Christopher B. Perry

Hemepeptide models for hemoproteins: the behavior of N-acetylmicroperoxidase-11 in aqueous solution.

The acetylation of the hemeundecapeptide prepared by proteolysis of cytochrome c yields a species di(N-acetyl)-microperoxidase-11, NAcMP11, that is monomeric in aqueous solution at least for concentrations below 20 microM, in contrast to MP11 itself, which aggregates because of intermolecular coordination of Fe(III) by the N-terminal amino group or the amino group of the side chain of Lys-13. The present report complements a report by Peterson and co-workers on the preparation and properties of NAcMP11 (Inorg. Chem. 35 (1996) 6885). We show that NAcMP11 has six spectroscopically observable pH-dependent transitions at 1.90 +/- 0.03, 3.37 +/- 0.01, 4.6 +/- 0.1, 5.4 +/- 0.03, 9.56 +/- 0.01 and 12.4 +/- 0.03. The first is probably due to displacement of one of two H2O molecules from the coordination sphere of Fe(III) by the C-terminal Glu-21 carboxylate (giving the axial ligand combination RCOO-/H2O); as the pH is raised, His-18 is deprotonated and coordinates the metal (His/H2O). The next two transitions are due to ionization of heme propionic acid groups; the penultimate is caused by the ionization of Fe(III)-bound H2O (His/OH-); and the final transition is from ionization of His-18 to form a histidinate (His-/OH-). The EPR spectrum of NAcMP11 at pH 0.7 is consistent with a mixture of a di(aqua) and a mono(aqua) species. Both the aqua complex of NAcMP11 (at pH 7.6) and the hydroxo complex (at pH 11.0) are in equilibrium between a quantum-mechanically admixed spin state (S = 3/2, 5/2) and a low-spin state (S = 1/2). The crystal field parameters of the two complexes (which are similar) as derived from the EPR spectrum are reported. The EPR spectrum at pH 13.8 shows that the hydroxo-histidinate complex of NAcMP11 undergoes a slow reaction, possibly to form a di(hydroxo) complex with displacement of the histidinate ligand, or a dimerization with the histidinate acting as bridging ligand. The coordinated H2O molecule in NAcMP11 is readily replaced by an exogenous ligand, and binding constants for coordination of cyanide, imidazole, azide and chloride are reported. NAcMP-11 is shown to display similar physical and chemical properties to the analogous octapeptide, NAcMP-8, but is easier to prepare; this makes NAcMP-11 a useful alternative model for the hemoproteins.

DFT studies of trans and cis influences in the homolysis of the Co-C bond in models of the alkylcobalamins.

Density functional theory (DFT) calculations (BP86/6-31+G(d,p)) and an analysis of the electron density using Baders quantum theory of atoms in molecules (QTAIM) are used to explore factors that influence the bond dissociation energy (BDE) of the Co-C bond in models for the cofactor in the coenzyme B12-dependent enzymes. An increase in the basicity of L in [L-Co(III)(corrin)-CH3](n+), L = NH3, NH2(-), and NH(2-), causes an elongation of the trans Co-C bond, but this does not necessarily cause the BDE to decrease. The bond between the metal and the N-donor of L, Co-Nα, usually becomes shorter after Co-C homolysis as the resulting five-coordinate product permits the metal ion to move toward L. This contraction increases with the basicity of L and stabilizes the five-coordinate product. The BDE is found to correlate well with two variables, the basicity of L and the difference in the Co-Nα bond length between the five-coordinate product and the six-coordinate ground state. When L is a naturally occurring amino acid or a model for its metal-coordinating side chain, the BDE is found to be moderately dependent on L and decrease with an increase in the softness of the donor atom of L. Sulfides produce a BDE < 30 kcal mol(-1), whereas neutral alcohol donors produce a stronger Co-C bond with a BDE of 34-35 kcal mol(-1). All other ligands are associated with a trans Co-C bond that is almost invariant in strength and with a BDE of 31-33 kcal mol(-1). Models of the type [H3N-Co(III)(N4)-CH3](n+), where N4 = bis(dimethylglyoxime), porphyrin, corrin, and corrole, show that the nature of the tetraaza equatorial ligand can change BDE values by over 8 kcal mol(-1); the BDE when N4 = bis(dimethylglyoxime) is significantly larger than for the other three systems, among which differences in BDE are quite small (2.4 kcal mol(-1)). The differential stabilization of the five-coordinate product by the shrinking of the Co-Nα bond (in corrin and in corrole) or its elongation (in porphyrin and in bis(dimethylglyoxime)) is an important factor in determining the BDE of these systems. Corrin has the longest and weakest Co-C bond; this, together with a significant contraction of the Co-Nα after homolysis, is likely to be the origin of its relatively low BDE.

cis influence in models of cobalt corrins by DFT and TD-DFT studies.

