The adsorption and aggregation of beta amyloid (1-16) fragment at the air/ water interface was 
investigated by the combination of second harmonic generation (SHG) spectroscopy, Brewster 
angle microscopy (BAM), and molecular dynamics simulations (MD). The Gibbs free energy of 
surface adsorption was measured to be -10.3 kcal/mol for bulk pHs of 7.4 and 3, but no adsorption 
was observed for pH 10-11. The 1-16 fragment is believed not to be involved in fibril formation 
of the beta amyloid protein, but it exhibits interesting behavior at the air/water interface, 
as manifested in two time scales for the observed SHG response. The shorter time scale (minutes) 
reflects the surface adsorption, and the longer time scale (hours) reflects rearrangement and 
aggregation of the peptide at the air-water interface. Both of these processes are also evidenced 
by BAM measurements. MD simulations confirm the pH dependence of surface behavior of the beta 
amyloid, with largest surface affinity found at pH = 7. It also follows from the simulations 
that phenylalanine is the most surface exposed residue, followed by tyrosine and histidine in 
their neutral form.