformations is important for understanding the interactions of peptaboils with biological membranesand explaining their ability to form voltage-gated channelsin black bilayer membranes(1). Theoretical studies suggestthat the Aib residue should adopt an a-helical conformation if the substituents on the C, atom are arranged in a tetrahedral symmetrical geometry and a 31a-helical conformation for an asymmetric geometry of the substituents (2). So far, the X-ray structure determinations of Aib containing oligopeptideshave shown that depending on the chain length and the content in Aib residues,the peptides preferably form P-bends, a seriesof consecutiveP-bendswhich give rise to right oi’ left-handed 3lo-helices(3-6), a-helical structures (1,7) or structures consisting of a- and 3to-helical regions (8). However, despite the growing number of structure determinations on Aib containing peptides, the existing data basisdoesnot yet allow a satisfactory explanation of their stereochemistry and reliable predictions of their structures. 0006-291X/86 Copyright All rights
$1.50 0 1986 by Academic Press, oJ’ reproduction in any ,fbbrrn
Inc. reserved.
870
Vol.
136,
No.
3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
We now report the successful crystal structure determination OtBu (I) (Abbreviations: nine).
Z = benzyloxycarbonyl,
(I) is an end-protected
several trichotoxin
RESEARCH
COMMUNICATIONS
of Z-Aib-Aib-Aib-Ala-Ala-Aib-
OtBu = tertiary
butylester,
peptide with a sequence corresponding
Ala=L-ala-
to residues 7-12 of
A-50 sequence analogues (9). For all isolated A-50 components this re-
gion comprises exclusively Aib and Ala residues (9). However with the exception of residue Ala 10 in trichotoxin
(corresponding
to Ala 4 in (I)), every Aib residue in this fragment is
exchangable by Ala and vice versa. This variability heterogeneity of trichotoxin,
contributes
to the pronounced micro-
i.e. to a complex pattern of sequence analogues. MATERIALS
AND METHODS
(I) was synthesized by fragment coupling of the oxazolone of Z-Aib-Aib-Aib-OH (10) and H-Ala-Ala-Aib-OtBu. The latter was synthesized by stepwise elongation of H-Aib-OtBu with Z-Ala-OH using N-ethyl-N’-(3-dimethylaminopropyl)~carbodiimide hydrochloride and I-hydroxybenzotriazole as coupling reagents and N-terminal deprotection with hydrogen and palladium on charcoal (10%) as catalyst. (I) was characterized by elemental analy sis, mass spectrometry and 13C nmr spectroscopy. Single crystals of (I) were obtained from a methanol/water solution. The title compound crystallizes in space group P2rZr2r with cell dimensions a=11.598, b=12.831, c=27.859 A and Z=4. One molecule of methanol was found associated with each peptide molecule after structure determination. A single crystal with the approximate dimensions 0.1x0.2x0.6 mm was used for the collection of X-ray intensity data (CAD-4 diffractometer, w - 28 scans, Cu & radiation). Of the 4322 reflections measured up to a resolution of 0.82 A, 3935 reflections with I > l.Su(1) were considered observed and were used in the crystallographic refinement. The structure was solved by direct methods using the program MULTANBO (11). An E map with the highest figure of merit revealed a stereochemically sensible 38-atom fragment. Successive Karle (12) recycling followed by Fourier techniques allowed the location of all non-hydrogen atoms including one molecule of crystalline methanol. Least squares refinement on F was performed using the program SHELX76 (13) with anisotropic temperature factors for the non-hydrogen atoms. Hydrogen atoms were refined isotropically as rigid groups with fixed bond distance of 1.08 A. The hydrogen atom of the OH group in methanol could not be located on difference Fourier maps and was not included in the refinement. The refinement converged to a final R=0.074 for 3909 reflections (26 reflections with IIFo] - lFc]l > 4up excluded in the final cycles). RESULTS AND DISCUSSION A perspective
view of the hexapeptide is shown in Fig.
1. The conformation
of (I) is
described by the backbone torsion angles d,$, w (14) given in Table I. The title compound adopts a secondary structure
corresponding
to a right-handed
from four consecutive lo-atom hydrogen bonded p-turns 310 conformation
31e-helii.
of type III (15). Despite the overall
of (I) the 4,J, angles of the first residue Aib 1 are very close to the ideal
values for an a-helix (4 = -57O, + = -47O).
