ASSESSING THE EFFECT OF LIGAND PROXIMITY ON CHIROPTICAL AND OTHER PROPERTIES IN COBALT(III) MODEL, TYROSINE-LIKE COMPLEXES

Article abstract of DOI:10.1135/cccc19952097

A series of new cobalt(III) octahedral complexis of the general formula Na (ohb-aa=N-(o-hydroxybenzyl)amino acid anion; amino acid= glycine, (S)-α-alanine, α-aminoisobutyric acid, (S)-valine, (S)-norvaline and (S)-leucine) were prepared by th

Full text of DOI:10.1135/cccc19952097

Cobalt(III) Model, Tyrosine-Like Complexes
2097
ASSESSING THE EFFECT OF LIGAND PROXIMITY ON CHIROPTICAL
AND OTHER PROPERTIES IN COBALT(III) MODEL, TYROSINE-LIKE
COMPLEXES
a
b
Frantisek JURSIK and Ronald D. ARCHER
a Department of Inorganic Chemistry,
Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
b Department of Chemistry,
University of Massachusetts, Amherst, MA 01003, U.S.A.
Received May 22, 1995
Accepted September 10, 1995
A series of new cobalt(III) octahedral complexes of the general formula Na[Co(ohb-aa)2] (ohb-aa =
N-(o-hydroxybenzyl)amino acid anion); amino acid = glycine, (S)-α-alanine, α-aminoisobutyric acid,
(S)-valine, (S)-norvaline and (S)-leucine) were prepared by the charcoal catalyzed reaction of the appro-
priate ligand with [Co(NH3)6]3+ in alkaline aqueous solution. Complexes obtained have, regardless of
the amino acid used, the same facial all-trans symmetry (CoN2O4 chromophore) with the vicinal ef-
fects as the entire source of the optical activity. 13C NMR spectra reveal that the leucine derivative
has, due to the steric reasons, a different ground state structure. Absorption maxima reflect a positive
inductive effect from the amino acid side chain carbon atoms. Complexes of the ligands bearing elec-
trophobic alkyl groups exhibit more negative E1/2 in comparison with the glycine derivative, reduc-
tion of which occurs at a more positive potential. Reduction potentials do not correlate with
cobalt(III) Lewis acidity modulated by ligands.
The fact that numerous proteins such as transferrins, catechol dioxygenases and purple
acid phosphatases bind iron1–3, has necessarily raised questions concenrning the ident-
ity of the actual ligands coordinated to iron(III) in these important species. Although
the iron coordination centers in some transferrins are now well established4–6, iron(III)
coordination by tyrosine originating from different part of these proteins7–12, together
with the observations that the protein functional groups that are not governed in iron
coordination play an important role in determining kinetic and thermodynamic proper-
ties of transferrins13, makes the study of proximal ligand influences particularly important
at the present time. Thus, we report the results of a study of the structural diversity of
tyrosine-like ligands on the stereochemistry, spectral and redox properties of their octa-
hedral cobalt(III) complexes with the aim of applying the results obtained to the coor-
dination chemistry of transferrins or more generally to other iron tyrosinate proteins.
Substitution inert cobalt(III) complexes have been used to avoid the dissociation events
which complicate the study of the more labile iron(III) species.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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Jursik, Archer:
EXPERIMENTAL
Electronic absorption spectra were recorded with either a Perkin–Elmer Lambda Array model 3840
UV-VIS or a Specord M 40 (Zeiss, Jena) spectrophotometer. 13C NMR spectra were obtained in D2O
and recorded on Bruker 400 instruments with tetramethylsilane as the external standard. Circular
dichroism spectra were measured on an Aviv 62DS spectrometer. Redox measurements were per-
formed using a Polarograph PA4 (Laboratorni pristroje, Praha) in 0.1 M NaClO4 against a saturated
calomel electrode.
All chemicals used were purchased from Aldrich.
