Rapid screening of dyes for self-aggregation, adsorption and metabolic integrity – Quantitative metrics as a prelude to biological studies

Abstract

The traditional development of dyes for biological applications typically begins with synthesis, is followed by spectroscopic/photophysical characterization, and culminates in testing with cultured cells or in animals. The present work describes a set of facile, low-technology, invitro, largely quantitative absorption spectrophotometric assays in aqueous solution as a prelude to biological studies. The assays include the following: (1) Self-aggregation in phosphate-buffered saline (PBS), with or without the presence of 3 % bovine serum albumin (BSA), via reciprocal change in concentration and pathlength; (2) Adsorption to solid materials as biomolecular surrogates, including cellulose, chitosan, polyethylene (PE), styrene-divinylbenzene beads (XAD-4), polyacrylic beads (XAD-7), anion exchange beads (IRA-67), cation exchange beads (CG-50), polytetrafluoroethylene (PTFE) membranes, and Nylon membranes; (3) Hydrophilicity/hydrophobicity assessment, by measurement (not calculation) of logP values; and (4) Metabolic integrity, by exposure to a rat liver homogenate. Twenty-two dyes were examined in the assays. The dyes represent diverse classes (acridine, anthraquinone, azo, benzimidazole, coumarin, cyanine, flavonoid, naphthalene, phthalocyanine, polyene, tetrapyrrole, triarylmethine, xanthene), polarity (anionic, cationic, nonionic, hydrophobic, amphipathic), and absorption (ultraviolet, visible, near-infrared). As one example, indocyanine green (ICG), an FDA-approved dye for invivo use, (i) aggregates at the concentration of 5 μM but disaggregates in the presence of BSA; (ii) in PBS is very adsorptive (>80 % binding) to various materials (cellulose, chitosan, PE, IRA-67, CG-50, PTFE, Nylon) and quite adsorptive (50–80 % binding) to XAD-4 and XAD-7, (iii) exhibits only marginal preference for water versus 1-octanol (1.5:1), and (iv) is largely digested by rat liver homogenate. A single dye can be examined in the span of one day. The quantitative metrics should prove valuable for establishing biologically relevant features of diverse dyes by simple studies in aqueous solution.