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Conformations of cyclo-l-his-l-ser, cyclo-l-his-l-asp, and cyclo-l-his-l-his
Tetrahedron
Lettera
No. 6, pp 403 - 406, 1972.
CONFORMATIONS OF CYCLO-L-HIS-L-SER, Ziauddin, Department
of
(Received
P.m.r. dine
side
chain
solutions
prefers
the xl
a-protons
ward diketopiperazine
c-L-Asp-L-His dues affects
Table
I presents
the added bulk, populated
dominant
one.
imidazole
the xl
current
conformation,
We have now examined aqueous
functional
side
its
side
of the other rotamer,
of
constant chain
serine
is
side
two staggered
although
solutions
ring
to-
of phenylalanine, of c_-L-Ser-L-His,
preference
the second
shielding
of aromatic
peptides,
including
near ~1 = 60’
side
resi-
chains
is
also
are noteworthy.
are based.
of the coupling
of the
dipeptides
In these
the histi-
aromatic
to be more stable
the diketopiperazine
analysis6 state
chains.
residue
aS coupling
that
in cyclic
conformational
of the conformation
group toward
= 60’
being
ring
60616
6 January 1972)
show that
.
indicate
= 60’
This
consistently
this
Illinois
c-Gly-L-His
the evidence
group2.
to see if
Chicago,
in OK for publication
dipeptide
on which our conclusions
as the more stable
-c-L-Ser-L-His,
rotamer,
of the aromatic
derivative, the xl
the cyclic
= 60’
The 2.6 Hz.
hydroxyl
received
in Great Britain.
AND CYCLO-L-HIS-L-HIS1
of Technology,
favored
features
the data
form of c-L-Ser-L-His
the 0-acetyl
of
between
the tendency
and certain
conformation,
Institute
Printed
and C. A. gush
is
2-5
interactions
c-L-Ser-L-His.
of
ring,
and C-L-His-L-His
-c-L-His-L-His, apparent,
by the
and tryptophane
tyrosine,
Illinois
of
CYCLO-L-HIS-L-ASP,
K. D. Kopple
in DSA 16 December1971;
studies
of the glycine
Chemistry,
Preee.
Pergamon
of
the serine
almost
completely
ring
is
forms.
somewhat less
indicates still
three
= 60’
For the cation that,
in spite
times
For the free
favored,
in the cationic
in the xl
(shown in I).
constants chain
residue
is again
of
as densely
base of the pre-
No. 6
Cyclic Dipeptides Table I. B-ProtonResonancesin Hi.stidine-Containing Seryl or Aspartyl c-L-His-LSer
OAc-Ser
Asp
His
pHd
Histidyl
JaB__-6
J aB
k
--&
Jut?
--6
Ja8 ---
7-8
3.32
5.4
3.59
3.1
11.8
3.16
---
3.16
3
3.53
2.6
3.93
2.6
12.2
3.20
5.0
3~.46 5.9
7-8
3.53
6.0
4.08
3.5
11.5
3.14
4.8
3.25
6.1
3
4.14
4.7
4.26
3.2
11.6
3.31
5.5
3.40
6.1
10
1.38
10.5
2.46
2.9
16.5
3.13
---
3.13
---
5-6
2.10
8.0
2.62
3.8
16.4
3.32
---
3.32
---
3
2.75
___
2.75
---
___
3.38
---
3.38
---
10
2.40
7.4
2.90
4.3
3
3.18
--- . 3.18
---
Reference: 2,2-dimethyl-2-silapentane-5-sulfonate Ser and OAc-Ser derivativesrun at 220 MHz, others at 60 MHz.
