Fluorescence of quinacrine mustard conjugated to proteins

Abstract

Proteins are readily labeled by quinacrine mustard to yield conjugates whose spectral properties are well-suited for fluorescence studies. Data on these conjugates and on the parent compound, quinacrine, are presented including lifetimes, quantum yields, and corrected excitation and emission spectra. Polarization studies using the Perrin-Weber equation show that rotational relaxation times can be obtained with quinacrine mustard conjugates. Such conjugates had lifetimes ranging from 4 to 13 ns and quantum yields from about 0.1 to 0.3. Quinacrine mustard is a useful reporter group, as shown by the changes in fluorescence parameters of conjugates undergoing conformational changes induced by denaturants. An excited state pK a ∗ of 4.9 was identified for quinacrine, but the protonation was suppressed in the mustard conjugate of serum albumin until the N-F transition took place. The properties of the mustard conjugates are discussed in terms of potential uses and compared with properties of other types of fluorescent conjugates.