Conformationof 'Eyco(-L-Pro-Gly-)3 And Its Ca2' And Mg2 Complexes

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Proc. .Nat Acad.
Sci. USA
Vol. 79, pp. 4519-4522, July
1982
Chemistry
Conformationof 'eyCo(-L-Pro-Gly-)3 and its
Ca2'
and
Mg
2
complexes
(cyclic hexapeptide/ionophore/octahedral coordination/x-ray crystallography)
GOPINATH
K-ARTHA,
KOTTAYIL
I.
VARUGHESE,
AND
SABURO AIMoTo
Center
for
Crystallographic Research, Roswell Park
Memorial
Institute, Buffalo, New York
14263
Communicated by Elan
R
-
Blout,
Apri
12, 1982
ABSTRACT
The synthetic hexapeptide
cyclo(-L-Pro-Gly-)3
is
an ionophore that shows interesting conformational changes upon
binding metal ions. X-ray crystallographic studies of this peptide
show that when it is crystallized from an ethanol/ethyl acetate
mixture the ring takes up an asymmetric conformation containing
one cis
peptide
bond. In crystals of a
Ca"s
complex, the cation is
sandwiched between two peptide molecules that differ markedly
in conformation. However, both exhibit threefold symmetric
forms, with all six peptide bonds in. the molecule occurring in the
usual trans conformation. The
Ca"'
is octahedrally surrounded
by six glycyl carbonyl oxygens from the two peptides at an average
distance of 2.26
A
and can easily be released by the disruption -of
the
peptide
sandwich. In the magnesium complex, the peptide
forms a 1:1 complex with the ion. The Mg2
is
octahedrally
co-
ordinated to three glyc yl carbonyls and three water oxygens. The
average coordination distance between magnesium and the pep-
tide oxygens
is
2.03
A
and that between magnesium and water
oxygen-is 2.11
A.
The two peptide molecules in the asymmetric unit
have similar conformations and have approximate threefold
symmetry.
Naturally occurring cyclic peptides such as
enniatin (1-3) and
valinomycin (4,
5)
are known to bind ions and mediate their
transport across natural and prepared
biological
membranes;
these peptides have been
the subject of extensive investiga-
tions. The conformational changes that facilitate ion binding and
transport can be studied in considerable detail by using x-ray
crystallographic techniques, and such studies yield insight into
the mechanism
of ion
binding and transport through
mem-
branes at the molecular level. These studies and. the interpre-
tation of -the results are much simplified by the use of simple
model peptides that mimic these properties and can easily be
synthesized in the laboratory. One such synthetic peptide that
mimics the ion binding properties of antamanide (6) and en-
niatin is. the hexapeptide cyclo(-L-Pro-Gly-)3
[hereinafter
de-
noted
(PC)3],
which has been extensively studied by Blout and
co-workers by
spectroscopic
and computed
potential
energy
techniques (7,
8).
From
their studies they concluded that (PG)3
conformation
varies significantly writh
the
nature
of the
me-
dium.
In
nonpolar medium
as
well
as
when forming
complexes
with
cations,
(PG)3 takes
up a
threefold
symmetric
conforma-
tion, and in
polar solvents this changes
to an
asymmetric con-
formation
with
one
of the peptide bonds
in
the cis configuration.
Even
though crystallographic results of
a
few cyclic hexapep-
tides have been reported,
most
of these
contain two
f-turns,
with the peptide rings having at least an approximate twofold
or inversion symmetry. The
threefold
nature
of the chemical
sequence of (PG)3 and the existence of alternating proline
res-
idues
exert
conformational
restrictions
that make the usual dou-
Table 1. Crystal data
Molecular
formula
Space
group
Cell constants,
a, b, and
c
in
A, g in0
(PG)3
C2jH30N606.
3/H20
P3212
a
=
11.379(3)
c
=
32.93(1)
Z
6
No. of peptide
molecules in
the asymmet-
ric unit
1
20,m.
Cu Ka, °
120
No. of reflections
used in struc-
ture determi-
nation and
refinement
1,478
Final R factor, % 8
(PG,)3'Ca
C21H30N606.
%/Ca(C104)2.
3H20
P63
a = 12.366(1)
c = 20.830(1)
4
2/3
150
1,633
8
(PG)35Mg
C21H30N60,6,
i/X(C104)2.
%/2H20
P21
a
=
12.677(2)
b
=
12.340(5)
c
=
21.502(2)
P
=
92.69(1)
4
2
150
6,107
10
ble 3-turn structure with intramolecular hydrogen bonds
im-
possible for this hexapeptide.
We
report here the conformational
features of this peptide and the stoichiometry and geometry of
two of its cation complexes.
EXPERIMENTAL
(PG)0
was synthesized according to the procedure described by
Deber and Blout (9) and crystals were obtained from
ethanol/
ethyl acetate mixture by slow evaporation. The crystals
of cal-
cium and magnesium complexes with the
peptide
were ob-
tained from aqueous medium by the addition of calcium
or
magnesium perchlorate to the solution. The diffraction data
were
measured
at room
temperature on an Enraf Nonius au-
tomatic
diffractometer.
The
crystal data
are
given
in
Table
1.
The crystal
structure
of both (PG£)3 and
(PG)3-Ca
were
solved
by multisolution techniques (10). (PG)3'Mg
had
97
nonhydrogen
atoms
in an asymmetric unit
and all
attempts
to
solve the
struc-
ture
by direct methods
were
unsuccessful. The
cation
positions
were
located from the sharpened
Patterson
function and the
structure was
solved by
a
number of
cycles
of
structure
factor
and
Fourier
calculations. The
positional
and thermal
parame-
ters were
refined by block
diagonal least-squares techniques.
The
crystallographic details
and
atomic
parameters will be
pub-
lished elsewhere.
(PG)3 Molecule.
Some of
the
main
features of (PG)3
are
shown
in
Fig. 1. This is
not a
symmetric structure
and has
no
/3-turns
Abbreviation:
(PG)3, CyClO(-L-PrO-Gly-)3.
451.9
The publication
costs of this article were
defrayed
in
part by page charge
payment.
This article
must
therefore
be
hereby marked
"advertise-
ments"
in accordance with 18
U. S. C.
§1734
solely
to indicate
this
fact.

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