18 The Synthesis, Thermodynamic Behavior, and Biological Properties of Metal-Ion-Specific Sequestering Agents for Iron and the Actinides Downloaded by MICHIGAN STATE UNIV on August 14, 2013 | http://pubs.acs.org Publication Date: December 22, 1980 | doi: 10.1021/bk-1980-0140.ch018
1
K E N N E T H N. RAYMOND , WESLEY R. HARRIS, C A R L J. CARRANO, and FREDERICK L . WEITL Department of Chemistry and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, C A 94720
Certain types of anemia require regular transfusions of whole blood, since the victims of these diseases cannot correctly manu facture their own. One such disorder, β-thalassemia, is described in more detail elsewhere in this publication (1). Because the body lacks any mechanism for excreting significant amounts of se rum iron, the iron contained in transfused blood (-250 mg per pint) can accumulate to lethal levels. At this time the only treatment for Cooley's anemia is continual transfusion and so some way must be found to efficiently remove this excess iron. Thus there is an obvious need for a drug which can selectively bind iron in vivo and facilitate its excretion. Anderson has described the current use of desferrioxamine Β for clinical iron removal in the treatment of β-thalassemia (1). Desferrioxamine Β belongs to the class of compounds called sidero phores (2), which were discussed by Neilands (3). Siderophores are produced by microorganisms for the purpose of binding exo genous ferric ion and facilitating its transport across the cell membrane. These compounds generally utilize either hydroxamic acid or catechol groups to bind ferric ion and form very stable, high spin, octahedral complexes. Desferrioxamine Β is one of the hydroxamate type siderophores (Figure 1). It is a linear molecule consisting of alternating units of succinic acid and 1,5-diaminopentane, which combine to give three hydroxamic acid groups. Al though the effectiveness of desferrioxamine therapy has been im proved, there appear to be fundamental limitations to its poten tial for iron removal which are probably inherent to hydroxamates in general. This led to our interest in the second major type of sidero phores, the catechols. The best known member of this class of compounds is enterobactin, a cyclic triester of 2,3-dihydroxybenzoylserine, shown in Figure 2. Although there were indica tions that enterobactin formed very stable f e r r i c complexes (4, 5), the formation constant of f e r r i c enterobactin had never been 1
Author to whom correspondence should be addressed. 0-8412-0588-4/80/47-140-313$05.00/0 © 1980 American Chemical Society In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
I
R
0
2
HOOC H
II
5
0
2
3
2
ÇOOH
OH
I
Figure 1.
9
H"
IC00H
-C-N-C-H
2
({H,,, '4
n R
Il
I
V
RHODOTORULIC ACID
0 OH 3
CH C-N
3
(R = CH , R' = R" = R'" = H)
FERRICHROME
H—ι
Structural formulas of representative types of hydroxamate siderophores
2
ο
3
CH, I 0=C I HO-N
2
CONH / \ (CH ) ( C H ) ,
(R = H, R' = CH )
AEROBACTIN
2
-0
2
FERRIOXAMINE B
2
CONH / \ (CH Î ( C H )
H-C-N-C-CH,—C-CH
2
4
N-OH
(ÇH )
5
-0.0^
2
(CH )
CH, I C=0
3
H-N\
Downloaded by MICHIGAN STATE UNIV on August 14, 2013 | http://pubs.acs.org Publication Date: December 22, 1980 | doi: 10.1021/bk-1980-0140.ch018
I
0
II
N-CCH, HO
M
2
m Ο Ο
> 2! Ο
r ο ο
Ο
H
g
m
Ο Χ
2!
>
Ο
2
18.
Sequestering
RAYMOND E T AL.
Agents
315
determined. Therefore, our f i r s t p r i o r i t y i r o n a f f i n i t y of e n t e r o b a c t i n .
was
to determine the
Downloaded by MICHIGAN STATE UNIV on August 14, 2013 | http://pubs.acs.org Publication Date: December 22, 1980 | doi: 10.1021/bk-1980-0140.ch018
S o l u t i o n E q u i l i b r i a of F e r r i c E n t e r o b a c t i n The potentiometric t i t r a t i o n curve of f e r r i c e n t e r o b a c t i n , shown i n F i g u r e 3, has a sharp i n f l e c t i o n a f t e r the a d d i t i o n of s i x e q u i v a l e n t s of base. Such a break i n d i c a t e s that the s i x phenolic oxygens from the three dihydroxybenzoyl groups are d i s placed by f e r r i c i o n i n the f e r r i c e n t e r o b a c t i n complex. This i n t e r p r e t a t i o n i s f u r t h e r supported by the absorbance maximum at 490 nm (ε 5600), which i s very s i m i l a r to simple t r i s ( c a t e c h o l a t o ) i r o n ( I I I ) complexes (6, 7). The very low pH at which comp l e x a t i o n o f e n t e r o b a c t i n occurs, with v i r t u a l l y complete complex formation by pH 6, i s a strong i n d i c a t i o n o f a very s t a b l e com plex. However, the t i t r a t i o n i s prematurely terminated at pH 3.8 by the p r e c i p i t a t i o n of a purple n e u t r a l i r o n complex (whose com p o s i t i o n and s t r u c t u r e w i l l be discussed l a t e r ) which makes i t impossible to determine the s t a b i l i t y constant of f e r r i c entero b a c t i n from potentiometric data alone. Instead, the s t a b i l i t y constant of e n t e r o b a c t i n has been de termined s p e c t r o p h o t o m e t r i c a l l y by competition with EDTA, as des c r i b e d by the equation: , 3
Κ
1
F e ( e n t ) " + EDTA *" + 6H
Fe(EDTA)
+ H ent 6
(1)
It i s necessary to take advantage of the strong pH dependence of Eq. 1. At n e u t r a l or b a s i c pH, t h i s e q u i l i b r i u m l i e s completely on the s i d e of f e r r i c e n t e r o b a c t i n . At pH 5, however, a measur able d i s t r i b u t i o n of f e r r i c i o n i s obtained with l e s s than a t e n f o l d excess of EDTA. The intense charge t r a n s f e r band of f e r r i c e n t e r o b a c t i n provides a convenient way of determining the concen t r a t i o n o f F e ( e n t ) " , and the remaining concentrations are o b t a i n ed from mass balance c o n s i d e r a t i o n s . Using the l i t e r a t u r e value for the formation constant of f e r r i c EDTA (8), one can c a l c u l a t e a value of the proton-dependent e q u i l i b r i u m constant 3
*
3
b
[Fe(ent) -J[H+]
6 =
1 0
-9.7