The conformational
next two residues Aib 2, Aib 3 agree well with a 3 re-helical structure JI = -309,
The helix results
backbone angles for the (ideal values: 4 = -60°,
whereas the backbone torsion angles for Ala 4 and Ala 5, while still acceptable 871
Vol.
136,
BIOCHEMICAL
No. 3, 1986
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
Fig. 1.: The molecular structure of (I). The backbone is indicated by thick lines, intramolecular hydrogen bonds by broken lines.
for a 3ro conformation, deviate significantly from the ideal values. The torsion angles for the last residue Aib C-terminal
lie in the left-handed helical region. This feature has been observed for the
6
Aib residue in some other peptides with a terminal ester group (16,17). Thus it
appears that solely dihedral angles provide an insufficient criterion for the classification of a helical structure,
a fact already observed with other Aib peptides (8). The four Aib residues
show significant deviations of the w angles (Table I) from the ideal value of a trans planar peptide ( 1800). The effects of the non-planar distortion of the structure
of the peptide unit on the stability
are discussed in (2). The &e-helical character of (I) becomes obvious from
the pattern of intramolecular
Four 4 + 1 hydrogen bonds (Fig.
hydrogen bonds.
1) are
formed in (I) with the NH groups of residues Aib 3, Ala 4, Ala 5 and Ala 6 interacting with the CO groups of Z, Aib 1, Aib 2 and Aib 3 respectively.
The geometrical parameters of
these bonds are given in Table II. The valency geometry around the C,
Table I:
atom
in the four Aib
Backbone torsion angles (“1 of (I1
Residue
4
JI
Aib 1 Aib 2 Aib 3 Ala 4 Ala 5 Aib 6
-57.6 -58.4 -57.3 -65.8 -75.4 50.5
-49.7 -3G.O -30.2 -18.0 -24.8 38.5
For definitionof the backboneseeFig. 872
1
W
-162.5 -172.1 -171.9 -177.6 -177.1 -173.0
Vol.
136,
BIOCHEMICAL
No. 3, 1986
AND
Table
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
II
Parameters of the hydrogenbondsformedbetweenNH/OH groupsand CO groupsin (I)
Donor (N/O)
Acceptor (0)
Aib 3 (N) Ala 4 (N) Ala 5 (N) Aib 6 (N) Aib 1 (N) Aib 2 (N) Methanol (0)
Z Aib Aib Aib Ala Ala Ala
N/O...0
(A)
N - H...O [“)
3.04 3.10 2.96 3.12 2.88 3.12 2.81
1 2 3 4o 5a 5
131.6 157.9 150.8 154.8 167.0 164.0 -6
o Intermolecularhydrogenbondformedbetweenmolecules relatedby translationalongthe a-axis(l+x,y,s) b The position of the H atom of the OH group in methanol has not been determined.
residuesis asymmetric (Table III). Following the nomenclature given in (2) we designateas Cf and C$ the Cs carbonsin Aib that occupy the sameposition asthe Cs and the hydrogen atom respectively in L-amino acids. For residuesAib 1, Aib 2 and Aib 3 we observedthat the N-C,-Cf
and the C-Ca-Ci angles are significantly smaller than the tetrahedral value
(109.45O),while N-C&‘{
and C-C&~
are significantly bigger. This behaviour is expected
from theoretical calculations (2) for a right-handed 3rs-helix. An opposite asymmetry is found for Aib 6 which lies in the left-handed helical region. Since an a-helical conformation should show no asymmetry, the valency geometry at C, in Aib 1 agreeswith the hydrogen bonding interactions (3ie-helical) of this residue and contradicts its o-helical 4, II, angles. The 310-helix formed by (I) is parallel to the crystallographic a-axis. As seenfrom Fig. 2 this arrangement permits the NH groupsof Aib 1 and Aib 2 to form intermolecular hydrogen bonds with the CO groupsof Ala 4 and Ala 5 respectively of a moieculerelated by translation along the a-axis (Table II). Thus the moleculesare connected in a head-to-tail fashion with
Table
III
Bond angles (“) defining the valency geometry around the Aib C, atoms Residue Aib Aib Aib Aib
1 2 3 G
N-C./$ 107.0 107.2 108.9 111.1
C-C/Tf 108.8 106.4 107.2 109.4
873
N-C,--C,B 110.6 111.4 111.0 108.1
of (11
c-c/3:: 109.6 111.5 110.6 106.6
Vol.