Ligand Synthesis
N-((o-Hydroxybenzyl)amino acid) ligands abbreviated as ohb-aa (aa is the glycine (Gly), (S)-α-
alanine (Ala), α-aminoisobutyric acid (Ibu), (S)-valine (Val), (S)-norvaline (Nva) and (S)-leucine
(Leu)) were prepared by the reduction of the Schiff bases synthesized in situ from salicylaldehyde
and the sodium salts of the appropriate amino acid with NaBH4. Acidification of the reaction mixture
to pH 5–6 afforded the desired product, which was washed with water, acetone and characterized by
elemental analysis, which gave satisfactory results.
TABLE I
Elemental analysis, electronic absorption spectra (cm–1) and reduction potentials (V) of Na[Co(ohb-aa)2]
complexes
Calculated/Found
νmax (ε)
Compound
E1/2
% C
% H
% N
1st band
2nd band
Na[Co(ohb-Gly)2] . 0.5 H2O
C18H19N2CoNaO6.5 (449.2)
48.12 4.26
48.20 4.30
6.23
6.30
14.660 (118)
18.149 sh
25.660 (2.530) –0.18
26.220 (2.530) –0.34
26.300 (2.543) –0.33
Na[Co(ohb-Ala)2] . 2 H2Oa
47.63 5.20
47.33 5.27
5.55
5.64
14.382 (125)
18.160 sh
C
20H26N2CoNaO8 (504.3)
Na[Co(ohb-Ibu)2] . H2O
22H28N2CoNaO7 (514.4)
51.37 5.49
51.29 5.52
5.45
5.49
14.440 (128)
18.169 sh
C
Na[Co(ohb-Nva)2] . H2O
24H32N2CoNaO7 (542.4)
53.14 5.95
52.98 6.00
5.16
5.15
14.455 (131)
18.173 sh
26.380 (2.542)
–
C
Na[Co(ohb-Val)2] . 1.5 H2O
24H33N2CoNaO7.5 (551.4)
52.27 6.03
52.18 6.09
5.08
5.00
14.453 (130)
18.180 sh
26.400 (2.550) –0.32
26.440 (2.549) –0.38
C
Na[Co(ohb-Leu)2] . 1.5 H2O
26H37N2CoNaO7.5 (579.5)
53.88 6.44
53.70 6.46
4.83
4.79
14.502 (131)
18.188 sh
C
a
Protonated (sample dissolved in approximately 0.1 M HClO4): 14.454 (120), 26.482 (2.074).
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Cobalt(III) Model, Tyrosine-Like Complexes
Synthesis of Na[Co(ohb-aa)2] Complexes
2099
All cobalt(III) complexes were prepared using [Co(NH3)6]Cl3 as the cobalt(III) precursor. A typical
preparation of valine complex which describes the general method of synthesis, ohb-Val (4.42 g, 20 mmol)
and NaOH (1.60 g, 40 mmol) were dissolved in H2O (150 ml). To this solution solid [Co(NH3)6]Cl3
(2.67 g, 10 mmol) together with charcoal were added. The reaction mixture was stirred and heated
on the water bath until NH3 evolution ceased, cooled to room temperature and filtered. The filtrate
was evaporated to dryness on the water bath. The brown residue was dissolved in dry CH3OH; the
solution was filtered and evaporated to dryness using a rotary evaporator. The product was purified
by chromatography on an alumina column an ethanol–water 60 : 40 (v/v) mixture as eluant. Evapora-
tion of the main band eluate to dryness in vacuo yielded the complex of the desired composition
(Table I). The remaining complexes were prepared in the same manner replacing N-(o-hydroxy-
benzyl-Val) with the appropriate ligand. Isomeric purity of the individual complexes was checked by
TLC (alumina precoated plates, 2-propanol–water 60 : 40 as the solvent).
RESULTS AND DISCUSSION
Characterization of Complexes
Complexes of the general formula Na[Co(ohb-aa)2] were prepared by the displacement
of ammonia ligands from [Co(NH3)6]Cl3 with the proper ligand in alkaline solution in
the presence of charcoal. Although several isomers are possible for each of these com-
plexes, only one isomer was obtained in each case as revealed by the chromatographic
results. Their absorption spectra (Fig. 1, Table I), which exhibit the same absorption
pattern, show a shoulder on the low-energy side of the intense charge-transfer band.
