Increased blood plasma concentrations of the aminothiol homocysteine (Hcy) are associated with a variety of disease states including those which cause impaired renal function, many forms of cardiovascular disease, and neurodegenerative diseases such as Alzheimer's. Therefore, Hcy has the potential to be a significant diagnostic biomarker. Routine monitoring of Hcy plasma concentration is encumbered by the time and resources required to quantify Hcy using currently accepted instrumental analysis methods. As part of the continuing effort to develop a quick, reliable, inexpensive, and user-friendly test to quantify Hcy at the point of care, we have designed a series of novel colorimetric and fluorescent chemical probes based on bridged viologen structures. The absorbance at 540 nm for the para-bridged bis-nitrile viologen probe (pCN) was found to be proportional to the concentration of Hcy analyte, with LOD = 2.17 μM and LOQ = 6.10 μM where unhealthy Hcy plasma concentrations are > 15 μM. The mechanism of reactivity between pCN and Hcy encompasses a dynamic set of reactions which involve pimerization of radical probe species and thioether adduct formation of pCN with Hcy. Preliminary results with fluorometric analogs of the bridged viologen probes are also presented.

The most abundant biological thiols, homocysteine (Hcy), cysteine (Cys) and glutathione (GSH) have been the subject of intense research due to their association with a wide range of diseases. They play a key role in maintaining the redox status of biological systems. Selective detection methods for these thiols are challenging due to their similar structures and properties. Current commercially available detection methods use separations, fragile and expensive enzymatic or immunogenic materials and complex instrumentation. This has led to a global effort towards developing simple and inexpensive optical probes and indicators selective for specific biological thiols. Highly selective chemical probes and simple methods for detection and potential quantification of Hcy and GSH in their natural biological media have been developed. These indicators and methods are relatively simple and inexpensive for potential application at point of care. The selective detection of Hcy using novel asymmetric viologen chemical probes at room temperature is described as well as the use of commercially available materials under photochemical conditions. These probes respond linearly proportional to increasing Hcy concentrations, potentially enabling the monitoring of Hcy levels in human plasma. Additionally, new methods for the selective determination of GSH in human plasma, as well as its quantification in whole blood deposited on filter paper (dried blood spots), is also presented herein.

Homocysteine is a natural occurring aminothiol. It is an intermediate product in the metabolism of methionine. Methionine is an essential amino acid required for protein synthesis. Metabolic irregularities disrupt homocysteine levels in plasma. Elevated homocysteine levels are directly linked to folate and cobalamin (vitamin B12) deficiencies, and are an independent risk factor for cardiovascular diseases. High homocysteine levels have also been associated with Alzheimer's, osteoporosis, renal failure, cancer, birth defects and pregnancy complications. The association of elevated homocysteine levels with cardiovascular disease and other diseases has generated great interest in the detection of homocysteine. An optical method for the detection of homocysteine has been developed using fluorescein mono- and dialdehydes. Selectivity for homocysteine was achieved based on the characteristic differences between 5- and 6-membered ring heterocyclic amines formed upon the reaction with fluorescein mono- and dialdehydes. 6-membered ring homocysteine-derived thiazinane-4-carboxylic acids were found to be more basic than 5-membered cysteine-derived thiazolidine-4-carboxylic acids. Fluorescence enhancement in response to homocysteine was thus attained by tuning pH and excitation wavelengths. Furthermore, the design and synthesis of a more sensitive fluorophore, fluorescein tri aldehyde has been accomplished based on the aforementioned findings to enable the detection of homocysteine at physiological levels. Calculations of Mulliken charges revealed that the formation of thiazinanes results in modulation of the electron density on the fluorophore leading to higher fluorescence.