Application of 2D IR correlation analysis to phase transitions in Langmuirmonolayer films

The surface pressure-dependent conformational state of a monolayer film of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water (A/W) i nterface was studied using infrared external-reflection spectroscopy and tw o-dimensional infrared (2D IR) correlation analysis. When the IR spectra of the DPPC monolayer was collected using polarized IR radiation, a band spli tting was observed in both the antisymmetric (nu(a)) and symmetric (nu(s)) methylene CH2 stretching modes that was not observed with unpolarized radia tion. This band splitting was interpreted as being due to the presence of c o-existing ordered and disordered conformational states, however, definitiv e identification of conformational sub-bands is problematic due to the low signal-to-noise inherent in the polarized IR spectra. To further investigat e the spectral changes observed in the C-H region, 2D IR correlation analys is was applied to a set of pressure-dependent unpolarized IR spectra of the DPPC monolayer. When these unpolarized spectra were analyzed using 2D IR m ethods, the 2D asynchronous correlation spectrum of the DPPC monolayer clea rly showed that cross peaks attributable to the nu(a) and nu(s) CH2 bands b oth split into two components, in agreement with the polarized IR monolayer spectra. Since band splitting in 2D IR spectra may be due to several cause s, computer simulations were undertaken to help elucidate the exact cause o f the observed splitting in the DPPC 2D asynchronous spectrum. Synthetic mo nolayer IR spectra were calculated for two limiting cases. The first was a 'frequency shifting' model in which a single band underwent a simple freque ncy shift. The second limiting case was an 'overlapped peaks' model in whic h an overall vibrational band was calculated as the sum of two individual s ub-bands whose frequencies remained constant, but whose relative intensitie s changed through the simulated monolayer transition. The results of the co mputer simulations indicated that a simple frequency shift could be disting uished in the 2D asynchronous spectrum by the presence of a quartet of cros s peaks, two with positive correlation intensities, and two with negative. In addition, a curved elongation of these cross peaks along the diagonal wa s associated with this frequency shift. In contrast, the 2D asynchronous sp ectrum for two overlapped peaks resulted in a correlation intensity cross p eak doublet, one positive and one negative with no elongation along the dia gonal. The experimentally measured 2D IR asynchronous correlation spectrum for the DPPC monolayer closely resembled the computer-simulated spectra for the 'overlapped peaks' model. Therefore, the origin of the band splitting in the nu(a) and nu(s) CH2 bands in the 2D asynchronous spectrum is due to overlapping sub-bands that represent the ordered and disordered conformatio nal states of the monolayer. Furthermore, these results also support the in terpretation that the sub-bands observed in the polarized monolayer IR spec tra are correlated with ordered and disordered monolayer states. (C) 2000 E lsevier Science B.V. All rights reserved.