Time-dependent density-functional theory and density-functional theory are applied to study the cis influence of the equatorial macrocycle in vitamin B(12) derivatives. A series of dicyanocobalt corrinoids, CN-[Co(III)-corrin]-CN, where the C(10)H of the corrin ring is replaced by different substituents, X, is considered. The calculated UV-visible absorption spectra, the charge distribution obtained from a Bader QTAIM analysis of the electron density, the CN stretch frequencies of the axial cyano ligands and the electron densities at some bond critical points are compared. The main absorption bands in the UV-visible spectra depend on the electron donating or withdrawing power of X, as assessed from its Hammett σ(p) constants. For X with a stronger electron donating power than H, the other properties do not change appreciably. However, when σ(p)(X) > σ(p)(H), these properties vary linearly with the electron withdrawing power of the substituent. This helps explain the experimental observation that substitution of the axial ligand is more difficult and proceeds more slowly with the increase of the electron withdrawing power of the C(10) substituent.

The preparation of N-acetyl-Co(III)-microperoxidase-8 (NAcCoMP8) and its ligand substitution reactions: A comparison with aquacobalamin (vitamin B12a)

The synthesis of the Co(III) porphyrin octapeptide N-acetyl-Co(III) microperoxidase-8 (NAcCoMP8) is described. NAcCoMP8 provides a means of comparing and contrasting the chemistry of Co(III) porphyrins and corrins to assess the influence of the macrocycle. Log K values, and ΔH and ΔS, for the coordination of anionic (CN(-), N3(-), NO2(-), HSO3(-)) and neutral (pyridine, N-methylimidazole, methoxylamine and hydroxylamine) ligands by aquacobalamin (H2OCbl(+)) and NAcCoMP8 are reported. Anions bind more strongly to H2OCbl(+) than to NAcCoMP8 while the converse is true for the neutral ligands. Density Functional Theory (DFT) calculations and QTAIM analyses suggest the bonding between Co(III) and these ligands is predominantly ionic although anionic ligands induce a significant covalency, the extent of which is important for the stability of the complex. The CoL bond length (L=an anion) in a Co(III) corrin, while longer than in a Co(III) porphyrin, is shorter than might be expected as assessed from CoL bond lengths when L=neutral ligand. It is likely that this stems from coulombic interaction between L and the residual charge at the metal center (2+ in corrin; 1+ in porphyrin). Co(III) in H2OCbl(+) is more labile towards substitution by CN(-) than NAcCoMP8 but the converse is true when the entering ligand is neutral N-methylimidazole. The extent of participation of the incoming ligand in the transition state of the reaction is controlled by the log K value so the nature of the incoming ligand determines in which of these two macrocyclic systems Co(III) is the more labile.

The Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B12a) and Its Ligand Substitution Reactions

The synthesis of a Co(III) corrole, [10-(2-[[4-(1H-imidazol-1-ylmethyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]cobalt(III), DPTC-Co, bearing a tail motif terminating in an imidazole ligand that coordinates Co(III), is described. The corrole therefore places Co(III) in a similar environment to that in aquacobalamin (vitamin B12a, H2OCbl(+)) but with a different equatorial ligand. In coordinating solvents, DPTC-Co is a mixture of five- and six-coordinate species, with a solvent molecule occupying the axial coordination site trans to the proximal imidazole ligand. In an 80:20 MeOH/H2O solution, allowed to age for about 1 h, the predominant species is the six-coordinate aqua species [H2O-DPTC-Co]. It is monomeric at least up to concentrations of 60 μM. The coordinated H2O has a pKa = 9.76(6). Under the same conditions H2OCbl(+) has a pKa = 7.40(2). Equilibrium constants for the substitution of coordinated H2O by exogenous ligands are reported as log K values for neutral N-, P-, and S-donor ligands, and CN(-), NO2(-), N3(-), SCN(-), I(-), and Cys in 80:20 MeOH/H2O solution at low ionic strength. The log K values for [H2O-DPTC-Co] correlate reasonably well with those for H2OCbl(+); therefore, Co(III) displays a similar behavior toward these ligands irrespective of whether the equatorial ligand is a corrole or a corrin. Pyridine is an exception; it is poorly coordinated by H2OCbl(+) because of the sterically hindered coordination site of the corrin. With few exceptions, [H2O-DPTC-Co] has a higher affinity for neutral ligands than H2OCbl(+), but the converse is true for anionic ligands. Density functional theory (DFT) models (BP86/TZVP) show that the Co-ligand bonds tend to be longer in corrin than in corrole complexes, explaining the higher affinity of the latter for neutral ligands. It is argued that the residual charge at the metal center (+2 in corrin, 0 in corrole) increases the affinity of H2OCbl(+) for anionic ligands through an electrostatic attraction. The topological properties of the electron density in the DFT-modeled compounds are used to explore the nature of the bonding between the metal and the ligands.

