Fluorescence correlation spectroscopy shows that monomeric polyglutamine molecules form collapsed structures in aqueous solutions

We have used fluorescence correlation spectroscopy measurements to quantify the hydrodynamic sizes of monomeric polyglutamine as a function of chain length (N) by measuring the scaling of translational diffusion times (τ_D) for the peptide series (Gly)-(Gln)_N-Cys-Lys₂ in aqueous solution. We find that τ_D scales with N as τ_oN^ν and therefore ln(τ_D) = ln(τ_o) + νln(N). The values for ν and ln(τ_o) are 0.32 ± 0.02 and 3.04 ± 0.08, respectively. Based on these observations, we conclude that water is a polymeric poor solvent for polyglutamine. Previous studies have shown that monomeric polyglutamine is intrinsically disordered. These observations combined with our fluorescence correlation spectroscopy data suggest that the ensemble for monomeric polyglutamine is made up of a heterogeneous collection of collapsed structures. This result is striking because the preference for collapsed structures arises despite the absence of residues deemed to be hydrophobic in the sequence constructs studied. Working under the assumption that the driving forces for collapse are similar to those for aggregation, we discuss the implications of our results for the thermodynamics and kinetics of polyglutamine aggregation, a process that has been implicated in the molecular mechanism of Huntington's disease.