Spectroscopic study of squaraines as protein-sensitive fluorescent dyes

A series of new symmetrical and asymmetrical squaraines were synthesised and efficiency of their use as fluorescent probes for the specific detection of proteins was studied. Spectral-luminescent properties of the squaraines were measured in presence of bovine serum albumin (BSA), human serum albumin (HSA), ovalbumin, avidin from hen egg white (AVI), lysozyme, and trypsin. All investigated squaraines show considerable (in 24–190 times) emission increase in the presence of BSA. At the same time the fluorescent response of the studied dyes in the presence of other albumins is significantly lower – emission enhances up to 24 times. The 3-oxo-substituted indolenine dye 9(74) demonstrates sufficient fluorescence increasing value and emission intensity level in the presence of BSA as well as of HSA and ovalbumin. Dyes containing N-carboxyalkyl group demonstrate sufficient emission enhancement (up to 16 times) and noticeable fluorescent signal in the presence of avidin from hen egg white. Squaraines slightly increase or even decrease their emission intensity in the presence of hydrolases lysozyme or trypsin.