The co-ordination of ligands by iron porphyrins: a comparison of ligand binding by myoglobin from sperm whale and the haem undecapeptide from cytochrome c

The effect of the protein on the ligand binding properties of a ferric porphyrin was investigated by determining spectrophotometrically the temperature dependence of the binding constants, log Keq, and hence the thermodynamic parameters ΔH and ΔS, for coordination of F−, N3−, SCN−, CN− and imidazole by the haemundecapeptide from cytochrome c, known as N-acetyl microperoxidase-11 (NAcMP11), and ferric myoglobin from sperm whale. The axial binding site of the iron porphyrin in NAcMP11 is relatively open to the solvent, but in metMb it is buried in the protein and surrounded by tightly-packed distal amino acid residues. Whereas the N-donor ligands ethanolamine, glycine, pyridine, ammonia and imidazole are readily coordinated by NAcMP11, only the latter is coordinated by metMb, although the affinity of the haemoprotein for imidazole is nearly two orders of magnitude smaller than for NAcMP11 because of a much less favourable reaction enthalpy. Examination of the crystal structures of metMb and its imidazole complex suggests that steric factors are largely responsible. Of all the halides, only F− is coordinated by metMb. In contrast with imidazole, the anionic ligands N3−, SCN−, F− and CN− are coordinated more strongly by metMb than by NAcMP11. In the case of the first two, the difference in affinity is enthalpically driven, although there is a compensating entropic effect; this is attributed to the difference in the polarity of the environment of the porphyrin, with the relatively apolar environment of the haemoprotein stabilising the complex due to an electrostatic interaction between the anionic ligand and the residual positive charge at the metal centre. The effect is not generally true for all anionic ligands. Thus, although both F− and CN− are coordinated more strongly by metMb than by NAcMP11, the difference in affinity now stems largely from an entropic effect. Hence, whereas very significant differences in affinity for an exogenous ligand exist between the relatively exposed ferric porphyrin of the haempeptide and the buried ferric porphyrin of the haemoprotein, such differences cannot be attributed to a single factor only.

Probing the nature of the Co(III) ion in corrins: comparison of reactions of aquacyanocobyrinic acid heptamethyl ester and aquacyano-stable yellow cobyrinic acid hexamethyl ester with neutral N-donor ligands.

Equilibrium constants (log K) for substitution of coordinated H(2)O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the corrin interrupts the normal delocalized system of corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) shows that complexes of the aliphatic amines with SYCbs produce shorter and stronger Co-N bonds with less ionic character than the Co-N bonds of these ligands with the cobester. Conversely, the Co-N bond to the aromatic N donors is shorter, stronger, and somewhat less ionic in the complexes of the cobester than in those of the SYCbs. Therefore, the distinction between the harder Co(III) in ACSYCbs and softer Co(III) in ACCbs, reported previously for anionic ligands, is maintained for neutral N-donor ligands.

Isoamylcobalamin-acetone-water (1/0.385/12.650).

The title compound, [Co(C(5)H(11))(C(62)H(88)N(13)O(14)P)].0.385C(3)H(6)O.12.650H(2)O, contains the isoamyl (3-methylbutyl) anion bonded to the Co(III) ion through a C atom. The compound is thus a structural analog of the two biologically important vitamin B(12) coenzymes adenosylcobalamin and methylcobalamin. The lower axial Co-N bond length [2.277 (2) A] is one of the longest ever reported for a cobalamin and reflects the strong sigma-donor ability of the isoamyl group.

Hydrogen-bonding patterns in enaminones: (2Z)-1-(4-bromophenyl)-2-(pyrrolidin-2-ylidene)ethanone and its piperidin-2-ylidene and azepan-2-ylidene analogues.

The title compounds, namely (2Z)-1-(4-bromophenyl)-2-(pyrrolidin-2-ylidene)ethanone, C12H12BrNO, (I), (2Z)-1-(4-bromophenyl)-2-(piperidin-2-ylidene)ethanone, C13H14BrNO, (II), and (2Z)-2-(azepan-2-ylidene)-1-(4-bromophenyl)ethanone, C14H16BrNO, (III), are characterized by bifurcated intra- and intermolecular hydrogen bonding between the secondary amine and carbonyl groups. The former establishes a six-membered hydrogen-bonded ring, while the latter leads to the formation of centrosymmetric dimers. Weak C-H...Br interactions link the individual molecules into chains that run along the [011], [101] and [101] directions in (I)-(III), respectively. Additional weak Br...O, C-H...pi and C-H...O interactions further stabilize the crystal structures.

Diisopropylphosphitocobalamin-acetone-water (1/3.48/7.56).

The diisopropylphosphite ligand in the title diisopropylphosphitocobalamin compound, [Co(C(68)H(102)N(13)O(17)P(2))].3.48C(3)H(6)O.7.56H(2)O, coordinates to the Co(III) atom via its P atom. The crystal structure is isomorphous with that of other cobalamins that adopt packing type II [Gruber, Jogl, Klintschar & Kratky (1998). Vitamin B(12) and B(12) Proteins, edited by Kräutler, Arigoni & Golding, pp. 335-347. New York: Wiley-VCH], with a Co-P bond length [2.227 (1) A] similar to that found in other phosphitocobalamins. The structural trans influence in cobalamins is discussed.