The shoulder is positioned where a second component of the first T1g octahedral com-
plex absorption band is anticipated for a trans-CoN2O4 complex. (The third expected
d–d transition is buried under the same charge-transfer band, which occurs at lower
2.0
1
A
2
1.0
0.0
300
400
500
600
700
800
λ
, nm
max
FIG. 1
Absorption spectra of the deprotonated (1) and protonated (2) Na[Co(ohb-Ala)2]
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

2100
Jursik, Archer:
energy than normal for cobalt(III) complexes because of the low-energy π*-levels of
the aromatic systems.) It should be noted that the absorption spectra pattern is not
affected (except for values of both molar absorption coeficients cf. Fig. 1), contrary to
cobalt(III) amino alcohol complexes14,15, by the protonation. The splitting of the T1g
band into two separate peaks suggests that all of the isomers prepared have a trans(N)
geometry, either with facial or meridional topology for the chelate rings. (Splitting into
separate peaks would not be expected for a cis(N) geometry). To distinquish between
the two trans(N) topologies, 13C NMR spectroscopy has been used, and the results are
presented in Table II, from which it follows that the isomers (except for leucine – vide
infra) give NMR spectra corresponding to structures with the respective carbon atoms
in both ligands in chemically equivalent environments (aminoisobutyric acid has two
nonequivalent methyl groups). This equivalency is satisfied only for the symmetrical
“all-trans” trans(N)-facial geometry*(I) with several planes of symmetry.
Coordination of the secondary N-atoms of unsymmetrical ohb-aa ligands leads to the
nitrogen atoms fixed stereochemistry and when both N-atoms adopt an (R) absolute
configuration, a minimum of intramolecular nonbonding interactions (based on Dreiding
models) occur between the amino acid side chains and the H-atoms of the hydroxy-
benzyl moiety. Interactions gradually increase in the series of N-atom configurations:
(R,R) < (R,S) < (S,S).
Although these interactions are generally reduced for the (R,R) configurations, in-
creasing the steric volume of the substituents on the chiral atoms leads to steric strain,
which is most striking in the case of the (S)-leucine ligand, where the facial isomer
would have large nonbonding interactions between the isobutyl groups and the H-atoms
of CH2N. To reduce this potential steric crowding, the two facially coordinated
* This geometry has been recently estimated by X-ray analysis of [Co(ohb-Leu)2]– anion (ref.16).
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Cobalt(III) Model, Tyrosine-Like Complexes
2101
o-hydroxybenzyl-(S)-leucine ligands are forced to deviate from their equivalent posi-
tions. As a result, the two ligands are in nonequivalent environments and the leucine
complex shows double the number of signals observed for the less hindered systems.
Since the optical activity of the complexes described arises primarily from the N- and
C-vicinal effects (vide infra), the optical activity of the leucine complex is very similar
to that of the other systems.
With respect to the geometry of these complexes, the results clearly demonstrate that
o-hydroxybenzylamino acidato ligands coordinate with geometric specificity giving
rise to facial isomers. Furthermore, the fact that only facial “all-trans” isomers were
obtained, even for the glycine complex that is free of any steric interactions, suggests
that the coordination mode of the two flexible tyrosine-like tridentate ligands in these
complexes, except for leucine, is primarily dictated by the mutual orientation of six-
membered chelate rings rather than by the steric requirements of amino acid nonpolar
side chains. This tendency, i.e. the in-plane coordination of angle expanding six-membered
chelate rings is common feature of metal complexes with fused chelate rings17,18
.
