We have studied surface and spectroscopic properties of Photosystem II core complex (PS LI CC) for the first time in monolayers at the nitrogen/water interface. A new instrument was thus specially built to perform absorption and fluorescence spectroscopic measurements directly at the nitrogen/water interface. The effect of initial surface density, incubation time, and compression speed have been studied. When PS II CC was spread at an initial surface pressure of 5.7 mN/m and immediately compressed at a speed of 40 nm(2)/moleeule.min, it retained its native spectroscopic characteristics. Even though a slower speed of compression (10 nm(2)/molecule.min) produced more homogeneous films, the absorption maxima suffered a blue shift, indicating denaturation of PS II CC. Compression at a speed of 80 nm(2)/molecule.min produced aggregates of intact PS II CC as indicated by Delta V-A isotherms, absorption spectra, and fluorescence micrographs. We also conclude that spreading of PS II CC at an initial surface pressure of 0.6 mN/m followed by a 30 min incubation time is inadequate to maintain PS II CC surface and spectral properties. Indeed, pi-A and Delta V-A isotherms measured in that condition showed transitions which suggested that PS II CC underwent physical changes during compression. Moreover, absorption and fluorescence maxima were blue shifted, indicating that PS II CC is denatured under that condition.