Competitive binding of Chlorin p6 and Dansyl-l-Proline to Sudlow's site II of human serum albumin.

The binding of chlorin p6, a model photosensitizer for photodynamic therapy (PDT), to the Sudlow's site II of Human Serum Albumin (HSA) has been monitored by different spectroscopic methods. Displacement of Dansyl-l-Proline (DP) from its conjugate with HSA is manifested in the spectral shift and decrease in its fluorescence intensity as well as the emergence of component with lifetime of 2-3ns, which is characteristic of free DP. As DP is known to bind specifically to the Sudlow's site II of human serum albumin, its displacement by chlorin p6 indicates the residence of the photosensitizer in the same site, in addition to Sudlow's site I. The binding constants for Sudlow's site II, determined by the stopped-flow technique, are found to be two orders of magnitude smaller than that for Sudlow's site I.