136,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
Fig. 2.: Head-to-tail arrangement of (I) in the crystal. For clarity, hydrogen atoms have been omitted for the hexapeptide, however not for methanol. Intermolecular hydrogen bonds are indicated by broken lines, intramolecular hydrogen bonds by dotted lines. View down the b-axis.
a hydrogen bonding pattern corresponding to an infinitely long helical column. Other 31ehelical peptides also pack in a similar fashion (6,17). Interestingly, only the right-handed helical part of (I) participates in the net of hydrogen bonds (intra- or intermolecular) in the crystal. The left-handed C-terminal region (Aib 6) flips out of the helical column and is not involved in any hydrogen bonding interactions. A further hydrogen bond is formed between
L
Fig. 3.: Stereo drawing View down the a-axis.
of the crystal packing.
874
For clarity, hydrogen atoms are omitted.
Vol.
136,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
the methanol molecule and the CO group of Ala 5. The packing of the infinitely long helical columns is such that they form layers parallel to the ab-plane (Fig. 3). In these layers the column-column
interactions are hydrophobic with each column interacting with two columns
of opposite directionality.
Energy calculations indicate that this antiparallel
arrangement
is the most favourable (18). Layers are separated from each other by layers of methanol molecules. REFERENCES 1.
Fox, R.O. and Richards, F.M. (1982) Nature 300, 325-330
2.
Patenon, Y., Rumsey, S.M., Benedetti, E., NCmethy, G. and Sheraga, H.A. (1981) J. Am. Chem. Sot 103, 2947-2955
3.
Shamala, N., Nagaraj, R. and Balaram, P. (1977) Biophys. 79, 292-298
Biochem. Res. Comm.
4.
Bosch, R., Jung, G. and Winter, W. (1983) Acta Cryst.
5.
Shamala, N., Nagaraj, R. and Balaram, P. (1978) J.C.S. Chem. Comm., 996-997
C39, 776-778
6.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Acta Cryst.
7.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Biopolymers
24, 961-978
8.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Biopolymers
24,979.999
9.
Przybylski, M., Dietrich, I., Manz, I. and Briickner, 11, 569-582
C41, 1821.1825
H. (1984) Biom. Mass Spectrom.
10.
Leplawy, M.T., Jones, D.S., Kenner, G.W. and Sheppard, R.C. (1960) Tetrahedron 11, 39-51
11.
Main, P., Fiske, S.J., Hull, S.E., Lessinger, L., Germain, G., Declercq, J.-P. and Woolfson, M.M. (1980) MULTAN80. A System of Computer Programs for the Automatic Solutionof Crystal Structures from X-ray Diffraction Data. Univs. of York, England and Louvain, Belgium
12.
Karle, J. (1968) Acta Cryst.
13.
Sheldrick, G.M. (1976) SHELX76. Univ. of Cambridge, England
14.
IUPAC-IUB 3479
15.
Crawford, J.L., Lipscomb, W.N. and Schellman, C.G. (1973) Proc. Nat. Acad. Sci. USA 70, 538-542
16.
Smith, D.G., Pletnev, V.Z., Duax, W.L., Balasubramanian, T.M., Bosshard, H-E., Czerwinski, E. W., Kendrick, N.E., Mathews, F.S. and Marshall, G.R. (1981) J. Am. Chem. Sot. 103, 1493-1501
17.
nancis, 1505
18.
Hol, W.G.J., Halie, L.M. and Sander, C. (1981) Nature 294, 532-536
B24, 182-186 A Program for Crystal
Structure Determination.
Commission on Biochemical Nomenclature (1970) Biochemistry
9,3471-
A.K., Iqbal, M., Balaram, P, and Vijayan, M. (1983) Biopolymers 22, 149%
875
136,
May
14,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
1986
COMMUNICATIONS Pages
CRYSTAL STRUCTURE A HEXAPEPTIDE Michael Kokkinidis,
870-875
OF Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBn, FRAGMENT OF TRJCHOTOXIN Demetrius Tsernoglou and Hans Briickner *
EMBL, Meyerhofstrasse 1, Postfach 10.2209, D-6900 Heidelberg, FRG * Institute of Food Technology, University of Hohenheim, D-7000 Stuttgart 70, FRG Received
March
13,
1986
SUMMARY. The crystal structure of Z-Aib-Aib-Aib-Ala-Ala-Aib-OtBu, an end-protected hexapeptide with a sequencecorresponding to residues7-12 of several trichotoxin A-50 sequenceanalogueshas been determined by X-ray crystallography. The hexapeptide adopts a right-handed &o-helical conformation consistingof four consecutivep-turns of type III. The helix is stabilized by four intramolecular hydrogen bonds. In the crystal the moleculesare connected head to tail with intermolecular hydrogen bonding interactions among translationally related moleculesthus forming infinitely long helical columns. The column-column interactions in the crystal are hydrophobic and occur predominantly between antiparallel directed columns. 0 1986 Academic Press, Inc.
