Human Cardiac Troponin I: A Langmuir Monolayer Study

Human cardiac troponin I (cTnI) is the preferred biomarker in the assessment of myocardial infarction. It is known to interact with troponin C and T to form a trimeric complex. Whereas small amounts are found in the cytoplasm, most of cTnI is in the form of a complex with actin located in myofilaments. To understand these interactions of cTnI better, we first investigated the surface chemistry of cTnI as a Langmuir monolayer spread at the air−water interface. We investigated the optimal conditions for obtaining a stable Langmuir monolayer in terms of changing the ionic strength of the subphase using different concentrations of potassium chloride. Monolayer stability was investigated by compressing the cTnI monolayer to a specific surface pressure and keeping the surface pressure constant while measuring the decrease in the molecular area as a function of time. Aggregation and/or domain formation was investigated by using compression−decompression cycles, in situ UV−vis spectroscopy, Brewster angle microscopy (BAM), and epifluorescence microscopy. To ensure that the secondary structure is maintained, we used infrared reflection−absorption spectroscopy (IRRAS) directly at the air−subphase interface. It was found that cTnI forms a very stable monolayer (after more that 5000 s) that does not aggregate at the air−subphase interface. The cTnI molecules maintain their secondary structure and, on the basis of the low reflectivity observed using BAM measurements and the low reflection−absorption intensities measured with IRRAS spectroscopy, lie flat on the subphase with the α-helices parallel to the air−subphase interface.