Secondly, a signi®cant red shift of the emission maximum is
observed as the temperature is raised. This shift results from
an increase in the number of accessible vibration levels and/or
from modi®cations in solvent relaxation effects (Lakowicz,
1999) as more thermal energy becomes available. Modi®cation
in ®ne structure (Fig. 3) might also result from changes in
sample rigidity. Therefore, temperature-dependent fuorescence
studies appear of high interest to probe dynamics in
crystalline proteins. For example, we have obtained evidence
that solvent transitions such as the glass transition (Vitkup et
al., 2000; Weik et al., 2001) can be followed in this way (not
shown). In particular as opposed to endogenous fuorophores
located in the interior hydrophobic part of the protein,
exogenous fuorophores can be used to probe the solvent
channels and the two behaviours can be compared. These
results will be published elsewhere.