The sterically hindered amino acid a-aminoisobutyric acid (Aib), a constituent of the peptide antibiotic alamethicin and related microbial peptides (peptaboils) has provoked a great deal of interest into the conformational parameten of these molecules.From the biological point of view an insight into these conformations is important for understanding the interactions of peptaboils with biological membranesand explaining their ability to form voltage-gated channelsin black bilayer membranes(1). Theoretical studies suggestthat the Aib residue should adopt an a-helical conformation if the substituents on the C, atom are arranged in a tetrahedral symmetrical geometry and a 31a-helical conformation for an asymmetric geometry of the substituents (2). So far, the X-ray structure determinations of Aib containing oligopeptideshave shown that depending on the chain length and the content in Aib residues,the peptides preferably form P-bends, a seriesof consecutiveP-bendswhich give rise to right oi’ left-handed 3lo-helices(3-6), a-helical structures (1,7) or structures consisting of a- and 3to-helical regions (8). However, despite the growing number of structure determinations on Aib containing peptides, the existing data basisdoesnot yet allow a satisfactory explanation of their stereochemistry and reliable predictions of their structures. 0006-291X/86 Copyright All rights
$1.50 0 1986 by Academic Press, oJ’ reproduction in any ,fbbrrn
Inc. reserved.
870
Vol.
136,
No.
3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
We now report the successful crystal structure determination OtBu (I) (Abbreviations: nine).
Z = benzyloxycarbonyl,
(I) is an end-protected
several trichotoxin
RESEARCH
COMMUNICATIONS
of Z-Aib-Aib-Aib-Ala-Ala-Aib-
OtBu = tertiary
butylester,
peptide with a sequence corresponding
Ala=L-ala-
to residues 7-12 of
A-50 sequence analogues (9). For all isolated A-50 components this re-
gion comprises exclusively Aib and Ala residues (9). However with the exception of residue Ala 10 in trichotoxin
(corresponding
to Ala 4 in (I)), every Aib residue in this fragment is
exchangable by Ala and vice versa. This variability heterogeneity of trichotoxin,
contributes
to the pronounced micro-
i.e. to a complex pattern of sequence analogues. MATERIALS
AND METHODS
(I) was synthesized by fragment coupling of the oxazolone of Z-Aib-Aib-Aib-OH (10) and H-Ala-Ala-Aib-OtBu. The latter was synthesized by stepwise elongation of H-Aib-OtBu with Z-Ala-OH using N-ethyl-N’-(3-dimethylaminopropyl)~carbodiimide hydrochloride and I-hydroxybenzotriazole as coupling reagents and N-terminal deprotection with hydrogen and palladium on charcoal (10%) as catalyst. (I) was characterized by elemental analy sis, mass spectrometry and 13C nmr spectroscopy. Single crystals of (I) were obtained from a methanol/water solution. The title compound crystallizes in space group P2rZr2r with cell dimensions a=11.598, b=12.831, c=27.859 A and Z=4. One molecule of methanol was found associated with each peptide molecule after structure determination. A single crystal with the approximate dimensions 0.1x0.2x0.6 mm was used for the collection of X-ray intensity data (CAD-4 diffractometer, w - 28 scans, Cu & radiation). Of the 4322 reflections measured up to a resolution of 0.82 A, 3935 reflections with I > l.Su(1) were considered observed and were used in the crystallographic refinement. The structure was solved by direct methods using the program MULTANBO (11). An E map with the highest figure of merit revealed a stereochemically sensible 38-atom fragment. Successive Karle (12) recycling followed by Fourier techniques allowed the location of all non-hydrogen atoms including one molecule of crystalline methanol. Least squares refinement on F was performed using the program SHELX76 (13) with anisotropic temperature factors for the non-hydrogen atoms. Hydrogen atoms were refined isotropically as rigid groups with fixed bond distance of 1.08 A. The hydrogen atom of the OH group in methanol could not be located on difference Fourier maps and was not included in the refinement. The refinement converged to a final R=0.074 for 3909 reflections (26 reflections with IIFo] - lFc]l > 4up excluded in the final cycles). RESULTS AND DISCUSSION A perspective
view of the hexapeptide is shown in Fig.
