The ninhydrin test, also called Kaiser test, is a qualitative colorimetric method developed by Kaiser et al. in 1970 to detect the presence of free primary amines on a solid-phase peptide synthesis (SPPS) resin. It enables real-time monitoring of amino acid coupling, evaluation of reaction completion and detection of coupling failures.
Chemical principle. Ninhydrin (2,2-dihydroxyindane-1,3-dione, CAS 485-47-2) reacts with primary amines in two steps: (1) condensation of two ninhydrin molecules with the free amine to form the Ruhemann complex (diketohydrindylidene diketohydrindamine, intense purple colour absorbing at 570 nm); (2) concomitant release of CO2, NH3 and an aldehyde. Secondary amines (Pro, Hyp) give a yellow/orange complex at 440 nm, visually distinguishable. The reaction requires heating at 100°C for 3-5 min.
Kaiser procedure. For routine SPPS testing: take 5-10 resin beads, wash 3× with DMF and 3× with EtOH/DCM, transfer to clean glass tube. Add a few drops of three prepared solutions: (A) 5% ninhydrin in EtOH, (B) 80% phenol in EtOH, (C) pyridine/KCN 0.01 M. Heat at 100°C for 3-5 min. Intense blue/purple result = free amines present = incomplete coupling, recycle. Light yellow or colourless result = blocked amines = complete coupling, proceed to Fmoc deprotection.
Limitations and alternatives. The Kaiser test does not detect secondary amines quantitatively (false negatives on Pro/Hyp), requires heating (risk of thermal degradation), and is semi-quantitative. Modern alternatives include: TNBS test (2,4,6-trinitrobenzene sulfonic acid, orange colour), chloranil test (p-chloranil, specific for secondary amines), NF31 test, Bromophenol blue test (spectro 600 nm), or online UV/Vis Fmoc deprotection monitoring (dibenzofulvene-piperidine absorption at 301 nm). The latter, quantitative and non-destructive, equips modern synthesisers (CEM Liberty Blue, Biotage Initiator+).
