Lanthanide complexes of histidine in which the ligand is protonated

1NORG.

NUCL.

CHEM.

LETTERS

LANTHANIDE

Vol.

COMPLEXES

7,

OF

and

369-372,

HISTIDINE

IS

Alan D. Jones, Department of Chemistry Trent Polytechnic, Nottingham, NGI 4BU

pp.

1971.

IN

Pergamon P r e s s .

WHICH

THE

Printed in Great Britain.

LIGAND

PROTONATED

David R. Williams, Department of Chemistry, The University, St. Andrews, Scotland.

Biology,

(Receivedl7 Novem~r1970} Potentiometric series

of

have

(Ln.hist)

to

time

that

such

acid

systems

now

there

give

is

that

complexes

(III)

ions

as

similar

a general

call

based

priority

treatment

upon

been

reported

probes

for

calcium

experiments for

more

is

at

computing

the

~C

that

the

this

was

trivalent

increasing in

(4)

1:1

the

interest

and

entropies,

the

systems,

and

so we

and

were

37 ° .

supplement

protonating

first

in

biological

for

complexes

bonds

temperature, to

a

lanthanide-amino

histidyl-proton

(II)

results P ZhS parameters

into

(hist.H)

protonate

blood

~C

25 °

(Ln)-histidine

for

ion

there

at

to

believe

metal

Since

ZhG, ZhH and to

possible

We

divalent

(2,3)

performed

lantnanide was

(I)

have

though

ago.

theories

it

Ln.hist.H.

even

investigations

trivalent

revealed

years

lanthanide have

calorimetric

representative

complexes

reported

and

have

these

to

we

Further, reinforce

given i:I

complexes

P of

lanthanide-histidyl. TABLE.

pKs,

stepwise

enthalpies

heat capacities

and

changes

in

molar 2+ for the addition of a proton to Ln.hist at

25°and 37 ° and I = 3.00 M CIO 4.

Ln(III)

PKLn.hist. H

(kJ mol -I)

25 °

37 °

25 °

37 °

La

7.65

7.67

0.08

-0.92

~

(KJ mol-ldeg -I) 25 °

~c

atP31 °

37 °

(J mol-ldeg -I)

146

150

84

Pr

7.41

7.35

1.97

0.29

135

140

140

Nd

7.37

7.25

2.13

0.88

133

136

105

Sm

7.32

6.81

8.20

4.77

113

115

286

Gd

7.08

6.36

11.25

9.79

97

90

122

Dy

6.76

6.47

5.10

5.61

112

106

-42

Er

6.69

6.41

5.52

3.51

109

iii

167

Yb

7.17

6.84

5.40

2.84

119

122

213

369

370

IANTHANIDE COMPLEXES OF HISTIDINE

7.5

.

~

Vol. 7, No. 4

C)

=

A

= 37 °

25 °

7.0

pK

6.5

i0

-~H (kJ

mo1-1)

150

AS (kJ

mol -ideg-I j

100

I

I

I

I

La

Pr

Nd

Sm

I Gd

*

I

I

Dy

Er

Yb

Vol. 7, No. 4

IANTHANIDE COMPLEXES OF HISTIDINE

The pKs and stepwise enthalpies

and entropies

371

of adding a proton to a

i:i complex at 25 ° and 37 ° are shown in the figure and noted in the table. Clearly

the earlier members

and this basic character proton-complex increasing

of the series

decreases

bond is both enthalpy and entropy

the temperature

stabilised.

lanthanide

the main histidyl-lanthanide

heat change.

remaining

bond involves

to be the pyridine

histidyl

- lanthanide

complexes

the proton.

possible errors deviations, solutions

that the

ring since the pKs of the histidine

~Cp,

(6).

for

Ln.hist.H 3+

from ~ C

= [~(Z~H)]/ZkT and are noted in the table. The P are large (up to 20~) and the least squares standard

in ~ C

P and the heat capacity

of lanthanide

changes

for other complexes

present

in

(III) and histidyl ~, are currently being computed

so that eventually more extensive the moment,

=

bonding must

This latter group appears

in the sums of the molar heat capacities, Ln.hist 2+ + H +

that

are more stable

(5),one can conclude

type nitrogen of the imidazole

have been calculated

the proton

group on the

complex are similar to those of this nitrogen in uncomplexed The changes

stabilization.

(for HSAB reasons)

the carboxylic

acetate complexes

amine grouping attracts

Clearly,

If one assumes

the lanthanide

than their corresponding

In general,

at a position that is remote to the

ion.

ligand and notes that some degree of h i s t i ~ l a m i n e also be present because

The

to find that adding a proton to a positively

an exothermic

must become bonded to the histidyl charged

is increased.

gives less enthalpy but more entropy

It is rather surprising charged complex produces

positively

form the most basic complexes,

as the temperature

it is interesting

conclusions

may be made.

to note that the ~ C

However,

for

values have two maxima P

(at Sm and Y b) and a minimum in the Gd-Dy region. Z~S of complexing ules hydrating

have been ascribed

lanthanide

waters are progressively being so, it appears adding protons changes

com~lexesj

Thus,

These

results,

of hydration belonging reaction,

to the lan-

the resultant ~H, ]kq

is really the summation of the change in

of the metal ion and that of its complex. importance

This

which refer to

P follow the same pattern because of

for a complexing

vs atomic number patterns

upon the relative

the series.

removed as li~ands are added to the ions.

that our Z~H, ~S and ~ C

in the number of water molecules

p hydration

trends in Z~H and

in the number of water molec-

(Ill) ions as one crosses

to lanthanide

thanide complexes. and ~ C

to changes

Similar

of these two changes,

the familiar maxima and minima or smoothe curves.

Clearly,

depending

this may produce either

372

IANTHANIDE COMPLEXES OF H1ST1DINE

Vol. 7, No. 4

REFERENCES

1,

A.D. JONES and D.R. WILLIAMS, J. Chem. Soc. (A), 1970, paper O/1154.

2.

R.H. CARLSON and T.L. BROWN, Inorg. Chem., 1966, ~, 268.

3.

D.D. PERRIN and V.S. SHARMA, J. Chem. Soc. (A), 1967, 724.

4.

I. WADS~ in 'Biochemical Academic press, 1969.

5,

A. SONESSON, Acta Chem. Scand., 1958, 12, 165.

6.

D.R. WILLIAMS, J. Chem. Soc. (A), 1968, 2965.

Microcalorimetry',

Edited by H.D. Brown.