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Resonance Raman electronic absorption and circular dichroism spectra and pH titration curves are reported for the trianionic ferric complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide (MDHB). The spectral signatures of the enterobactin and MDHB complexes are virtually identical and differ from those of the catechol complex in ways that reflect the influence of the amide group on the electronic structure. Excitation in either the visible charge-transfer bands or the near-ultraviolet pi-pi* bands enhances Raman bands associated with benzene ring modes, although the relative enhancements differ markedly in the two regions. The data stronly support a structural model in which iron is bound exclusively to the phenolate oxygen atoms in all three complexes.

The structure of the carbonmonoxy-hemoglobin (COHb) photoproduct has been studied on the picosecond time scale with resonance Raman spectroscopy, by tightly focusing the 30-ps pulses of a synchronously pumped mode-locked cavitydumped dye laser on ajet stream of COHb solution. The spectrum of the photoproduct is similar to that of deoxy Hb, but the frequencies 1603 cm-1 (depolarized), 1552 cm-' (anomalously polarized), and 1542 cm-t (depolarized) are 2-4 cm-' lower than those of deoxy Hb. Similar low frequencies are observed for a species believed to be the bis-tetrahydrofuran adduct of Fei" octaethylporphyrin, containing in-plane high-spin Fe",. These results indicate that in the COHb photoproduct the Fe" is already high-spin but is closer to the heme plane than in deoxy Hb. Photodissociation from a quintet ligand-field excited state of COHb is suggested. The frequency shifts relative to deoxy Hb persist when the laser pulses are lengthened to 20 ns. The apparently slow relaxation to the fully out-of-lane heme conformation of deoxy Hb is suggested to be associate with change of the globin tertiary structure.The efficient photodissociation of CO from carbonmonoxyhemoglobin (COHb) has long been exploited to study the kinetics of ligand binding to the freshly generated deoxy Hb (1). With the advent of lasers generating ultrashort pulses, the dynamics of the photodissociation itself have been under active investigation (2-10). The initial absorbance change upon laser excitation is very fast, the rise time being reported as <1 ps (2) or 11 ps (3); the difference is possibly related to the laser wavelength (3). It is presumed, although not proven, that CO dissociates in this fast process. The optical spectrum of the initial photolysis product (4) resembles that of deoxy Hb, but the absorption bands are broader. The broadened spectrum persists for at least 680 ps.The structure of this initial photoproduct is of considerable interest. It can be probed with resonance Raman (RR) spectroscopy, which is capable of monitoring heme structure inside heme proteins by virtue of the large enhancements observed for the porphyrin vibrational modes when the laser excitation wavelength is brought into resonance with the intense heme absorption bands (11). The frequencies of some of these modes vary in response to changes in heme geometry and electronic structure (12). The influence of porphyrin core expansion (13,14), and of electron donation into the porphyrin i7r* orbitals (12,15,16), have been well characterized. Three recent studies (5-7), using 5-to 10-ns laser pulses to photolyze COHb and simultaneously to generate RR spectra, have established that on this time scale the RR bands are characteristic of deoxy Hb. Lyons et al. (7), however, reported slight but definite frequency lowerings of the observed bands relative to deoxy Hb, and this has been confirmed by W. H. Woodruff (personal communication).We have now determined that this spectrum is already developed within 30 ps of COHb photolysis and that it persists to at ...

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