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Interleukin-8 has been shown by X-ray crystallography and NMR to be a homodimer, suggesting that this is the form which binds to its receptor. Here we measure, for the first time, the monomer-dimer equilibrium of interleukin-8 using analytical ultracentrifugation and titration microcalorimetry and find that it dissociates readily to monomers with an equilibrium dissociation constant of 18 +/- 6 microM at 37 degrees C. The present findings suggest that the monomer is the form which binds to the receptor. Comparison of experimental and structure-based calculated thermodynamics of interleukin-8 dimerization argues for limited subunit conformational changes upon dissociation to monomer.

The multiexponential decay of tryptophan derivatives has previously been explained by the presence of rotamers having different fluorescence lifetimes, but it has been difficult to correlate rotamer structure and physical properties. New time-resolved and static data on dipeptides of the type Trp-X and X-Trp, where X is another aminoacyl residue, are consistent with the rotamer model and allow some correlations. That a dominant rotamer of Trp-X zwitterion has the -NH3+ group near the indole ring was inferred from absorption and fluorescence spectra, titrimetric determination of pKa values, photochemical hydrogen-deuterium-exchange experiments, decay-associated spectra, quantum yields, and decay kinetics. Analysis of the lifetime and quantum yield data for Trp dipeptides, especially X-Trp, suggests that static self-quenching is not uncommon. Highly quenched and weak components of the fluorescence do not contribute to the calculated mean lifetime, thus resulting in apparent static quenching. We propose the term quasi-static self-quenching (QSSQ) to distinguish this phenomenon from quenching due to ground-state formation of a dark complex. Mechanisms of quenching and the structure of statically quenched rotamers are discussed. The occurrence of QSSQ supports the idea that rotamers interconvert slowly. A major perceived deficiency of the rotamer model, namely, the apparent inability to predict reasonable rotamer populations from fluorescence decay data, may result from the presence of statically quenched species, which do not contribute to the fluorescence.

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