Circular dichroism (CD) spectra (Fig. 2, Table III) exhibit two high intensity peaks
with negative signs in the T1g absorption band region. The fact that both peaks have the
same sign indicates that the CD spectra of this region are dominated by the vicinal
effects (generally two negative Cotton effects are observed in the T1g region for the
(S)-amino acids19) because configurational chirality effects would lead to two CD
bands of opposite sign for the trans-CoN2O4 chromophore. However, it must be em-
phasized that the CD band intensity significantly increases when an amino acid is a part
of a linear rigid tridentate ligand that coordinates to a metal forming five- and six-mem-
TABLE II
Resonance frequency assignments (δ, ppm) in the 13C NMR spectra of Na[Co(ohb-aa)2] complexes
aa
α-C
β-C
γ-C
δ-C
CH2N
CO−2
Cipso–CH2 Cipso–O
–
Gly
55.4
61.7
66.4
–
–
–
53.4
51.5
47.9
186.0
186.6
190.4
126.2
128.6
128.5
164.0
163.0
162.9
α-Ala
Ibu
16.6
23.0
27.0
30.6
30.4
–
–
–
–
Val
71.1
66.0
19.3
18.6
–
52.9
51.5
185.6
185.7
128.6
128.3
162.8
162.8
Nva
13.9
14.3
Leu
63.7
64.8
51.4
52.3
25.6
26.2
22.1 22.8
23.5 23.7
40.4
42.6
186.8
188.9
125.6
128.6
163.0
163.7
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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Jursik, Archer:
bered chelate rings18. This suggests that the strain imposed by a rigid six-membered
fused chelate ring exerts strong influence on the vicinal effect, the intensity of which is
related to the distortions of the octahedron angles. In this connection, it should be noted
that the carbon-atom vicinal effect thoroughly dominates the CD spectra of heteroleptic
amino acid linear-triamine cobalt(III) chiral complexes in which the deviation of donor
atoms from the Cartersian coordinates is the result of six-membered fused chelate ring
formation20. Thus, it can be concluded that CD spectra for this type of cobalt(III) com-
plex show a close relationship between the chelate ring size and the vicinal effect in-
tensity. However, it must be emphasized that the vicinal effect reflects other, not well
understood contributions from intra- and interligand interactions, the importance of
which varies as expected. When the degree of these interactions which diminish in the
series Leu > Val > Ala (based on Dreiding models), are taken into account, a possible
reason for the gradual decrease in the CD peaks intensities (Ala > Val > Leu) becomes
apparent. Besides this, the CD peaks intensities of the [Co(ohb-aa)2]– complexes are
also affected by the protonation of phenolic oxygen atoms (Fig. 2, Table III). Deproto-
nated forms display, in comparison with their conjugate acids, rather enhanced CD
magnitudes. The observed CD peak intensity dependence on pH probably arise from
conformational changes.
The CD spectra of the Na[Co(ohb-aa)2] complexes also display a two high intensity
positive peaks congruent with the intense charge-transfer bands at about 300 and 400 nm
(Table III). These peaks are also observed in the CD spectrum of iron(III) transfer-
rin21,22
.
Amino Acid Side Chain Effects
The absorption maxima of the homologous complexes under investigation are sensitive
to the number of carbon atoms in the amino acid side chain (Table I). The absorption
+15
∆ε
+10
+5
0
FIG. 2
–5
Circular dichroism spectra of the deprotonated
(− − −) and protonated (−−−) trans(N)-facial (“all-
trans”) Na[Co(ohb-Ala)2] isomer
300
500
700
λ
, nm
max
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Cobalt(III) Model, Tyrosine-Like Complexes
2103
maxima of both the T1g and CT transitions shift to higher energy as the ring substituents
become more electron donating, suggesting that the ligand-field strength at the co-
balt(III) is inductively affected by the nature of the amino acid located in the tyrosine-
-like ligand moiety (Fig. 3). Since the first absorption band energies of homologous
complexes of the same central atom reflect the ligand-field strength, these results demon-
strate that the ligand donor ability increases in the series Ala < Ibu < Val = Nva < Leu
(the absorption of the glycine complex, which occurs at higher energies is in line with
the greater ligand-field strength obtained with the unsubstituted chelate rings because
of the absence of steric strain). As far as the energy of the CT band is concerned (Fig. 3),
it follows that the increasing electron density (associated with increasing ligand ba-
sicity) on the cobalt atom makes the charge-transfer t2g to π*-transition more favorable.
