Structural properties of gp41 fusion peptide at a model membrane interface.

We explore the structure and orientation of N-terminal (23 amino acids) of HIV gp41 envelop protein at the interface of a phospholipid monolayer. Using surface specific sum frequency generation, we probe the response of the Amide I vibrational states of the protein, and compare the experimental results to the modeled response of several secondary structures that have previously been reported in literature. To obtain the modeled response, we derive the line-shape expressions under cumulant expansion within the Brownian oscillator model, and express the intensities of the theoretical response according to the components of the Raman tensors and the infrared transition dipole moments of the relevant Amide I modes, under different orientation angles. This approach enables us to identify one plausible secondary structure among those considered, and allows us to determine the orientation angle, under which the protein inserts into the monolayer. We discuss the relevance of our findings toward understanding the functionality of this polypeptide at the membrane interface.