High-potential iron-sulfur proteins (HiPIPs) 2 are found in photosynthetic purple nonsulfur bacteria. 3 The three-dimensional structure of Chromatium Vinosum HiPIP features two short segments of R-helix, three strands of antiparallel β-pleated sheet, and a small helix near the N-terminus. 4 The cubane [Fe 4 S 4 ] cluster is attached covalently to the polypeptide matrix through Fe-S γ bonds to cysteines 43, 46, 63, and 77. The side chains of Tyr19, Phe48, Trp60, Phe66, Trp76, Trp80, and other nonpolar residues encapsulate the cluster in a hydrophobic cavity that is inaccessible to solvent. 5 Tyr19, which contacts the [Fe 4 S 4 ] core, has been suggested to play a particularly important structural role. 6 In both oxidation states, the cysteinyl and core inorganic sulfur atoms are involved in H-bonding interactions with peptide NH protons. 7 We have studied intramolecular electron transfer in C. V. HiPIPs in which surface histidines at positions 18, 42, 50, and 81 (42 is wild type; 18, 50, and 81 are mutants in which 42 is Gln) 8 were modified by coordination of Ru(bpy) 2 (im) 2+ (Figure 1A). 9,10 The calculated [Fe 4 S 4 ] 2+ :[Ru(HisX)] 3+ electronic couplings in these four Ru-modified proteins vary dramatically, even though the closest Fe-Ru ET distances fall in a narrow range (Figure 1B-E). Thus these Ru-HiPIPs provide a particularly rigorous test of the tunneling-pathway model of ET in proteins. 11,12 Kinetics of ET from [Fe 4 S 4 ] 2+ to Ru(HisX) 3+ (X ) 18, 42, 50, 81) were measured by transient absorption changes following intramolecular quenching of electronically excited Ru-HiPIPs. 13 Rate constants (k ET ) and driving forces (-∆G°) are set out in Table 1. It is likely that the reorganization energy (λ) is below 1 eV for these reactions, since NMR work has established that the solution structures of oxidized and reduced C. V. HiPIP 14 are similar. 15 A value in the 0.6-0.9 eV range [16][17][18] accounts for the very small changes in rate that accompany driving-force (X ) 42, 81) and temperature (X ) 50) 19 variations. Thus the observed rates closely approximate coupling-limited k ET values.

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Seven 2-(N-cycloalkylamino-1,3-dithiocarbamoyl)-carboxylic acids and seven mesoionic 2-N-cycloalkylamino-5-alkyl-1,3-dithiolium-4-thiolates have been conveniently synthesized. They were characterized by elemental analysis, LR., U.V., mass and 'H NMR spectrometry, plus I3C N M R spectrometry in some cases. Important questions concerning the structure of the mesoionic compounds, relevant to mesoionic compounds in general, are addressed. In particular, we refer to the degree of separation of regions of positive and negative charge, bond orders, electron and charge delocalization and aromaticity. In this discussion we cite some of our X-ray diffraction and theoretical studies. We conclude that there are regions of positive and negative charge in which there is delocalization of electrons and charge with bond orders between 1 and 2. However, the shared regions are separated by what are essentially single bonds. Thus, they, and mesoionic compounds in general, should not be considered as formally aromatic. This is supported by estimates of Bird Aromaticity indices-experimentally based for two of the title mesoionic compounds-of the order of 50: substantially less than values for thiophene and pyrrole and much less than values for benzene and pyridine. Finally, based on this discussion, we introduce a new notation for the structures of mesoionic compounds which we believe more accurately represents both the bonding situation and the chemical patterns of reactivity of such compounds.