Fourier transform (FTIR) difference spectra of pepsin minus diazoacetylnorleucine methyl ester (DAN) or minus diazoacetyl-l-phenylalanine methyl ester (DAP) modified pepsin, respectively, demonstrated that Asp-215 is not deprotonated in pepsin. The FTIR difference spectrum of pepsin minus 1,2-epoxyparanitrophenoxypropane (EPNP) modified pepsin demonstrates that Asp-32 is present in pepsin as CO-2 anion. The position of the v(C O) vibration demonstrates that no (O ... H ... O)- hydrogen bond between Asp-215 and Asp-32 is formed. Furthermore, no H3O+ is present in the active center. Studies of the complex of pepsin with the inhibitor pepstatin prove that the inhibitor removes the water from the active site and Asp-32 becomes protonated. © 1994.
Iliadis, G., Zundel, G., & Brzezinski, B. (1994). Aspartic proteinases - Fourier transform IR studies of the aspartic carboxylic groups in the active site of pepsin. FEBS Letters, 352(3), 315–317. https://doi.org/10.1016/0014-5793(94)00979-1