Hydrogen-bonding to M-O2 and M = O in porphyrin models equipped with an intramolecular carboxyl group.

288

Abstracts

DO21

HYDROGEN-BONDING TO M-02 AND M=O IN PORPHYRIN MODELS

EQUIPPED WITH AN INTRAMOLECULAR CARBOXYL GROUP.

C. K. Chang,a Y. Liang,a G. Aviles,a E. Schmidta and S.-M. Pengb

aDepartment of Chemistry, Michigan State University, E. Lansing, Ml 48824 of Chemistry, National Taiwan University, Taipei, Taiwan

bepartment

Reduction and activation of dioxygen in hemoproteins generally involve proton transfers during O-O bond cleavage in the catalytic cycle. Porphyrin models equipped with intra- molecular proton donors are useful in probing the reactivity of dioxyheme as well as other heme-substrate complexes. Previously, Co(ll)porphyrin naphthoic acid 1 was shown to have a very high QJ affinity constant [l], but the close proximity of -COOH to the porphyrin methine carbon facilitates an unusual ring degradation yielding an oxaporphyrin.

/ @ \/ 00 HO 6. \ Hz0 N N 33 &I) \ ‘OH ,N’ N /0 1

To prevent this verdoheme-like formation, a porphyrin Kemp acid has been synthesized in which a carboxyl group overhangs above the heme binding site and imposes its proton on any mono- or diatomic heme substrates. The free base porphyrin proves to be an excellent ditopic ligand for water (FIG 1): NMR measurements in CDCl3 gave K = 67.5 Mt. The crystal structure of Zn(ll)-MeOH complex reveals the interaction between the methanolrc proton and the carboxyl group (FIG. 2). The Co(ll) complex exhibits a 02 affinity as high as 1 (Pi/2 = 0.03 torr in DMF at -45°C). Significantly, it is stable in CH2Cl2 and electro chemically mediates the 4-electron reduction of 02 when coated on graphite electrodes. Using the porphyrin Kemp acid, H-bonding effects on ferry1 and metal-oxo species as probed by IR and resonance Raman spectroscopy provide a benchmark of the frequency shift expected. For example, vanadyl VO has a vibrational frequency of 978 cm-l in the presence of H-bonding versus 992 cm-l without it. The characterization of H-bonded Fe=0 is important for understanding the reaction intermediates in the catalytic cycle of cytochrome c oxidase.

porphyrin. FIGURE 2. Crystal structure of Z 1. C. K. Chang and ii. Kondylis, J. Cbem. Sot. Chem. Commun., 316 (1986)