Fresh evidence to identify the donor group which is protonated in lanthanide-histidinate-proton complexes in aqueous solution

INORG. N[)CI,.

FRESH

Cli[%M. LETTERS

EVIDENCE

TO

Vol. 8, pp. 1009-1012, 1972. Pergamon

IDENTIFY

THE

DONOR GROUP WHICH

[ANTHANI DE -H I ST I DI NATE-PROTON COMPLEXES

A l a n D. Jones D e p a r t m e n t of Chem. Trent ~ l y t e c h n i c Nottingham NGI 4BU

Press. Printed

IN

IS

in

Great

PROTONATED

Britain.

IN

AQUEOUS SOLUTION

David R. W i l l i a m s D e p a r t m e n t of C h e m i s t r y The U n i v e r s i t y St. Andrews, S c o t l a n d K Y I 6 9ST

and Biol.

(Received 30 May 1 ~ 2 )

We have

previously

ZhCp) of f o r m i n g N

reported

protonated

the t h e r m o d y n a m i c s

(pK, ZN3o, ZhH°, ZhS° and

lanthanide(III)~istidinate

(~H2~..H--COO--Ln3+.H+)

and shown t h e

proton

complexes

t o be b o n d e d e i t h e r

to

the

H2 primary

amine nitrogen

secondary

the

amino-acid

part

amine n i t r o g e n of the i m i d a z o l e

tentatively However,

of

to suggest

in aqueous

accordance

with

ZhG° s o m e t i m e s

the

op~se

that

solution,

each other

evidence

and c o n s e q u e n t l y

of ZhH° and &S ° .

now arises

of p r o t o n a t i n g

Cu.histidinate2, The s t a n d a r d

as~ragine,

rather

enthalpies

Cu.histidinate acid,

and e n t r o p i e s

Hence,

pK e v i d e n c e that was solvated

led us

protonated. and so, in

than from ~

alone.

of the

+ , Cu.H-histidinate2,

cysteine

the. p r o t o n

1009

picture

comparisons

re~rts

to

trends

a more realistic

of p r o t o n a t i n g

in the table and the n i t r o g e n to which

the

ZhG° do not exhibit

from our recent

aspartic

or to

(4,5), ZhH° and &S ° c o n t r i b u t i o n s

+ thermodynamics

(I-3).

are e x t e n s i v e l y

law'

from hH ° and ZhS° patterns

Additional

ring

ligand

the latter n i t r o g e n

s~cies

'com~nsation

that are true r e f l e c t i o n s is evident

it was

of the

and p h e n y l a l a n i n e these

is added

s~cies

(6,7).

are listed

in L n - h i s t i d i n a t e 2+

1010

TIIE DONOR GROUP

can now be i d e n t i f i e d

Vol. 8, No. 12

(a) by c o m p a r i n g the m a g n i t u d e s of z3S° values,

and (b)

by g r a p h i c a l l y e x a m i n i n g the ~i ° values. (a) All

the Z3S° v a l u e s

for p r o t o n a t i n g p r i m a r y a m i n e groups, e i t h e r

a m i n o - a c i d s or in their metal c o m p l e x e s , whereas

for i m i d a z o l e p r o t o n a g i o n s

range 6 - 1 1 J J K

-1

mol

-1

K -1 m o 1 - 1

~o

for

are in the range 9-112 J K

mol

-I

( s e c o n d a r y amines), ~ S ° o c c u p y the

protonating

Ln'histJdinate

thus i n d i c a t i n g p r i m a r y amine i n v o l v e m e n t .

p r i m a r y a m i n e ~ S ° s are h i g h e r t h a n those n u m b e r of w a t e r m o l e c u l e s

-I

in

2+ a r e

97-146

We b e l i e v e that

for s e c o n d a r y amines b e c a u s e a larger

is l i b e r a t e d w h e n p r o t o n a t i n g the n i t r o g e n s of the

former. (b) ~I ° of p r o t o n a t l o n also i n c l u d e c o n t r i b u t i o n s

from h e a t s of amine

d e s o l v a t i o n and so it is prudent to c o n s i d e r the same base w h e n m a k i n g enthalpic comparisons. any c h a n g e s

Additionally,

in the b a s e ' s

It m a t e r i a l i s e s

due a l l o w a n c e must aJso be made

for

formal c h a r g e that o c c u r d u r i n g c o m p l e x i n g .

that only

for the case of p r i m a r y a m i n e D r o t o n a t J o n of

h i s t i d i n a t e and its e o m p } e x e s do we o b t a i n a linear r e l a t i o n s h i p betwo(:n ~ { o and

formal c h a r g e

(see figure).

