1constant for internal return in 70% dioxane is estimated to be 40 times larger than k4. The fact that k4 is an apparent dissociation rate constant for an oriented complex held together by secondary valence forces makes it impossible to accurately compare k4 to the limiting values for internal return (k-,). The observation of a large isotope effect of 15% does not require that k 4 be much less than k-, since KirbyZo has observed similar effects of 5-16% (20) Craze, G.-A,; Kirby, A. J.; Osborne, R. J. Chem. Soc., Perkin Trans. 2 1978, 357.for SN2 displacement reactions involving formaldehyde methyl phenyl acetals. The large negative p of -4 that is observed in 70% dioxane suggests that if the reaction is SN2-like, the incoming and leaving groups are widely separated in space. This large amount of effective charge on the central carbon would be expected to have a significant effect on the zero-point energies for the two states under consideration, giving rise to the large isotope effect. If displacement reactions involving acetals commonly exhibit late, loose transition states such as this, large isotope effects such as those that are observed with glycosides would be anticipated. Abstract:The fluorescence decay kinetics of aqueous tryptophan and 3-methylindole have been determined as a function of pH and temperature by using a picosecond dye laser-single photon counting system with a time resolution of 50 ps. At pH 11, tryptophan exhibits a single exponential decay, with a lifetime of 9.1 ns at 18 O C . However, at pH 7 the decay is faster and definitely nonexponential; the values obtained from a biexponential fit to the data at pH 7 are T ] = 0.43 ns, 7 = 3.32 ns, a n d f = 0.19 at 18 O C . The behavior of a 3-methylindole closely resembles that of tryptophan at pH 11. A model for the photophysics of aqueous tryptophan is presented in which the excited-state decay constant at pH 11 (where the amino acid side chain is not protonated) is given by the superposition of three independent processes: fluorescence, intersystem crossing, and photoionization; of these processes only photoionization is temperature sensitive (E' = 51 kJ mol-'). In the region pH 4-8, where tryptophan exists in the zwitteridn form, a new nonradiative process is introduced, which involves intramolecular proton transfer from the -NH3+ group to the excited indole ring. The apparent activation energy for intramolecular quenching (E' = 16 kJ mol-') suggests that it is a predominantly diffusion-controlled process. It is proposed that the nonexponential decay observed for aqueous tryptophan at pH 7 arises from transient terms in the rate constant for intramolecular quenching. Quantum yields calculated from this model compare well with experimental values. IntroductionThe fluorescence of proteins is usually dominated by that of the tryptophan residues.' Both the fluorescence lifetime and quantum yield of a tryptophan residue are strongly influenced by the nature of its local environment, and this sensitivity is widely exploited through the use of tryp...

Raman and infrared spectra of 2-line and 6-line ferrihydrite and schwertmannite were recorded. The spectra of the ferrihydrites are compatible with octahedrally coordinated Fe 3+ . The spectra of schwertmannite are consistent with Fe(O,OH) 6 octahedra in some of which an OH − ion has been replaced by a SO 4 2− ion which acts as a bridging bidentate ligand to two Fe 3+ ions. The Raman spectra show that under heating by the He-Ne laser beam 2-line ferrihydrite and schwertmannite transform to hematite via a maghemite intermediate, whereas 6-line ferrihydrite transforms directly to hematite. The different thermal transformation behaviour of the two ferrihydrites accords with recent reports that the two forms are structurally different and suggests that it is the disordered component of 2-line ferrihydrite that transforms to maghemite on heating. In the case of schwertmannite the loss on heating of SO 4 2− ions in the form of SO 3 and H 2 O entails the simultaneous loss of OH − ions and the conversion of some OH − ions to O 2− . The loss and resulting rearrangement of ligands around those Fe 3+ ions originally liganded by SO 4 2− results in a fraction of Fe 3+ achieving the tetrahedral coordination that is associated with maghemite.

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