FIG. 4.

Experimental (black) and DFT (red) spectra for fully protonated, singly ¹³C-labeled Ac-FA5K. DFT lines are labeled according to the dominant contributor to the mode; “R” indicates a phenyl ring-breathing mode, “B” indicates a lysine –${\mathrm{NH}}_3^{+}$ bending mode, and 1-7 indicate amide I modes (labeled as in Figure 1).