Abstract: The present invention provides optionally substituted compounds of the formula I or salts thereof; wherein R1 is O or S when double bonded to the ring or is OH, SH, or a protected equivalent, when single bonded to the ring, R2 is hydrogen or more preferably an C1-C10 organic group attached by a carbon atom, X is H, O, OO, S or SS R3 is absent where X?H, is hydrogen or is a hydroxyl or thiol protecting group, R4 is a hetero- or preferably homo-cyclic aryl group, optionally substituted with a further group R5 and groups T1 are each, independently, absent, hydrogen or an S—R6 group, where any/each R6 is independently an organic group of molecular weight up to around 500 amu. The invention further provides a method for the synthesis of such compounds and a method of treatment comprising administering such compounds to a mammalian subject.

Abstract: An immunoassay, e.g. ELISA, method and kit for determining (preferably quantitatively) an analyte adsorbed at a surface or present in a liquid sample, comprising binding the analyte to a solid phase, attaching a marker to the analyte, and detecting marker attached to the solid-phase. The invention proposes to use a combination of marker and detection (e.g. an enzyme-substrate combination) which is capable of producing a precipitate on a solid phase which carries the marker and to detect the binding of analyte to the solid phase by in-situ determining the change in surface mass of the solid phase due to the formation of the precipitate. Ellipsometry is an example of a technique suitable for determining the change of surface mass of the solid phase, which could be made of a silicon- or chromium-sputtered glass slide The invention shortens the assay time and/or improves the assay sensitivity, and allows to measure extremely low surface concentrations of analytes of interest.