Znorg. Chem. 1988, 27, 203-205
203
The solvent sensitivity ( B = v ~ ~ / E of* the ~ MLCT ~ ~ ~ ) transition energy for the [M(C0)4]dppand [M(CO),I,dpp complexes decreases slightly in the order M = Mo > W > Cr. The B value for [Mo(CO),]dpp of 3058 is an intermediate value between the extremes for [Mo(CO),]bpm ( B = 4100, bpm = 4,4'-bipyrimidine) and [Mo(CO),]abpy ( B = 560, abpy = azo2,2'-bi~(pyridine)).'~~~On the basis of comparative B values with other [Mo(CO),]L complexes, this result suggests ddp is less effective at ?r back-donation than L = bpy, bpm, bpym, 2,2'bipyrazine (bpz), or 3,3'-bipyridazine (bpdz).18-20 The B value for [Mo(CO),],dpp of 3256 also falls between extremes ([Mo(CO),],bpym, B = 41 10; [Mo(CO),],abpy, B = 1460) and most closely approximates the value of 31 10 reported for the isomeric 2,5-bis(2-pyridyl)pyrazine (dppz) ligand complex [Mo(C0)4],dppz.18-20 These results are consistent with previous solvatochromic studies which suggested that metal centers bound to stronger bases such as pyrimidines and pyrazines (greater G 0 5 10 donation) are more polarizable in the ground state than metal CA-3/10-6mol d m 3 centers bound to weaker bases. The B values of the bimetallic dpp complexes (except Cr-Cr) are larger than those of the reFigure 1. Plots of kO(z,,A)[Zn2+]-1/mol-' dm3s-l vs [A-]/lOd mol dm-3 for L-tryptophan (0),L-phenylalanine (A),and glycine (0),where Aspective monometallic dpp complexes, consistent with two podenotes the anionic form of amino acid. larizable metal centers.'* The B value decreases for heterobimetallic complexes in the order Mo-W > Mo-Cr > W-Cr as could be predicted from B values of the monometallic complexes. to the heme and chlorophyll^.^^' The last step of the heme The energy of the MLCT absorption in a given solvent debiosynthesis is the incorporation of iron(I1) into protoporphyrin creased in the order M = Mo > W > Cr for [M(CO),]dpp IX. The reaction does not occur readily and requires the enzyme complexes and Mo-Mo > Mo-W > Mo-Cr > W-W > W-Cr of f e r r o ~ h e l a t a s e . ~Large ~ metal ions such as mercury(II), 2 Cr-Cr for [M(CO),]dpp[M'(CO),] complexes. The decreasing cadmium(II), or lead(I1) have been found to catalyze the meenergy trend for the monometallic series is consistent with that talloporphyrin for other complexes of group 6 metal carbonyls.20~21 The present paper describes a catalytic effect of amino acids I3C N M R spectra for dpp and bimetallic dppbridged complexes on the rate of metalloporphyrin formation. The reaction of zinc(I1) were recorded in deuteriated dichloromethane (Table IV). The with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine(H2TPPS4; assignment of dpp signals is based on those for bpy and bpym.16,22 charges on porphyrin and metalloporphyrin are omitted Assignment of signals for homobimetallic complexes is based on throughout) was found to be accelerated by amino acids (HA: dpp, and heterobimetallic signals are interpreted from the relative glycine (Gly), L-a-alanine (Ala), L-valine (Val), L-phenylalanine amount of shift from each metal in the homobimetallic complex. (Phe), L-tyrosine (Tyr), and L-tryptophan (Trp), where H A deAll I3C signals in the bimetallic complexes shifted downfield (vs notes neutral form of amino acid). The catalytic effect of amino dpp), indicating stronger a-donating effects predominate vs ?racids is discussed in terms of hydrophobic interaction of the amino back-bonding contributions. The relative order of downfield shift acid side chain with porphyrin. The enhanced rate constant in for a carbon in homobimetallic complexes ( M = Cr > W > Mo) the presence of amino acids is correlated with the hydrophobicity parallels previous results for bpym complexes.16*22 scale of amino acid r e ~ i d u e . ~ Acknowledgment. We gratefully acknowledge the generous Kinetic studies were carried out in the pH range 6.0-7.0 financial support of this work from the Research Corp. (N,N'-bis(2-~ulfonatoethyl)piperazine (PIPES) buffer, 5.0 X mol dm-3), at various concentrations of amino acid (1 X to Registry No. [Cr(CO),],dpp, 111468-79-2; [W(CO),],dpp, 11146880-5;[Mo(CO),]dpp [W(CO),], 111468-81-6; [ Mo(CO),] dpp [Cr(CO),], mol dm-3) and of zinc(I1) (1 X lo4 to 2 X mol dm-3) 4X 111468-82-7; [W(CO),]dpp[Cr(CO),], 111468-83-8; [Cr(CO),]dpp, at 25 OC and Z = 0.1 mol dm-3 (NaN03). The reaction was 111468-84-9; [Mo(CO)4]2dpp,105969-43-5; [Mo(CO),]dpp, 105969started by mixing a solution containing zinc(I1) and amino acid 42-4; [W(CO),]dpp, 11 1489-99-7; w(co)6, 14040-1 1-0; Cr(CO),, with an H2TPPS4solution (1.05 X 10" mol dm-3). The change 13007-92-6; Mo(CO),, 13939-06-5. in absorbance at 421 nm (Soret band of Zn"(TPSS4)) was monitored as a function of time. The conditional formation rate constant of Zn"(TPPS4), ko,involving concentrations of hydrogen, zinc(II), and amino acid was determined from the first-order Contribution from the Department of Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga, 840 Japan, and Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya, 464 Japan
Importance of Hydrophobic Interaction in Metalloporphyrin Formation Masaaki TabataIa and Motoharu Tanaka*lb Received July 30, 1987
Kinetic studies of metalloporphyrin formation may be helpful to understand the in vivo metal incorporation processes leading *To whom correspondence should be addressed.
