Photophysical investigations of squaraine and cyanine dyes and their interaction with bovine serum albumin

Interaction of acridinedione dye with a globular protein in the presence of site selective and site specific binding drugs: Photophysical techniques assisted by molecular docking methods

Photophysical investigations and molecular docking studies of photoinduced electron transfer (PET) based fluorophores of acridine family with a globular protein, Bovine Serum Albumin (BSA) bound to non-narcotic drugs like phenylbutazone (PB) and flufenamic acid (FA) were carried out in aqueous solution. PB and FA are site specific and site selective drugs, wherein PB predominantly binds at the site (I) whereas FA selectively orients towards site (II) of BSA. Acridinedione (AD) dyes, both resorcinol and dimedone based are hydrophobic in nature and exhibits a combination of both hydrophobic and hydrogen-bonding interactions that are based on the binding sites in BSA. The extent of displacement of AD from the binding sites of BSA by PB and FA are elucidated and established from variation in the fluorescence lifetime and relative amplitude distribution of free and dye bound in site (I) and site (II). The extent of binding affinity of PB-BSA and FA-BSA in the presence of AD is minimal when compared to other site I and II drugs. This is attributed to AD dye bound to several amino acid residues present in BSA such that the dye prefers multiple binding sites in BSA even in the presence of FA and PB. Further, the dye bound to several amino acid residues of BSA ascertains the combination of hydrogen-bonding, hydrophobic interactions, pi-pi and pi-alkyl interaction apart from the binding through sites (I) and (II) from molecular docking methods. The combination of fluorescence tools with molecular modelling techniques provides an excellent approach in determining the stability of these complexes containing competitive guest molecules in the presence of a fluorescence probe and the binding characteristics of dye in a micro heterogeneous environment.

Far-red pentamethine cyanine dyes as fluorescent probes for the detection of serum albumins

Benzothiazole based cyanine dyes with bridged groups in the pentamethine chain were studied as potential far-red fluorescent probes for protein detection. Spectral-luminescent properties were characterized for unbound dyes and in the presence of serum albumins (bovine (BSA), human (HSA), equine (ESA)), and globular proteins (β-lactoglobulin, ovalbumin). We have observed that the addition of albumins leads to a significant increase in dyes fluorescence intensity. However, the fluorescent response of dyes in the presence of other globular proteins was notably lower. The value of fluorescence quantum yield for dye bearing a sulfonate group complexed with HSA amounted to 42% compared with 0.2% for the free dye. The detection limit of HSA by this dye was greater than 0.004 mg ml-1 which indicates the high sensitivity of dye to low HSA concentrations. Modelling of structure of the dyes complexes with albumin molecules was performed by molecular docking. According to these data, dyes could bind to up to five sites on the HSA molecule; the most preferable are the haemin-binding site in subdomain IB and the dye-binding site in the pocket between subdomains IA, IIA and IIIA. This work confirms that pentamethine cyanine dyes could be proposed as powerful far-red fluorescent probes applicable for highly sensitive detection of albumins.

Albumin-chaperoned cyanine dye yields superbright NIR-II fluorophore with enhanced pharmacokinetics

NIR-II fluorescence imaging greatly reduces scattering coefficients for nearly all tissue types at long wavelengths, benefiting deep tissue imaging. However, most of the NIR-II fluorophores suffer from low quantum yields and/or short circulation time that limit the quality of NIR-II imaging. Here, we engineered a supramolecular assembly of protein complex with lodged cyanine dyes to produce a brilliant NIR-II fluorophore, providing a NIR-II quantum yield of 21.2% with prolonged circulation time. Computational modeling revealed the mechanism for fluorescence enhancement and identified key parameters governing albumin complex for NIR-II fluorophores. Our complex afforded high-resolution microvessel imaging, with a 3-hour imaging window compared to 2 min for free dye alone. Furthermore, the complexation strategy was applied to an antibody-derived assembly, offering high-contrast tumor imaging without affecting the targeting ability of the antibody. This study provides a facile strategy for producing high-performance NIR-II fluorophores by chaperoning cyanine dyes with functional proteins.