No abstract

A series of complexes of the type [(bpy)2R~~~L-(Pro),-Co~~I(NH3)5]~+, n = 1-6, where L = 4-carboxy-4'-methyL2,2'-bipyridine, bpy = 4,4'-bipyridine, and Pro = l-proline, have been synthesized from the corresponding [ (bpy)zRuIIL] and [(NH3)5Co1I1(Pro),] components. The compounds were characterized by metal analyses, electrochemical measurements, and absorption spectroscopy. For n = 4-6 prolines, the C D spectra of the complexes show a polyproline I1 helical structure. Intramolecular electron transfer within these complexes was studied by generating the [ (bpy)2R~~IL*-(Pro),-Co~~~(NH3)5] intermediate by the reaction of e,, (generated by pulse radiolysis) with the [(bpy)2R~~~L-(Pro),-Co~~~(NH3)5] molecules. The driving force for this reaction is estimated to be lAGol N -1.1 V. The intramolecular electron transfer rates (k) and activation parameters (AH* (kcal/mol), AS* (eu)) found for these studies were (1.6 f 0.1) X lo7 s-l, 6.0 f 0.6, -6 f 2; (2.3 f 0.2) X los s-l, 9.2 f 0.4, -3 f 1; (5.1 f 0.4) X lo4 s-l, 9.4 f 0.2, -5.5 f 0.8; (1.8 f 0.1) X lo4 s-l, 9.0 f 0.4, -9f 1; and (8.9 f 0.6) X lo3 s-l, 8.8 f 0.4, -1 1 f 1 for n = 2-6, respectively. For n = 1 proline, k is >5 X lo8 s-* and no temperature dependence could be determined. The rates of intramolecular electron transfer decrease rapidly with distance for n = 1-3 prolines but show a surprisingly weak decrease with distance for the n = 4, 5, and 6 prolines, which exhibit the polyproline I1 helical structure. The electron-transfer pathways within these molecules and the relationship of the electron-transfer rates to the helical polyproline I1 structure are discussed.

Developing axons of the corpus callosum of mice are guided across the cerebral midline by a slinglike glial structure that forms transiently between the hemispheres. If the "sling" is cut at precallosal stages, the would-be callosal fibers whirl into paired neuromas adjacent to the longitudinal cerebral fissure. In experiments on such surgically acallosal animals, the aberrant commissural axons maintained a potential to regrow across the hemispheres at prenatal and early postnatal stages if they were presented with a properly aligned, glia-covered scaffold spanning the hemispheres.

The addition of Cu(I) to the random-coil peptide, C16C19-GGY, produces a self-organized, metal-bridged 4-helix bundle which displays an intense room-temperature luminescence at 600 nm. Emission, UV, and CD titrations along with X-ray absorption studies indicate that the luminescent cofactor is likely a Cu4S4 cluster in which each Cu atom is bridged by the side chains of two cysteine residues and has terminal N/O ligation.