Single molecule polyproline isomerization is studied by fluorescence quenching, induced by short-ranged electron transfer between TMR(5-carboxytetramethylrhodamine) and DMPD(dimethyl-p-phenylenediamine). To do this, we have prepared a polyproline(n=2,3) peptide with DMPD at its carboxylic end and TMR at its amino end. The electron transfer efficiency is measured by TCSPC(time-correlated single photon counting) in which accepter fluorescence lifetime is comparatively quenched according to the proximity of donor molecule. This allows us to measure local protein or peptide conformation at the level of a few residues. It also allows us to investigate the role of conformation in biological electron transfer. The figure above shows the major trans configuration on the left and the cis configuration on the right. The structures were determined by NMR NOE constraints. The trans configuration corresponds to the Pro-II helix and the cis configuration corresponds to the Pro-I helix.