1. The conformation
of (I) is
described by the backbone torsion angles d,$, w (14) given in Table I. The title compound adopts a secondary structure
corresponding
to a right-handed
from four consecutive lo-atom hydrogen bonded p-turns 310 conformation
31e-helii.
of type III (15). Despite the overall
of (I) the 4,J, angles of the first residue Aib 1 are very close to the ideal
values for an a-helix (4 = -57O, + = -47O).
The conformational
next two residues Aib 2, Aib 3 agree well with a 3 re-helical structure JI = -309,
The helix results
backbone angles for the (ideal values: 4 = -60°,
whereas the backbone torsion angles for Ala 4 and Ala 5, while still acceptable 871
Vol.
136,
BIOCHEMICAL
No. 3, 1986
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
Fig. 1.: The molecular structure of (I). The backbone is indicated by thick lines, intramolecular hydrogen bonds by broken lines.
for a 3ro conformation, deviate significantly from the ideal values. The torsion angles for the last residue Aib C-terminal
lie in the left-handed helical region. This feature has been observed for the
6
Aib residue in some other peptides with a terminal ester group (16,17). Thus it
appears that solely dihedral angles provide an insufficient criterion for the classification of a helical structure,
a fact already observed with other Aib peptides (8). The four Aib residues
show significant deviations of the w angles (Table I) from the ideal value of a trans planar peptide ( 1800). The effects of the non-planar distortion of the structure
of the peptide unit on the stability
are discussed in (2). The &e-helical character of (I) becomes obvious from
the pattern of intramolecular
Four 4 + 1 hydrogen bonds (Fig.
hydrogen bonds.
1) are
formed in (I) with the NH groups of residues Aib 3, Ala 4, Ala 5 and Ala 6 interacting with the CO groups of Z, Aib 1, Aib 2 and Aib 3 respectively.
The geometrical parameters of
these bonds are given in Table II. The valency geometry around the C,
Table I:
atom
in the four Aib
Backbone torsion angles (“1 of (I1
Residue
4
JI
Aib 1 Aib 2 Aib 3 Ala 4 Ala 5 Aib 6
-57.6 -58.4 -57.3 -65.8 -75.4 50.5
-49.7 -3G.O -30.2 -18.0 -24.8 38.5
For definitionof the backboneseeFig. 872
1
W
-162.5 -172.1 -171.9 -177.6 -177.1 -173.0
Vol.
136,
BIOCHEMICAL
No. 3, 1986
AND
Table
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
II
Parameters of the hydrogenbondsformedbetweenNH/OH groupsand CO groupsin (I)
Donor (N/O)
Acceptor (0)
Aib 3 (N) Ala 4 (N) Ala 5 (N) Aib 6 (N) Aib 1 (N) Aib 2 (N) Methanol (0)
Z Aib Aib Aib Ala Ala Ala
N/O...0
(A)
N - H...O [“)
3.04 3.10 2.96 3.12 2.88 3.12 2.81
1 2 3 4o 5a 5
131.6 157.9 150.8 154.8 167.0 164.0 -6
o Intermolecularhydrogenbondformedbetweenmolecules relatedby translationalongthe a-axis(l+x,y,s) b The position of the H atom of the OH group in methanol has not been determined.
residuesis asymmetric (Table III). Following the nomenclature given in (2) we designateas Cf and C$ the Cs carbonsin Aib that occupy the sameposition asthe Cs and the hydrogen atom respectively in L-amino acids. For residuesAib 1, Aib 2 and Aib 3 we observedthat the N-C,-Cf
and the C-Ca-Ci angles are significantly smaller than the tetrahedral value
(109.45O),while N-C&‘{
and C-C&~
are significantly bigger. This behaviour is expected
from theoretical calculations (2) for a right-handed 3rs-helix. An opposite asymmetry is found for Aib 6 which lies in the left-handed helical region. Since an a-helical conformation should show no asymmetry, the valency geometry at C, in Aib 1 agreeswith the hydrogen bonding interactions (3ie-helical) of this residue and contradicts its o-helical 4, II, angles. The 310-helix formed by (I) is parallel to the crystallographic a-axis. As seenfrom Fig. 2 this arrangement permits the NH groupsof Aib 1 and Aib 2 to form intermolecular hydrogen bonds with the CO groupsof Ala 4 and Ala 5 respectively of a moieculerelated by translation along the a-axis (Table II). Thus the moleculesare connected in a head-to-tail fashion with
Table
III
Bond angles (“) defining the valency geometry around the Aib C, atoms Residue Aib Aib Aib Aib
1 2 3 G
N-C./$ 107.0 107.2 108.9 111.1
C-C/Tf 108.8 106.4 107.2 109.4
873
N-C,--C,B 110.6 111.4 111.0 108.1
of (11
c-c/3:: 109.6 111.5 110.6 106.6
Vol.
