Electronic energy transfer between tyrosine and tryptophan in the peptides trp-(pro)n-Tyr

A series of peptides, Trp-(Pro)_n-Tyr, n = 0,…,5 were synthesized. Details of the synthetic method using the Merrifield method are given. The initial objective was to determine the characteristic distance R₀ for energy transfer from tyrosine to tryptophan. This objective became submerged in the question of the structure of the chain of prolyl spacers. The efficiency of the energy transfer diminishes as the number of intervening prolines increases. Quantitatively the fit is much better if we assume that the peptides have a polyproline I structure (cis) than if we assume a trans polyproline II structure. Proton nmr spectra on the other hand suggest that the n = 1,3,5 peptides are mixtures of rotamers. Our conclusions are similar to those of previous workers, i.e., that in water there is a transition from a mixture of structures for small n to an all-trans structure at some n > 5. This means that in water, at least, proline is a dubious spacer for energy-transfer studies.