Abstract
While some studies inferred that valid information can be retrieved for the refolding of proteins and consequent identification of folding intermediates in the stopped-flow spectrometry collapse phase, other studies report that these burst phase folding intermediates can be questioned, implying a solvent-dependent modification of the still unfolded polypeptide chain. We therefore decided to investigate the burst phase occurring for the α-synuclein (Syn) amyloid protein by stopped-flow spectrometry. Solvent-dependent modification effects indeed occurred for the Nα-acetyl-L-tyrosinamide (NAYA) parent small compound and for the folded monomeric ubiquitin protein. More complex was the burst phase analysis of the disordered Syn amyloid protein. While this amyloid protein was determined to be aggregated at pH 7 and pH 2, in particular, this protein at pH 3 appears to be in a monomeric state in the burst phase analysis performed. In addition, the protein at pH 3 appears to suffer a hydrophobic collapse with the formation of a possible folded intermediate. This folded intermediate seems to result from a fast contraction of the disordered amyloid polypeptide chain, which is proceeded by an expansion of the protein, due to the occurrence of solvent-dependent modification effects in a milliseconds time scale of the burst phase. Generally, it can be argued that both literature criteria of solvent-dependent modifications of the disordered Syn amyloid protein and of the formation of its possible folded intermediate are very likely to occur in the burst phase.