The serine g-protonsin the neutral form of c-L-Ser-L-Hisare shiftedupfield 0.6 and 0.3 ppm relative to c-L-Ser-L-Serin water (3.91 ppm). The proton with the larger coupling constant,which spends more time trans to the o-proton,has the greaterupfield shift. This is consistentwith ring current shieldingby the imidazolegroup, and indicatessignificant contributionsfrom the XL = 60' rotamer of the histidineside chain. Analysis of the couplingconstantdata indicatethat this is the most favored of the three staggered rotamers,althoughon the average only about half of the side chains are in this state. An X-ray crystallographic analysisof the structureof c-L-Ser-L-Tyrindicates Xl
= 60' for both the side chains,with an almost planar diketopiperazine ring7. The nmr
results presentedhere suggest that this (I) is probablythe most favored conformationOf c-L-Ser-L-Hisas well. However, this conformationalpreferencedoes not lead to any strong interactionbetween the functionalside chains, at least as judged by the pKa's of the the measured pKa's at peptides. For c-Cly-L-His,c-L-Ser-L-His,and c-OAc-L-Ser-L-His 3o", u = 0.085, are 6.3, 6.4, and 6.2, respectively. We have also determineda pH-rate profile for hydrolysisof the 0-chloroacetylderivativeand find no evidenceof a term for intramolecularimidazolecatalysis.
.
No. 6
485
II.
I. c-L-Asp-L-His.
The a-S
form of L-L-Asp-L-His tion
(0.7
c-Gly-Asp sidue
anion
is
= 300’
ring
important.
chains,
although
detectably,
the
anhydride S-protons
space
over
a6
This of
tyrosine3,
indicate
that
of
likelihood
the diketopiperazine
Gaussian
bands,
in one equivalent outside are well
evidence
of acid. error,
The hypochromism chain
at 30°, form,
1.26
(pH S-6),
between
a good hydrogen
ppm relative
closest to the
of the histidine
the preference
the oppositely to affect
for
charged
rexl
(Asp)
side
the imidazole
basicity
n = 0.085.
bears
coupling
constants
Again the proton
conformation
sufficient
a considerable
constants
resemblance
and the upfield
in which the imidazole
of
absorption side
chain
of
c-L-His-L-His,
interactions.
~2 4,500
is
assigned
In the case
of
c-L-His-L-His
which does
indicate
the ultraviolet
in c-L-His-L-His
CD bands,
6.3
the a-g
known in DNA and imply parallel examined
not
conforma-
to the
shifts
rings
are
of
sharing
ring.
and a band of
experimental
We have also
side
for
is
anionic
the predominant
coupling
(II).
between
of the
which allows
upfield,
= 60’
residue
is
in these
= 300°,
the peptide
a conformation
We have examined the ultraviolet and c-L-His-L-Asp
shifted
acid
= 300’
residue
the xl
in neutral
of
xl
attraction
being
in that
change
the aspartic
interaction
dipeptide,
or xl
form is
form of
the imidazole
the aspartic
= 180’
) is markedly
because
the coulombic
c-L-His-L-His. cyclic
J
xl
of
no significant
ppm, indicating
possibly
the pKa of
either
the favored
In the dipolar
is diminished,
constants
and N-H of
(highest
at 2.64
is
that
the carboxylate
imidazole
that
There
It seems likely
bond between to the
indicate
mole fraction).
2 and 60’.
coupling
planar
for
to histidine
evidence
c-L-Tyr-L-Tyr’.
were decomposed
is a 15% hypochromism
chain of
dichrosim
c-L-His-L-Ser
at 212 nm, at both
there
arrangements
led us to expect
as has been observed
The spectra
some side
circular
c-L-His-Gly,
interaction. aromatic
side
at pH 8, effects
chromophores8.
chain
In fact,
pH 8 and
Similar
of our compounds to of
into
185 nm.
interaction
in the
we do not observe
No. 6
486
evidence
of
side
chain
chromophore
any of the CD hands to the side L-L-His-L-Asp larger there
at basic
negative
coupling,
chain
that
to that histidyl
of c-L-Ala-Gly” is
are no CD bands which can be distinctly
of protonation
of the histidyl
protonation
of c-L-Orn-L-Orn
n-r*
near
region
side
Service,
National
the National
Institute
chromophoric
contribution
Science
was supported
of General
Foundation,
Medical
is
unambiguously c-L-His-L-Ser
we observe
slightly
than alanyl.
