1. Conductivity and u.v. and visible spectroscopic techniques were used to investigate the solution structure of the prosthetic group of the ferric haemoproteins (ferrihaem) in dimethyl sulphoxide, NN-dimethylacetamide, NN-dimethylformamide and sulpholane, and certain of their aqueous mixtures. 2. In neutral or acid dimethyl sulphoxide, chlorohaemin is monomeric and completely dissociated into Cl(-)ion and a ferrihaem species with dimethyl sulphoxide molecules in the fifth and sixth co-ordination positions on iron. 3. In neutral NN-dimethylacetamide and NN-dimethylformamide chlorohaemin is monomeric but is largely undissociated, giving different spectra from that of chlorohaemin in dimethyl sulphoxide. On acidification, dissociation occurs and the dimethyl sulphoxide type of spectrum results. 4. Studies in a fourth solvent, sulpholane, indicate that solvent co-ordinating power (ligand strength) rather than bulk dielectric constant is responsible for dissociation of chlorohaemin. 5. In neutral dimethyl sulphoxide-water mixtures chlorohaemin remains monomeric and completely dissociated, and spectra are independent of mixture composition, except at high water concentrations, when precipitation occurs. In alkaline dimethyl sulphoxide-water mixtures, where the complete solvent mixture range is accessible, ferrihaem is polymeric (probably dimeric) and spectra are dependent on solvent composition. A quantitative analysis indicates that the spectral changes are due to replacement by water of one molecule of co-ordinated dimethyl sulphoxide per ferrihaem aggregate, and do not involve a two-molecule replacement as has been suggested for the alkaline pyridine-water system.

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The aqueous ion Co(NH&(TSC)Z+, where TSC denotes trans-4-stilbenecarboxylate, appears in most respects to be a normal member of the carboxylatopentaamminecobalt(II1) family. The second ligand field band is masked, however, by a strong absorption at 320 mp, essentially identical with that for the free TSC molecule.The first singlet-singlet transition of the TSC moiety thus appears to function as an isolated chromophoric group of the complex. Irradiation of the 320-mp band of the free TSC ligand leads to trans to cis isomerization; however, irradiation of this same band in the complex produces cobalt(1I) in a quantum yield of 0.16 and both some free (unisomerized) and some oxidized TSC ligand. Other members of the carboxylatopentaamminecobalt (111) series are nearly inert to this wavelength of irradiation. Free TSC-ion shows a peak fluorescent emission at 400 mp, which is partially quenched in the presence of C O ( N H~)~( H~O )~+ and essentially completely so when the ligand is coordinated. It is concluded that an intramolecular excitation energy transfer occurs, probably to produce a charge transfer triplet excited state of the complex, which then undergoes redox decomposition. The suggested process is one of intersystem crossing from the first singlet excited state of the TSC ligand.

Studies of 'Li and 133Cs ion n.m.r. chemical shifts in dimethyl sulphoxide (DMS0)fwater mixtures, have been interpreted in terms of a series of competitive equilibria in which the solvation number of the ions for DMSO is half that for water. It is concluded that nitrate ions are preferentially solvated by DMSO and Li+ by water. The treatment has also been applied to 133Cs shift data for sulpholan and water mixtures, and it is concluded that caesium ions are solvated by the organic component.

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