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The human insulin (HI) protein was examined to elucidateits structureat the air–water interface. Optimal experimental conditionswere determined to prepare a homogeneous and stable human insulin(HI) Langmuir monolayer. HI insulin Langmuir monolayer can be usedto study interactions of HI with a membrane as Langmuir monolayersare used as an in vitro model of biological membranes. Surface pressureand surface potential-area isotherms were used to characterize theHI Langmuir monolayer. The compression–decompression cyclesand stability measurements showed a homogeneous and stable monolayerat the air–water interface. However, higher surface pressuresresulted in a higher decrease in area and less stability. In situUV–vis and fluorescence spectroscopy were used to verify thehomogeneity of the HI monolayer and to identify the chromophore residuesin the HI. Domain formation was examined through epifluorescence andBrewster angle microscopies. The conformation of HI was examined bycircular dichroism (CD) and Fourier transform infrared spectroscopy(FTIR) in the aqueous phase and at the air–water interfaceby infrared reflection absorption spectroscopy (IRRAS). HI was foundto exist as a monomer in 2-D. [ABSTRACT FROM AUTHOR]