However,
chromophores.
similar
Protonation
.
of the basic
research
although perturber
on the CD curve 11
to assign
to histidyl
as in the case
group does not make any distinct This
a stronger
assigned
or c_-L-Lys-L-Lys
220 nm, but,
Acknowledgement.
chain
impossible
The CD of c-L-His-Gly,
chromophore.
pH are similar
CD, indicating
is
and it
to that
The effect
observed
adds some positive
solutions,
in
CD in the
the protonated
imidazole
to the CD.
by grants
Sciences,
from the U. S. Public
GM-14069 and GM-47357,
Health
and from
GP-20053.
References Based on the Ph.D.
dissertation
of
Ziauddin,
IIT,
1971.
K. D. Kopple and M. Ohnishi,
J. Amer. Chem. Sot.,
91,
962 (1969).
K. D. Kopple
J. Amer. Chem. Sot.,
g,
6193
and D. H. Marr,
(1967).
G. Gawne, G. W. Kenner, N. H. Rogers, R. C. Sheppard and K. Titlestad, Peptides, 1968, E. Bricas, Ed. John Wiley and Sons, New York (1969) p. 5.
Ziauddin
6.
K. G. R. Pachler,
7.
C. F. Lin and L. E. Webb, abstracts graphic Association, August, 1971,
a.
C. A. Bush in “Basic Principles of Nucleic Academic Press, New York (in press).
9.
E. H. Strickland, 4168 (1970). J.
and K. D. Kopple,
Greenfield
10.
N.
11.
D. W. Urry,
J.
Org.
Spectrochimica
M. Wilchek,
Chem. 35,
Acta,
J.
g,
253 (1970).
581 (1964).
of the meeting of the American Crystallop. 56, and personal communication.
Horwitz
Acid
Chem. g,
Chemistry”
and C. Billups,
and G. D. Fasman,Biopolymers,
Ann. Rev. Phys.
in 28.
1,
477 (1968).
(P.O.P.Ts’o, J.
595 (1969).
Biol.
ed.)
Chem. 245,
Lettera
No. 6, pp 403 - 406, 1972.
CONFORMATIONS OF CYCLO-L-HIS-L-SER, Ziauddin, Department
of
(Received
P.m.r. dine
side
chain
solutions
prefers
the xl
a-protons
ward diketopiperazine
c-L-Asp-L-His dues affects
Table
I presents
the added bulk, populated
dominant
one.
imidazole
the xl
current
conformation,
We have now examined aqueous
functional
side
its
side
of the other rotamer,
of
constant chain
serine
is
side
two staggered
although
solutions
ring
to-
of phenylalanine, of c_-L-Ser-L-His,
preference
the second
shielding
of aromatic
peptides,
including
near ~1 = 60’
side
resi-
chains
is
also
are noteworthy.
are based.
of the coupling
of the
dipeptides
In these
the histi-
aromatic
to be more stable
the diketopiperazine
analysis6 state
chains.
residue
aS coupling
that
in cyclic
conformational
of the conformation
group toward
= 60’
being
ring
60616
6 January 1972)
show that
.
indicate
= 60’
This
consistently
this
Illinois
c-Gly-L-His
the evidence
group2.
to see if
Chicago,
in OK for publication
dipeptide
on which our conclusions
as the more stable
-c-L-Ser-L-His,
rotamer,
of the aromatic
derivative, the xl
the cyclic
= 60’
The 2.6 Hz.
hydroxyl
received
in Great Britain.
AND CYCLO-L-HIS-L-HIS1
of Technology,
favored
features
the data
form of c-L-Ser-L-His
the 0-acetyl
of
between
the tendency
and certain
conformation,
Institute
Printed
and C. A. gush
is
2-5
interactions
c-L-Ser-L-His.
of
ring,
and C-L-His-L-His
-c-L-His-L-His, apparent,
by the
and tryptophane
tyrosine,
Illinois
of
CYCLO-L-HIS-L-ASP,
K. D. Kopple
in DSA 16 December1971;
studies
of the glycine
Chemistry,
Preee.