136,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
Fig. 2.: Head-to-tail arrangement of (I) in the crystal. For clarity, hydrogen atoms have been omitted for the hexapeptide, however not for methanol. Intermolecular hydrogen bonds are indicated by broken lines, intramolecular hydrogen bonds by dotted lines. View down the b-axis.
a hydrogen bonding pattern corresponding to an infinitely long helical column. Other 31ehelical peptides also pack in a similar fashion (6,17). Interestingly, only the right-handed helical part of (I) participates in the net of hydrogen bonds (intra- or intermolecular) in the crystal. The left-handed C-terminal region (Aib 6) flips out of the helical column and is not involved in any hydrogen bonding interactions. A further hydrogen bond is formed between
L
Fig. 3.: Stereo drawing View down the a-axis.
of the crystal packing.
874
For clarity, hydrogen atoms are omitted.
Vol.
136,
No. 3, 1986
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
the methanol molecule and the CO group of Ala 5. The packing of the infinitely long helical columns is such that they form layers parallel to the ab-plane (Fig. 3). In these layers the column-column
interactions are hydrophobic with each column interacting with two columns
of opposite directionality.
Energy calculations indicate that this antiparallel
arrangement
is the most favourable (18). Layers are separated from each other by layers of methanol molecules. REFERENCES 1.
Fox, R.O. and Richards, F.M. (1982) Nature 300, 325-330
2.
Patenon, Y., Rumsey, S.M., Benedetti, E., NCmethy, G. and Sheraga, H.A. (1981) J. Am. Chem. Sot 103, 2947-2955
3.
Shamala, N., Nagaraj, R. and Balaram, P. (1977) Biophys. 79, 292-298
Biochem. Res. Comm.
4.
Bosch, R., Jung, G. and Winter, W. (1983) Acta Cryst.
5.
Shamala, N., Nagaraj, R. and Balaram, P. (1978) J.C.S. Chem. Comm., 996-997
C39, 776-778
6.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Acta Cryst.
7.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Biopolymers
24, 961-978
8.
Bosch, R., Jung, G., Schmitt, H. and Winter, W. (1985) Biopolymers
24,979.999
9.
Przybylski, M., Dietrich, I., Manz, I. and Briickner, 11, 569-582
C41, 1821.1825
H. (1984) Biom. Mass Spectrom.
10.
Leplawy, M.T., Jones, D.S., Kenner, G.W. and Sheppard, R.C. (1960) Tetrahedron 11, 39-51
11.
Main, P., Fiske, S.J., Hull, S.E., Lessinger, L., Germain, G., Declercq, J.-P. and Woolfson, M.M. (1980) MULTAN80. A System of Computer Programs for the Automatic Solutionof Crystal Structures from X-ray Diffraction Data. Univs. of York, England and Louvain, Belgium
12.
Karle, J. (1968) Acta Cryst.
13.
Sheldrick, G.M. (1976) SHELX76. Univ. of Cambridge, England
14.
IUPAC-IUB 3479
15.
Crawford, J.L., Lipscomb, W.N. and Schellman, C.G. (1973) Proc. Nat. Acad. Sci. USA 70, 538-542
16.
Smith, D.G., Pletnev, V.Z., Duax, W.L., Balasubramanian, T.M., Bosshard, H-E., Czerwinski, E. W., Kendrick, N.E., Mathews, F.S. and Marshall, G.R. (1981) J. Am. Chem. Sot. 103, 1493-1501
17.
nancis, 1505
18.
Hol, W.G.J., Halie, L.M. and Sander, C. (1981) Nature 294, 532-536
B24, 182-186 A Program for Crystal
Structure Determination.
Commission on Biochemical Nomenclature (1970) Biochemistry
9,3471-
A.K., Iqbal, M., Balaram, P, and Vijayan, M. (1983) Biopolymers 22, 149%
875