Tracking local conformational changes of ribonuclease A using picosecond time-resolved fluorescence of the six tyrosine residues.

The six tyrosine residues of ribonuclease A (RNase A) are used as individual intrinsic probes for tracking local conformational changes during unfolding. The fluorescence decays of RNase A are well described by sums of three exponentials with decay times (tau(1) = 1.7 ns, tau(2) = 180 ps, and tau(3) = 30 ps) and preexponential coefficients (A(1) = 1, A(2) = 1, and A(3) = 4) at pH 7, 25 degrees C. The decay times are controlled by photo-induced electron transfer from individual tyrosine residues to the nearest disulphide (-SS-), bridge, which is distance (R) dependent. We assign tau(1) to Tyr-76 (R = 12.8 A), tau(2) to Tyr-115 (R = 6.9 A), and tau(3) to Tyr-25, Tyr-73, Tyr-92, and Tyr-97 (all four at R = 5.5 +/- 0.3 A) at 23 degrees C. On the basis of this assignment, the results show that, upon thermal or chemical unfolding only Tyr-25, Tyr-92, and Tyr-76 undergo significant displacement from their nearest -SS- bridge. Despite reporting on different regions of the protein, the concordance between the transition temperatures, T(m), obtained from Tyr-76 (T(m) = 59.2 degrees C) and Tyr-25 and Tyr-92 (T(m) = 58.2 degrees C) suggests a single unfolding event in this temperature range that affects all these regions similarly.