Solution structure by site directed tryptophan fluorescence in tear lipocalin.

The solution structure of the G strand of human tear lipocalin was deduced by site directed tryptophan fluorescence (SDTF). The fluorescent amino acid, tryptophan, was sequentially substituted for each native amino acid in the sequence of the G strand. The fluorescent properties resolved alternating periodicity as predicted for beta sheet structure, twists in the beta sheet, strand orientation in the lipocalin cavity, and the relative depth of residues in the cavity. A distribution of microstates with various orientations of dipoles in the side chain environments of the G strand revealed mobility on the nanosecond time scale. SDTF is broadly applicable to most proteins and will complement x-ray crystallography, site directed spin labeling by electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) in the determination of solution structure.