It is interesting to speculate whether the charge-transfer spectra and other properties of
the natural transferrins may also be affected inductively by the amino acids in the vi-
cinity of the metal binding. According to our opinion, which is supported by the de-
pendence of natural transferrin CT band energy on the Lewis basicity of synergistic
TABLE III
Circular dichroism data for the trans(N)-facial (“all-trans”) Na[Co(ohb-aa)2] complexes
Complex
λmax, nm (∆ε)
Na[Co(ohb-Ala)2]a
Na[Co(ohb-Ala)2]b
Na[Co(ohb-Val)2]
Na[Co(ohb-Leu)2]
295 (+10.1), 391 (+17.1), 524 (–7.1), 690 (–8.0)
394 (+11.7), 526 (–3.8), 685 (–6.2)
299 (+8.9), 395 (+11.0), 520 (–6.8), 685 (–7.1)
299 (+8.7), 395 (+7.6), 524 (–6.9), 680 (–6.1)
a Deprotonated. b Protonated (sample dissolved in approximately 0.1 mM HClO4).
26.50
14.50
–3
–3
ν . 10
ν . 10
–1
–1
cm
cm
1
2
26.40
14.45
2
1
26.30
26.20
14.40
14.35
FIG. 3
Plots of the 1st and 2nd absorption band
maxima vs number of the amino acid side
chain carbon atoms (n). 1 (S)-norvalin, 2
(S)-valin
1
2
3
4
n
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

2104
Jursik, Archer:
anion23, at least the spectral properties reflect the character of proximal ligand uncoor-
dinated groups.
Additional insight into the electronic structure of the model complexes is provided
by electrochemical information. Results of polarographic reduction show (Table I) that
the half-wave potential of complexes bearing nonpolar alkyl groups exhibit more nega-
tive values in comparison with the glycine derivative suggesting that these groups ren-
der the metal center difficult to reduce. The less negative potential for the glycine
derivative may be thus related to relative ease with which the electron can penetrate the
complex ion relative to other amino acid derivatives with their electrophobic alkyl
groups. Because the reduction potentials do not correlate either with the amino acids
pK or with the Lewis acidity of the cobalt(III) center modulated by the ligands, it is
reasonable to assume that the reduction is controlled primarily by the complex solva-
tion which is aided by the lack of alkyl groups on the complex. As can be further seen
in Table I, the reduction potentials are, with the exception both of [Co(ohb-Gly)2]– and
[Co(ohb-Leu)2]– ions, virtually identical, especially considering the nonreversible (or
pseudoreversible) nature of the cobalt(III)/(II) redox couple that requires bond length
changes during the process. However, a closer look on the reduction potentials reveals
that the leucine derivative E1/2 is more negative than for the rest of the nonpolar series.
We ascribe this shift (which exceeds the estimated standard deviation of ± 0.01 V) to
the different ground state structure of the [Co(ohb-Leu)2]– ion, as is evident from both
its NMR spectrum and the results of single-crystal X-ray analysis16. If this complex ion
had and identical structure with the others, its extrapolated E1/2 value should be ap-
proximately –0.31 V.
CONCLUSIONS
Though the models used here are simplified in comparison with transferrins, the fact
that the iron coordination sites are very similar in individual type of transferrins6,24 in
spite of they differ in their function25–29 and the kinetic of iron release13 suggests that
the proximal uncoordinated groups as well as distal groups can play an important roles
(not yet studied) in the chemistry of transferrins at the molecular level. Present results
show that an increasing number of C-atoms is manifested in a spectral blue shift, which
may explain the well known color differences exhibited by iron-tyrosine proteins. In
addition to this, the presence of alkyl groups which increases the hydrophobic character
of complexes, shifts the reduction to more negative value, suggesting that the redox
potential of a metal can be altered by proximal ligands. One of the intriguing features
to emerge from this study is that the reduction potential is influenced by the polarity
(solvation) of the metal coordination site exerted by the proximal ligand substituent
rather than by the metal Lewis acidity. It should be pointed out in this connection that
the reduction potentials for the C- and N-terminal sites of human serum transferrin are
different30. This, together with the recent finding that the iron removal from lactoferrin
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Cobalt(III) Model, Tyrosine-Like Complexes
2105
causes significant conformational change in the N-terminal lobe, but not in the C-termi-
nal one4,31,32 makes the study of proximal ligand effects particularly important. Further
work concerning this problem is in a progress.
We are very grateful to Dr M. Krejcik from J. Heyrovsky Institute of Physical Chemistry of the
Academy of Sciences of the Czech Republic for the measurements of the reduction potentials.
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