Distance Dependence of Electron Transfer along Artificial β-Strands at 298 and 77 K

Photoinduced electron-transfer rate constants were measured for a series of binuclear metallopeptides consisting of a [Ru(bpy)2(cmbpy)]2+ electron donor tethered to a CoIII(NH3)5 electron acceptor by an oligovaline peptide chain (bpy = 2,2‘-bipyridine, cmbpy = 4-carboxy-4‘-methyl-2,2‘-bipyridine). These compounds were shown by 1H NMR to adopt the conformational properties found within the individual strands of a β-pleated sheet in both aqueous and methanol solutions. Emission lifetime measurements and HPLC product analysis show that the binuclear donor/acceptor compounds undergo photoinduced electron transfer (ET). The values of ket decrease with increasing donor/acceptor distance according to the expression ket = k‘ exp[−β(r − r0)]. A distance attenuation factor of β = 1.1 ± 0.4 A-1 is seen both in H2O at 298 K and in an ethanol−methanol glass at 77 K. The ET kinetics obtained at 77 K for 1−3 were single exponential, indicating that the compounds maintain a unique donor/acceptor separation and do not exi...