Distance dependence of intramolecular electron transfer across oligoprolines in [(bpy)2RuIIL.bul.-(Pro)n-CoIII(NH3)5]3+, n = 1-6: different effects for helical and nonhelical polyproline II structure

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.

Distance dependence of long‐range electron transfer through helical peptides

“…Boc-(Ala-Aib) 4 -OBzl and Boc-(Ala-Aib) 8 -OBzl were synthesized according to the method reported in the literature [24] where Boc stands for tertbutyloxycarbonyl and OBzl for benzyl ester. Chemical modifications of the peptides (8mer and 16mer) with the ferrocene moiety at the C-terminal and lipoic acid at the N -terminal were done similar to our previous work [10].…”

“…Especially, electron transfer through helices has attracted much attention because it is believed that α-helical segments play an essential role in mediating an electron and determining its direction in biological systems [6]. To study the nature of electron transfer, radiolysis [7,8] and photoinduced electron-transfer [9] studies in solution (donor-peptide-acceptor), electrochemical studies on self-assembled monolayer (SAM) systems (donor-peptide-metal) [10][11][12][13][14][15], and recently, single molecule measurements by scanning probe microscopy (metal-peptide-metal) [16][17][18], have been conducted intensively. Most of the researchers agree that a helical peptide is a good electron mediator and enables electron transfer over a long distance.…”

Activation entropy of electron transfer reactions

“…These two parameters (∆G g and V 12 ) were varied in fitting the experimental activation enthalpy ∆H † = 9.5 kcal/mol and the experimental activation entropy ∆S † /k B = −5.6 e.u. [41]. Note that the experimental quantity is an effective entropy, including contributions due to the electronic coupling element as well as solvation and inner-sphere vibrational modes [40].…”

Long-Range Electron Transfer Between Proline-Bridged Aromatic Amino Acids

“…This pathway is characterized by a low value of the descriptor of the exponential distance dependence of the electron transfer rate, β ΤΒ = 2.5 ± 0.1 nm -1 , suggesting that helical segments in proteins can function as efficient channels of long-distance electron transfer. Long-range electron transfer (LRET) between various oligoproline-bridged redox pairs has been studied over the past 10 years in several laboratories (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) with the aim of elucidating the parameters of LRET across a single peptide pathway. The choice of oligoprolines for such a study was dictated by the known ability of short H-(Pro)n-OH peptides to attain, in aqueous solution, a stable helical conformation similar to that of the 3X left-handed helix of alltrans poly-L-proline II (19)(20)(21)(22).…”