Two-dimensional infrared correlation analysis (2D-IR) was applied to a set of surface pressure-dependent unpolarized IR spectra of a monolayer film of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine (DPPC) at the air/water (A/W) interface. The experimentally measured asynchronous 2D-IR spectra were compared with synthetic spectra calculated by using an 'overlapped peaks' model vs. a 'frequency shifting' model. The results presented here show that when the experimentally observed monolayer IR spectra are acquired as a function of surface pressure, one model cannot be used exclusively for spectral interpretation. In this study, the monolayer IR spectra were divided into a low-pressure region subset (< 11 mN/m) and a high-pressure region subset (> 11 mN/m). When the monolayer IR spectra acquired as a function of surface pressure are analyzed by 2D correlation methods, the results strongly support the following conclusions: (1) the low-pressure subset, which encompasses both the liquid expanded (LE) and the liquid expanded/liquid condensed (LE/LC) regions of the DPPC monolayer isotherm, is best modeled by two overlapped peaks correlated with ordered and disordered conformational states of the monolayer film; and (2) the high-pressure subset, which reflects solely the liquid condensed (LC) phase of the monolayer isotherm, is best modeled by a single peak, which undergoes a minor frequency shift, and which may be primarily correlated with gradual packing of the liquid condensed structure. This interpretation of the 2D-IR correlation spectra is in agreement with the interpretation of sub-bands seen in polarized monolayer IR spectra previously reported by our laboratory.

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