  A fundamental question in protein folding is whether the coil to globule
collapse transition occurs during the initial stages of folding (burst-phase)
or simultaneously with the protein folding transition. Single molecule
fluorescence resonance energy transfer (FRET) and small angle X-ray scattering
(SAXS) experiments disagree on whether Protein L collapse transition occurs
during the burst-phase of folding. We study Protein L folding using a
coarse-grained model and molecular dynamics simulations. The collapse
transition in Protein L is found to be concomitant with the folding transition.
In the burst-phase of folding, we find that FRET experiments overestimate
radius of gyration, $R_g$, of the protein due to the application of Gaussian
polymer chain end-to-end distribution to extract $R_g$ from the FRET
efficiency. FRET experiments estimate $\approx$ 6\AA \ decrease in $R_g$ when
the actual decrease is $\approx$ 3\AA \ on Guanidinium Chloride denaturant
dilution from 7.5M to 1M, and thereby suggesting pronounced compaction in the
protein dimensions in the burst-phase. The $\approx$ 3\AA \ decrease is close
to the statistical uncertainties of the $R_g$ data measured from SAXS
experiments, which suggest no compaction, leading to a disagreement with the
FRET experiments. The transition state ensemble (TSE) structures in Protein L
folding are globular and extensive in agreement with the $\Psi$-analysis
experiments. The results support the hypothesis that the TSE of single domain
proteins depend on protein topology, and are not stabilised by local
interactions alone.
