Aggregation of alpha-synuclein (alpha-syn), a protein implicated in Parkinson's disease (PD), is believed to progress through formation of a partially folded intermediate. Using nanoelectrospray ionization (nano-ESI) mass spectrometry combined with ion mobility measurements we found evidence for a highly compact partially folded family of structures for alpha-syn and its disease-related A53T mutant with net charges of -6, -7, and -8. For the other early onset PD mutant, A30P, this highly compact population was only evident when the protein had a net charge of -6. When bound to spermine near physiologic pH, all three proteins underwent a charge reduction from the favored solution charge state of - 10 to a net charge of -6. This charge reduction is accompanied by a dramatic size reduction of about a factor of 2 (cross section of 2600 angstrom(2) (-10 charge state) down to 1430 angstrom(2) (-6 charge state)). We conclude that spermine increases the aggregation rate of alpha-syn by inducing a collapsed conformation, which then proceeds to form aggregates.