Expressions for thermodynamic equilibrium constants are set up for the ion-exchange equilibria of models defined in terms of operational variables. Formulas summarizing the methods for the computation of the requisite activity coefficients are given. In particular, the effects of the solvent, which is considered as an independently variable component, are taken explicitly into account.

An electron donor-acceptor-donor molecule consisting of two porphyrin donors rigidly attached to the two-electron acceptor N,N'-diphenyl-3,4,9,10-perylenebis(dicarboximide) acts as a light intensity-dependent molecular switch on a picosecond time scale. Excitation of the porphyrins within this molecule with subpicosecond laser pulses results in single or double reduction of the acceptor depending on the light intensity. The singly and doubly reduced electron acceptors absorb light strongly at 713 and 546 nanometers, respectively. Because these absorption changes are produced solely by electron transfers, this molecular switch effectively has no moving parts and switches significantly faster than photochromic molecules that must undergo changes in molecular structure.

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containin both c-and d,-type hemes28*29 or six C-type hemes per molecule80 Some reductases react with nitrite in their oxidized f0rm,3',~~ but all, in their reduced state, react with nitrite and form NO-bound heme species. The NO-bound d,-heme is six-coordinated with histidine as the other axial ligand,33 while the axial ligand for the siroheme remains undetermined. Most interestingly, the EPR spectrum for the NO-bound heme in the hexaheme nitrite reductase appears to be of the five-coordinate type.34 Consistent with this observed diversity, our present results make evident that a low-spin state for the initial complex formed between substrate and reductase heme does not have significant other axial ligand requirements. Acknowledgment. We thank the National Institutes of Health (Grant GM-38401 to W.R.S.) and the National Science Foundation (Grant DMB 9001 530 to B.H.H.) for support. Supplementary Material Available: Figures S 1 -S3 depicting a formal diagram of the core, an ORTEP plot of [K(222)]+ cryptand, and a drawing showing the relative orientation of nitrite ion and Fe-N, vectors and Tables SI-SV giving complete crystallographic details, final atomic coordinates, anisotropic thermal parameters, fixed hydrogen atom positions, and a complete listing of bond distances and angles for [K(222)][Fe(N02)-(TpivPP)].H20-C6HSCI (20 pages): listing of observed and calculated structure factor amplitudes (32 pages). Ordering information is given on any current masthead page. (28) Huynh, B. H.; Liu, M. C.; Moura, J. J. G.; Moura, I.; Ljungdahl, P. 0.: Milnck, E.; Payne, W. J.; Peck, H. D., Jr.; DeVartanian, D. V.; LeGall, J. J. Biol. Chem. 1982, 257, 9576. (29) Timkovich, R.; Cork, M. S.; Taylor, P. V. (34) Costa, C.; Moura. J. J. G.: Moura, 1.; Peck, H. D., Jr.; LeGall, J.; 0.; Siegel, L. M. Recent interest in the role of the solvent in electron-transfer reactions has focused on ultrafast photoinduced electron transfers,' highlighted by investigations into the primary events of bacterial photosynthesis.2 In contrast to photosynthesis, which exhibits ~ ~~ ~~ ~~ (I) (a) Brunschwig, B.; Ehrenson, S.; Sutin, N. J. Phys. Chem. 1987, 91, 4714. (b) Castner, E. W., Jr.; Bagchi, E.; Maroncelli, M.; Webb, S. P.; Ruggiero. A. J.; Fleming. G . R. Ber.