Role of small-scale mobility of protein globule in intensification of electronic interactions between photoactive groups

Lysozyme samples with different content of covalently bound anthracene-cont aining photoactive groups (PhAG) (from 0.27 to 2.0 PhAG per molecule) were obtained as products of reaction between isothiocyanatomethyl-anthryl and N H2 groups of protein. Spectral and polarization analysis of PhAG luminescen ce revealed that as the PhAG concentration in aqueous solution (0.01 M phos phate buffer, pH 7.4) of lysozyme is raised from 1 to 1.3 PhAG per protein molecule, there is a cooperative increase in the electronic interaction bet ween the photoactive groups. This gives rise to excimer formation and migra tion of energy of PhAG electronic excitation. Studies of polarized luminesc ence (PL) revealed that addition of up to 3 M guanidine hydrochloride (GHC) to aqueous solution of lysozyme induces an increase in the intramolecular mobility of the protein globule domains containing PhAG, although has virtu ally no effect on the compact globular structure of the protein. The increa se in the intramolecular mobility of the protein globule is accompanied by intensification of electronic interaction between PhAG. Further elevation o f the GHC content in the reaction medium causes unfolding of the lysozyme g lobule, a further increase in the intramolecular mobility, an increase in t he distance between PhAG, and a decrease in the efficiency of photophysical processes in the system.