Site-selective interactions: squaraine dye-serum albumin complexes with enhanced fluorescence and triplet yields

We report the effect of steric factors of a few squaraine dyes, bis(2,4,6-trihydroxyphenyl)squaraine (1), bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2), and bis(3,5-diiodo-2,4,6-trihydroxyphenyl) squaraine (3), on their binding with human (HSA) and bovine (BSA) serum albumins employing photophysical, chiroptical, biophysical, and microscopic techniques. These dyes interact with serum albumins very efficiently and exhibit site selectivity, involving synergistic effects of hydrophobic, hydrogen bonding, and electrostatic interactions. The association constants of these complexes have been determined and are found to be 4.9 x 10(6) and 4.1 x 10(5) M(-1), respectively, for the dyes 2 and 3 with BSA, while HSA showed relatively higher association constants of 6.0 x 10(6) and 9.9 x 10(5) M(-1). Highly clear distinction in site-selective binding can be ascertained from time-resolved fluorescence, displacement cum fluorimetry, and circular dichroism (CD) studies. The increased affinity toward the major binding site (site II, domain III) over the relatively smaller binding site (site I, domain II) in the serum albumin with the increasing size of the heavy atoms present in 2 and 3 as compared to 1 indicates the importance of steric factors thereby confirming that the dye structure has a predominant role in deciding site selectivity. The distance between the energy donor and acceptor was calculated using Forster theory, which agrees well with the reported site 1 binding agent dansylamine. In contrast, no energy transfer was observed between tryptophan (Trp-214) present in domain II of the albumins and the dyes 2 and 3, indicating that these derivatives bind less efficiently at site I due to steric conatraints but preferentially bind at site II. Laser flash photolysis studies of the dyes 2 and 3 in the presence of HSA exhibited ca. 2.5-fold enhancements in the triplet lifetimes and quantum yields when compared to that obtained in buffer. The uniqueness of these dyes is that they show substituent size-dependent selectivity at site II of serum albumins and signal the event through "turn on" fluorescence intensity as well as enhanced triplet excited state lifetimes and quantum yields, thereby indicating their potential use as NIR noncovalent protein labeling and photodynamic therapeutic agents.