PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER IN DICHROMOPHORE-APPENDED ALPHA-HELICAL PEPTIDES - SPECTROSCOPIC PROPERTIES AND PREFERRED CONFORMATIONS

The alpha-helical structure of donor-acceptor-substituted peptides 1 a nd 2 was established by circular dichroism and NMR spectroscopy. The p hotoproduction of a spatially separated radical ion pair was detected by transient absorption spectroscopy. The expected influence of the el ectric field along the helical pitch direction was established by moni toring S-1 lifetimes. The complex kinetics of the observed decays indi cate that even these rigid peptides exist in a distribution of conform ations. This assertion is further supported by single-photon counting measurements, NMR spectroscopy (COSY; NOESY and variable-temperature H -1 NMR) and extended MMS2 force-field model calculations. An estimate for the rotational thresholds of the appended chromophores was obtaine d, and the character of the electronic interaction between the appende d chromophores was examined by semiempirical single-paint calculations , Such calculations show the possible importance of through-bond inter actions between the chromophores. The backbone conformational regulari ties of 1 and 2 are sufficient to establish the previously reported in fluence of a electrostatic field on photoinduced electron transfer rat es, but side-chain conformational mobility of 1 and 2 imposes an inher ent limitation an experimental observations. (C) 1997 by John Wiley Si Sons, Ltd.