[K-35, a fluorescent probe for albumin study: optical properties]

Fluorescent probe N-(carboxyphenyl)imide of 4-(dimethylamino)naphthalic acid, K-35, is used as an indicator of structural changes of human serum albumin molecules in pathology. The probe occupies albumin binding pockets where the probe environment is of very high polarity; probably, the pocket(s) contains protein polar groups and water molecules. At the same time rather small Stokes shift of K-35 fluorescence spectrum shows that the polar group motion is of one-two order of value lower than mobility of polar molecules in polar fluids. K-35 fluorescence decay in HSA can be described as a sum of three exponentials with time constants close to tau1=9 ns; tau2=3.6 ns and tau3=1.0 ns. A difference between excitation maxima of these three decay components shows that environment of these three species of K-35 molecules has been different before excitation. Different r values are probably a consequence of non-identical structure of several binding sites, or a binding site(s) can have a variable conformation.