Motional Dynamics of a Buried Tryptophan Reveals the Presence of Partially Structured Forms during Denaturation of Barstar

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Abstract
A double mutant of the single-domain protein barstar having a single tryptophan (W53) was made by mutating the remaining two tryptophans (W38 and W44) into phenylalanines. W53 is buried in the core of barstar. Time-resolved fluorescence of the mutant barstar (W38FW44F) showed that W53 has a single fluorescence lifetime in the native (N) state and has three lifetimes in the molten globule-like low-pH (A) form. Quenching of fluorescence by either KI or acrylamide showed that W53 is solvent inaccessible in the N-state and fairly accessible in the A-form. The denaturation of W38FW44F by guanidine hydrochloride (GdnHCl) was monitored by several probes: near-UV and far-UV circular dichroism (CD), fluorescence intensity, and steady-state and time-resolved fluorescence anisotropy. While the unfolding transitions observed through CD and fluorescence intensity coincided with each other (midpoint ≈ 1.8 M GdnHCl), the transition observed through the steady-state fluorescence anisotropy was markedly different from others. Initially, the anisotropy increased with the increase in the concentration of GdnHCl and decreased subsequently. The midpoint of this titration was 2.2 M GdnHCl. Picosecond time-resolved fluorescence anisotropy showed that W38FW44F has a single rotational correlation time of 4.1 ns in the native (N) state and 1.5 ns in the unfolded (U) state (6 M GdnHCl). These could be explained as being due to the absence of motional freedom of W53 in the N-state and the presence of rotational freedom in the U-state. In the intermediate concentration region (1.8−3.0 M GdnHCl), the anisotropy decays showed at least two correlation times, ∼1 and 6−12 ns. These two correlation times are ascribed to partially structured forms leading to hindered rotation of W53. Thus, the usefulness of time-resolved fluorescence anisotropy in detecting partially folded structures is demonstrated.