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1985) The osmotic role of mannitol in the Phaeophyta: an appraisal. Phycologia 24: 35-47.Natural abundance l3C nuclear magnetic resonance spectroscopy (NMR) has shown mannitol to be the only major low molecular weight organic compound present in osmotically significant amounts within cells of the fo llowing marine brown algae: Alaria esculenta, Ascophyllum nodosum, Ectocarpus siliculosus, Fucus serratus, F. spiralis, F. vesiculosus, Halidrys siliquosa, Laminaria digitata, L. hyperborea, L. saccharina and Pilayella littoralis. Samples of the top-shore alga Pelvetia canaliculata were fo und to contain volemitol in addition to mannitol. Quantitative gas-liquid chromatographic analyses have confi rmed the presence of mannitol, at concentrations ranging fr om 83.3 to 314.0 mmol kg-1 (expressed in terms of intracellular water content) fo r plants maintained in a fu ll-strength (100%) sea water medium. A study of the changes in intracellular mannitol concentration of six marine brown macroalgae immersed in hyposaline and hypersaline media (20-1 50% EA I sea water medium) showed that mannitol concentration varied as a direct fu nction of salinity in all cases, providing support for the hypothesis that mannitol is intimately involved in osmotic adjustment in response to changes in external water status. Plants of the filamentous form P. littoralis maintained in darkness also showed a marked sensitivity of intracellular mannitol concentration to alterations in the external salt concentration. Overall, the data support an osmotic, rather than a respiratory role fo r the large internal pools of mannitol in such algal cells.

Natural-abundance "3C-nuclear magnetic resonance spectroscopy has shown glycerol to be the major osmotically significant low-molecular-weight solute in exponentially growing, salt-stressed cells of the yeasts Saccharomyces cerevisiae, Zygosaccharomyces rouxii, and Debaromyces hansenii. Measurement of the intracellular nonosmotic volume (i.e., the fraction of the cell that is osmotically unresponsive) by using the Boyle-van't Hoff relationship (for nonturgid cells, the osmotic volume is directly proportional to the reciprocal of the external osmotic pressure) showed that the nonosmotic volume represented up to 53% of the total cell volume; the highest values were recorded in media with maximum added NaCl. Determinations of intracellular glycerol levels with respect to cell osmotic volumes showed that increases in intracellular glycerol may counterbalance up to 95% of the external osmotic pressure due to added NaCl. The lack of other organic osmotica in "3C-nuclear magnetic resonance spectra indicates that inorganic ions may constitute the remaining component of intracellular osmotic pressure.

Most organic charge-transfer complexes, when dissolved in a solvent of low ionising-power, appear to exist almost entirely as neutral electrondonor-acceptor pairs. Such complexes involve very little transfer of charge in the ground state. This type of behaviour is even observed for complexes formed by the combination of electron donors of low ionisation potential with electron acceptors of high electron affinity which in the solid phase have a significant degree of ionic character 1. However, when such strongly interacting systems (or mixtures of their components) are dissolved in solvents of high ionising power, ionic species are formed. It is suggested that the driving force for this ionisation is simply the energy of solvation of the ions produced 2 . This may be represented as:where (AD)cT is the charge-transfer complex, and A-and D+ are the two ions formed by single electron transfer from the electron donor (D) to the electron acceptor (A).If the ionic products in solution are the result of solvation, it should be possible reversibly to change the position of equilibrium between the ionic and the neutral charge-transfer molecular species by altering the nature of the solvent.The absorption spectrum of an equimolecular mixture of N , N , N ' , N'tetramethyl-p-phenylenediamine (TMPD) and chloranil in acetonitrile is shown in Fig. l(a). The absorptions at 428,448 nm are assigned to the dianion of the hydroquinone derived from chloranil, and the bands with maxima at 568, 618 nm to TMPD+ 3. The broad, low-intensity band at 843 nm is the intermolecular charge-transfer band of the molecular complex 3 . On R. Foster and T.