Phosphorescence. Trp-151 phosphorescence studies provide further
support for hydrophobic stacking between the indole side
chain and the galactopyranosyl moiety of substrate (Fig. 7). In
the absence of sugar, the characteristic vibronic 0,0 band of
tryptophan appears at 408 nm, indicating that the indole group
is exposed to a polar environment (47, 48). Significantly, the 0,0
band of Trp-151 is reproducibly red-shifted 4 nm in the presence
of TDG, but no change is observed in the presence of glucose.
The spectral shift observed in the presence of TDG is consistent
with hydrophobic stacking between the indole group of Trp-151
and the galactopyranosyl ring of the lactose analog.