40'

~HIST \ N

\

\

\ \

30 - -

% \

\

\ \

o

"r

0 Cu(HIST)2

\

x. \

Cu(HIST) \

8 %\ C u ( H I S T ) z H \ \

20

I

\ \

\

\

\

0

x 0 \ Ln ( H I S T ) ' ~

0

-I

1

I

O Formal

I charge

8 6 2

Vol. 8, No. 12

TIIE DONOR GROUP

I011

TABLE Thermodynamic parameters

for p r o t o n a t i n g bases of a m i n o a c i d s and t h e i r

c o m p l e x e s in s o d i u m p e r c h l o r a t e m e d i a at 25 °. kJ mol -I and J K - 1 Base

mo1-1 r e s t ~ c t i v e l y .

Group protonated

Asparaginy i

Aspartyl

Cysteinyl

Tryptophy t

Ilist idyl

9.30

-COO

2.59

-N}{2

10.O1

-COO

-CO0

-~I °

50.5 5. iO

AS °

Ref.

8.9

7

32.4

7

23.6

112

7

4.07

23.9

- 2.3

7

2.34

7.3

20.4

7

i0.71

40.4

69.5

7

-NH 2

8.78

38.8

38.0

7

-CO0

2.4

0.3

50.6

7

-NH2

9.61

14.9

7

-COO-

2.75

20. I

7

-NH 2

9.92

38.4

61.1

8

-COO

2.75

3.35

41. ,t

8

9.63

40.4

48.9

8

6.97

36.6

10.5

8

-NIl2 \ -CO0

[ L n ( h i s t ) ]2+

pK

-NH2

-S

Phenylalanyl

Z ~ O and ZhSO are in u n i t s of

50.4 9.73

2.28

I .OO

40.2

8

7.6-6.7

O.O8-II.2

97-146

2

_NIl 2

or

/! [ C u ( h i s t ) ]+

-NH2

5.53

23. I

28.3

6

[Cu(h±st)2 ]

-NH2

6.86

23.7

51.9

6

[ C u ( h i s t ) 2H]+

-NH2

,1.89

22.1

18.9

6

~N

7.66

41.8

"~

7.42

39.0

2-hydroxymethyl imidazoIe

-

4 -hyd roxymottly I imidazole

\

//

6,44 II .2

9 9

THE DONOR GROUP

1012

Hence,

both

Ln.H.histidinate

entropy

3+ t h e

and e n t h a l p y

proton

is

arguments

attached

to the

Vol. 8. No. 12

now s u g g e s t primary

that

in

amine nitrogen.

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A.D.

JONES a n d D . R . W I L L I ~ I S ,

Inorg.

Nucl.

Chem.

Letts.,

[,

369-372

(1971) 2.

A.D.

JONES and D . R .

WILLIAMS, J .

Chem.

Soc.(A),

3138

(1970)

3.

A.D.

JONES and D.R.

WILLI&MS, J .

Chem. S o c . ( A ) ,

3159

(1971)

4.

G. OJELUND and

2691

(1968)

5.

D.J.

6.

D . R . WILLIAMS, J . C . S .

7.

R.D.

I.

IVES and P . D .

GRAHAM, D . R .

WADSO, A c t a Chem. S t a n d . , ~RSDEN, Dalton,

J.

Chem. S o c . , 790

22, 649

(1965)

(1972)

WILLIAhlS, a n d P . A .

YEO, J . C . S .

(1972) 8.

D.R.

WILLIAMS, J .

Chem. S o c .

(A),

1550

(1970)

9.

D . R . WILLIAMS, J .

Chem. 8 o c .

(A),

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(1968)

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II,

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