Conformationof 'Eyco(-L-Pro-Gly-)3 And Its Ca2' And Mg2 Complexes (Page 3 of 4) in pdf

Proc.

Natd.

Acad. Sci. USA

(1982)

4521

-90

FIG. 3. View of the

peptide

sandwich along the threefold axis with

the

top

peptide ring bonds shown

solid black. The octahedral

coor-

dination of the glyc] carbonyls

calcium is

seen.

tenon

thus

gives

added stability

the unexpected conformation

observed

molecule B,

which

all

six

carbonyls

point

the

same

side of the

peptide

ring,

with

three

of the

oxygens coor-

dinating

the

cation

and the other three forming

N-H"O

hydrogen bonds

with the peptide

itself.

sandwich of this

type

seems

be effective

sequestering

the

cation

from

the

solvent.

However,

dependingon

the

polarity ofthe surrounding

medium, the intrasandwich hydrogen bonds could be broken

and the peptide sandwich opened, resulting

conformational

changes ofmolecule

and the

consequent

release of the calcium

ion into

the solvent.

Peptide Mg Complex. The

magnesium

complex,

the other

hand, exhibits

1:1

stoichiometry for the peptide and the

ion,

the

cation

octahedrally coordinating

the three glycyl carbon-

yls of

the

peptide

and

three

water

molecules

(Fig. 5

Left). The

crystallographic

asymmetric unit contains

two

peptide mole-

cules, which show

very

similar coordination

geometry

and

con-

formation.

Both the molecules show noncrystallographic but

close threefold

symmetry

(Table 4),

with

the peptide confor-

mation

being

very

similar

that of molecule

of the Ca

com-

plex. Puckering of the prolines, however, differs

the

two

molecules.

It is

seen

from

Fig. 5

Right that the peptides, the

cations,

and the

water

molecules stack

form

infinite col-

umn

along

the

crystallographic 21

screw

axis.

The perchlorate

anions

are

located between these columns. The

average

coor-

dination distance of the Mg2+ and the

water

oxygens

2.11

whereas that between the

cation

and

the

peptide

oxygens

(2.03 A)

significantly

shorter. This difference

coordination

distance

may

be indicative of the

greater

basicity of

the

peptide

carbonyl.

Homologues

of (PG)3.

NMR

studies (24)

cyclo(-L-Pro-

Gly-)2 show

that

the

backbone of the peptide

made

trans-cis-trasw-cis

peptides like

most

other cyclic

tetrapeptides.

Studies (25)

cyclo(-L-Pro-Gly-)4

show that

chloroform

takes

up a

C4 symmetric

conformation stabilized by

y-turns

and

is made

of all trans peptide units. Crystal structure inves-

tigations (26)

on the

rubidium complex show that here also all

Table

Conformation angles (in degrees) of the

(PG)3

molecules

the calcium

complex

Mole-

cule

Residue

Pro

-64

144 -175 -7

20 -27 -34 -8

Gly

179 -177

Pro

-68

-24

177

6 -28

39 -34

Gly

-84 -157 -170

-90

'(85,

-181)

180

degrees

FIG. 4.

4,,

plot of molecule A, molecule

and the 3-y-turn

struc-

ture. Because all three molecules have threefold symmetry the

con-

formation of each molecule

can

be representedjust by

two

points

or one

vector.

The points denoted P and G

represent the

conformations

the

proline and

glycine

residues, respectively.

The

coordinates

are

given

parentheses.

The

peptide units containing prolines have nearly the

same

orientation with respect

the

threefold

axis

all

three

cases.

Proline restricts the rotation, but

the 4i

values of molecule A and

molecule B show

nearly 1700 difference.

glycine residues is

nearly the

same

but 4,values show large differences.

The

conformation

the 3-rturn structure

may

be visualized

being midway

between

the

structures of molecules A and B.

the peptide bonds

are

trans

but the molecule has deviated

from

symmetry.

The rubidium

ion

has

distorted octahedral

en-

vironment

and

coordinated

four glycyl carbonyl

oxygens

one

peptide, another glycyl

carbonyl

symmetry-related

peptide, and

water

oxygen

atom.

A magnesium

complex of

cyclo(-Gly-L-Pro-L-Pro-)2

has also been

reported (27) recently;

it is

discrete sandwich complex with approximately twofold

symmetry

relating the

two

peptides of the sandwiches.

thank Prof.

EMkan

Blout

for

starting

our

interest

in this

molecule

and Dr. Jake Bello for valuable discussions. This work

was

supported

by U.S. Public Health Service

Grant GM-22490 and

the New York

State Department of Health.

Table

Conformation angles (in degrees) of

the (PG)3 molecule

the magnesium complex

Mole- Resi-

cule dueno. Residue 4,

Xo XI

Pro

-63

142 -174

9 -18 20-13

Gly

79 -171 -173

Pro

-57

140

179

-9

7 -2

-5

Gly

7 -173 -179

Pro

-64

149 -172 -3

18 -28 25 -13

Gly

172 -176

Pro

-63

144-171

3-21

32-29

Gly

172

-175

Pro

-59

141

-178 18 -36

40 -30

Gly

82 -175 -179

Pro

-55

139 -178 9 -29 38 -32 15

Gly

174

177

Average

Pro

-60

143 -176

Average

Gly

180 -178

Chemistry:

Kartha

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

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