Two-way packing model for ribonuclease T1 fluorescence

摘要

The fluorescence intensity decay of ribonuclease T1 is biexponential at neutral pH. The lifetimes in nanoseconds and preexponential factors of the exponential components are 3.9 (81%) and 1.7 (19%). The mutations A22L, G23A, L26A, V67G, and V67D, which all neighbor tryptophan-59, have a fairly small effect on this biexponential decay. The lifetime of the long lived component varies from 3.7 to 4.2 nanoseconds and the preexponential varies from 75% to 92%. The emission maximum varies from 319 to 328 nanometers and the acrylamide quenching rate constant varies from 2.0 to 4.0×10{sup}8 M{sup}(-1)s{sup}(-1) for these mutations. Minimum perturbation mapping simulations of the tryptophan-59 side chain in wild type ribonuclease T1 show that the x{sup}2 side chain dihedral angle may adopt either a perpendicular or an antiperpendicular conformation. These computational and spectroscopic results lead us to propose a two-way packing model for tryptophan-59. This model predicts the relative free energies of the perpendicular and antiperpendicular conformations and acrylamide interaction site to indole ring distances for the wild type and mutant ribonucleases.