Direct Two-Dimensional Infrared Protein Spectroscopy of Beer Foam in H2O

Two-dimensional infrared (2D IR) spectroscopy is uniquely sensitive to protein secondary structure and dynamics, but the technique has historically been restricted to D2O solutions because the bending vibration of liquid H2O overlaps with the amide I band. The Hunt group has shown that 2D IR can be performed on protein solutions in H2O when the protein signal dominates over the residual water response, but these demonstrations have generally relied on purified or laboratory-prepared protein samples. Here we go one step further and acquire complete 2D IR spectra and related spectral dynamics directly on a natural, off-the-shelf, protein-rich aqueous sample, beer foam, with no isotopic exchange, purification, or chemical modification. Beer foam has a high surface-to-volume ratio and is naturally concentrating interfacial proteins. Using Grøn Tuborg as the reference system, we resolve α-helix, β-sheet, and β-sheet cross peaks, side-chain modes, and the amide I/II cross peak. Center line slope (CLS) analysis reveals an essentially static low-frequency β-sheet component coexisting with normally relaxing backbone modes, consistent with a solvent-shielded aggregate buried in the foam film. A survey of four additional beers (Guld Tuborg, Westmalle Dubbel, Westmalle Tripel, Guinness Draft) shows that this low-frequency β-sheet feature near 1610-1620 cm-1 recurs across very different beer styles. Foams of natural samples thus emerge as a route to 2D IR of protein films in H2O.