Pergamon
of
the serine
almost
completely
ring
is
forms.
somewhat less
indicates still
three
= 60’
For the cation that,
in spite
times
For the free
favored,
in the cationic
in the xl
(shown in I).
constants chain
residue
is again
of
as densely
base of the pre-
No. 6
Cyclic Dipeptides Table I. B-ProtonResonancesin Hi.stidine-Containing Seryl or Aspartyl c-L-His-LSer
OAc-Ser
Asp
His
pHd
Histidyl
JaB__-6
J aB
k
--&
Jut?
--6
Ja8 ---
7-8
3.32
5.4
3.59
3.1
11.8
3.16
---
3.16
3
3.53
2.6
3.93
2.6
12.2
3.20
5.0
3~.46 5.9
7-8
3.53
6.0
4.08
3.5
11.5
3.14
4.8
3.25
6.1
3
4.14
4.7
4.26
3.2
11.6
3.31
5.5
3.40
6.1
10
1.38
10.5
2.46
2.9
16.5
3.13
---
3.13
---
5-6
2.10
8.0
2.62
3.8
16.4
3.32
---
3.32
---
3
2.75
___
2.75
---
___
3.38
---
3.38
---
10
2.40
7.4
2.90
4.3
3
3.18
--- . 3.18
---
Reference: 2,2-dimethyl-2-silapentane-5-sulfonate Ser and OAc-Ser derivativesrun at 220 MHz, others at 60 MHz.
The serine g-protonsin the neutral form of c-L-Ser-L-Hisare shiftedupfield 0.6 and 0.3 ppm relative to c-L-Ser-L-Serin water (3.91 ppm). The proton with the larger coupling constant,which spends more time trans to the o-proton,has the greaterupfield shift. This is consistentwith ring current shieldingby the imidazolegroup, and indicatessignificant contributionsfrom the XL = 60' rotamer of the histidineside chain. Analysis of the couplingconstantdata indicatethat this is the most favored of the three staggered rotamers,althoughon the average only about half of the side chains are in this state. An X-ray crystallographic analysisof the structureof c-L-Ser-L-Tyrindicates Xl
= 60' for both the side chains,with an almost planar diketopiperazine ring7. The nmr
results presentedhere suggest that this (I) is probablythe most favored conformationOf c-L-Ser-L-Hisas well. However, this conformationalpreferencedoes not lead to any strong interactionbetween the functionalside chains, at least as judged by the pKa's of the the measured pKa's at peptides. For c-Cly-L-His,c-L-Ser-L-His,and c-OAc-L-Ser-L-His 3o", u = 0.085, are 6.3, 6.4, and 6.2, respectively. We have also determineda pH-rate profile for hydrolysisof the 0-chloroacetylderivativeand find no evidenceof a term for intramolecularimidazolecatalysis.
.
No. 6
485
II.
I. c-L-Asp-L-His.
The a-S
form of L-L-Asp-L-His tion
(0.7
c-Gly-Asp sidue
anion
is
= 300’
ring
important.
chains,
although
detectably,
the
anhydride S-protons
space
over
a6
This of
tyrosine3,
indicate
that
of
likelihood
the diketopiperazine
Gaussian
bands,
in one equivalent outside are well
evidence
of acid. error,
The hypochromism chain
at 30°, form,
1.26
(pH S-6),
between
a good hydrogen
ppm relative
closest to the
of the histidine
the preference
the oppositely to affect
for
charged
rexl
(Asp)
side
the imidazole
basicity
n = 0.085.
bears
coupling
constants
Again the proton
conformation
sufficient
a considerable
constants
resemblance
and the upfield
in which the imidazole
of
absorption side
chain
of
c-L-His-L-His,
interactions.
