The speed limit for protein folding measured by triplet-triplet energy transfer

A direct measure of intramolecular chain diffusion is obtained by the deter mination of triplet-triplet energy-transfer rates between a donor and an ac ceptor chromophore attached at defined points on a polypeptide chain. Singl e exponential kinetics of contact formation are observed on the nanosecond time scale for polypeptides in which donor and acceptor are linked by repea ting units of glycine and serine residues. The rates depend on the number o f peptide bonds (N) separating donor and acceptor and show a maximum for th e shortest peptides (N = 3) with a time constant (tau = Ilk) of 20 ns. This sets an upper limit for the speed of formation of the first side-chain con tacts during protein folding.