Fig. 5.

FRET between peptides labeled with NBD (donor) and TAMRA (acceptor). An equimolar mixture of unlabeled and donor peptides (3 μM each) in SDS (3 mM) was titrated with an increasing amount of acceptor-labeled peptide in the following way. Aliquots (125 μl to 1.0 ml) were removed and replaced with an equal volume solution containing donor peptide (3 μM) and the acceptor peptide (3 μM). The net effect is that unlabeled peptide is sequentially lowered in concentration and substituted with the TAMRA-labeled peptide, keeping the NBD peptide concentration constant. Theoretical curves (best fit) were generated by using a binomial distribution of donors in oligomeric ensembles and fit to monomer-dimer equilibrium for and TF2, and for a monomer-trimer equilibrium for TH1 and TF1. Plots of relative fluorescence (F/F₀) versus mole fraction of the acceptor (X_A) reveal that TH2 (▪) suffers the least quenching consistent with the lowest average association number, and TF2 (▾), TH1 (•), and TF1 (▴) show increasing association propensities.