~2 4,500
is
assigned
In the case
of
c-L-His-L-His
which does
indicate
the ultraviolet
in c-L-His-L-His
CD bands,
6.3
the a-g
known in DNA and imply parallel examined
not
conforma-
to the
shifts
rings
are
of
sharing
ring.
and a band of
experimental
We have also
side
for
is
anionic
the predominant
coupling
(II).
between
of the
which allows
upfield,
= 60’
residue
is
in these
= 300°,
the peptide
a conformation
We have examined the ultraviolet and c-L-His-L-Asp
shifted
acid
= 300’
residue
the xl
in neutral
of
xl
attraction
being
in that
change
the aspartic
interaction
dipeptide,
or xl
form is
form of
the imidazole
the aspartic
= 180’
) is markedly
because
the coulombic
c-L-His-L-His. cyclic
J
xl
of
no significant
ppm, indicating
possibly
the pKa of
either
the favored
In the dipolar
is diminished,
constants
and N-H of
(highest
at 2.64
is
that
the carboxylate
imidazole
that
There
It seems likely
bond between to the
indicate
mole fraction).
2 and 60’.
coupling
planar
for
to histidine
evidence
c-L-Tyr-L-Tyr’.
were decomposed
is a 15% hypochromism
chain of
dichrosim
c-L-His-L-Ser
at 212 nm, at both
there
arrangements
led us to expect
as has been observed
The spectra
some side
circular
c-L-His-Gly,
interaction. aromatic
side
at pH 8, effects
chromophores8.
chain
In fact,
pH 8 and
Similar
of our compounds to of
into
185 nm.
interaction
in the
we do not observe
No. 6
486
evidence
of
side
chain
chromophore
any of the CD hands to the side L-L-His-L-Asp larger there
at basic
negative
coupling,
chain
that
to that histidyl
of c-L-Ala-Gly” is
are no CD bands which can be distinctly
of protonation
of the histidyl
protonation
of c-L-Orn-L-Orn
n-r*
near
region
side
Service,
National
the National
Institute
chromophoric
contribution
Science
was supported
of General
Foundation,
Medical
is
unambiguously c-L-His-L-Ser
we observe
slightly
than alanyl.
However,
chromophores.
similar
Protonation
.
of the basic
research
although perturber
on the CD curve 11
to assign
to histidyl
as in the case
group does not make any distinct This
a stronger
assigned
or c_-L-Lys-L-Lys
220 nm, but,
Acknowledgement.
chain
impossible
The CD of c-L-His-Gly,
chromophore.
pH are similar
CD, indicating
is
and it
to that
The effect
observed
adds some positive
solutions,
in
CD in the
the protonated
imidazole
to the CD.
by grants
Sciences,
from the U. S. Public
GM-14069 and GM-47357,
Health
and from
GP-20053.
References Based on the Ph.D.
dissertation
of
Ziauddin,
IIT,
1971.
K. D. Kopple and M. Ohnishi,
J. Amer. Chem. Sot.,
91,
962 (1969).
K. D. Kopple
J. Amer. Chem. Sot.,
g,
6193
and D. H. Marr,
(1967).
G. Gawne, G. W. Kenner, N. H. Rogers, R. C. Sheppard and K. Titlestad, Peptides, 1968, E. Bricas, Ed. John Wiley and Sons, New York (1969) p. 5.
Ziauddin
6.
K. G. R. Pachler,
7.
C. F. Lin and L. E. Webb, abstracts graphic Association, August, 1971,
a.
C. A. Bush in “Basic Principles of Nucleic Academic Press, New York (in press).
9.
E. H. Strickland, 4168 (1970). J.
and K. D. Kopple,
Greenfield
10.
N.
11.
D. W. Urry,
J.
Org.
Spectrochimica
M. Wilchek,
Chem. 35,
Acta,
J.
g,
253 (1970).
581 (1964).
of the meeting of the American Crystallop. 56, and personal communication.
Horwitz
Acid
Chem. g,
Chemistry”
and C. Billups,
and G. D. Fasman,Biopolymers,
Ann. Rev. Phys.
in 28.
1,
477 (1968).
(P.O.P.Ts’o, J.
595 (1969).
Biol.
ed.)
Chem. 245,