(a) Saga University. (b) Nagoya University. Tanaka, M. Pure Appl. Chem. 1983, 55, 151-158, and references therein. Lavallee, D. K. Coord. Chem. Reu. 1985, 61, 55-96, and references therein. Jones, M. S.; Jones, 0. T. G. Biochem. J . 1970, 119, 453-462. Dailey, H. A,, Jr.; Lascelles, J. Arch. Biochem. Biophys. 1974, 160, 523-529. Camadro, J. M.; Labbe, P. Biochim. Biophys. Acta 1982,707,28&288. (a) Tabata, M.; Tanaka, M. Inorg. Chim. Acta Lett. 1980,40, X71. (b) Tabata, M.; Tanaka, M. Anal. Lett. 1980, 13(A6), 427-438. (c) Tabata, M.; Tanaka, M. Mikrochim. Acta 1982, 149-158. (d) Tabata, M.; Tanaka, M. J . Chem. Soc., Dalton Trans. 1983, 1955-1959. (e) Tabata, M. Analyst (London) 1987, 112, 141-144. Haye, S.E.; Hambright, P. Inorg. Chem. 1984, 23, 4777-4779. Nozaki, Y . ;Tanford, C. J . Biol. Chem. 1971, 246, 2211-2217. The hydrophobicity scale was experimentally determined by free energy of transfer of amino acids from water to organic solvent, and the value was recently used to analyze amino acid residues in globular proteins: Rose, G. D.; Geselowitz A. R.; Lesser, G.J.; Lee, R. H.; Zehfus, M. H. Science (Washington, D.C.) 1985, 229, 834-838.
0020-1669/88/1327-0203$01.50/0 0 1988 American Chemical Society
204 Inorganic Chemistry, Vol. 27, No. 1, 1988
Notes
Table I. Formation Rate Constant of Zn"(TPPS,) in the Presence of Amino Acids (HA) at I = 0.1 mol dm-' and 25 O C HA
kz,,/mol-l
H20 GlY Ala Val Phe TYr TrP
dm's-l
1.59 f 0.09 3.60 f 0.10 12.0 f 0.5 19.9 f 0.9 43.0 f 0.8 52.7 f 0.8 82.2 f 6.9
Ag,"/kJ mol-] 0.0 2.1
6.3 10.5 9.6 14.2
a hydrophobicity scale of amino acid side-chain: Nozaki, Y.; Tanford, C. J . Biol. Chem. 1971, 246, 2211-2217.
kinetic plot. The kinetics of Zn(TPPS4) formation in the presence of amino acid is of the form kO(Zn.A) = k1[Zn2+I + k2[ZnA+I + k3[ZnAzI
= kl[Zn2+] + k2Pl[Zn2+][A-]
+ k3P2[Zn2'][A-I2
(1)
(2)
where values PI and P2, for the overall formation constants of ZnA+ and ZnA2, were taken from the literature.'O Concentrations of Zn2+and A- were calculated by eq 3 and 4, where C,, and CHA
Cz, = [Zn2+] + [ZnA'] + [ZnAJ = [Zn2+](1+ P1[A-] + P2[A-I2) CHA
+
(3)
+
= [HA] [ZnA'] 2[ZnA2] = [A-]([H+]$-' + Pl[Zn2+] + 2P2[Zn2'][A-]2) (4)
are the total concentrations of zinc and amino acid, respectively, and K , is the proton dissociation constant of the amino acid. Formation of ZnA2 is a few percent at most under the present experimental conditions; for example, the distributions of Zn2+, ZnA+, and ZnAzfor glycine are 65%, 32%, and 3% at CHA = 2.79 X mol dm-, and pH 6.37. Zinc(I1) hydrolyzes to a small and extent at pH 6-7 (Kh = [ZnOH+][H+][Zn2+]-1= 10-8.8),11*12 the rate enhancement by the formation of Zn(OH)+ was negligible in the presence of amino acid and at pH
