Use of IR absorption of the carboxyl group of amino acids and their metabolites to determine pKs, to study proteins, and to monitor enzymatic activity

The absorption spectra of 20 amino acids (Gly, Ala, Val, Leu, Ile, Ser, Thr, Asp, Asn, Glu, Gln, Lys, His, Arg, Phe, Trp, Cys, Met, Pro, and hexafluorovaline) and some of their metabolites (α-ketoglutarate, oxalacetate, pyruvate, succinate, citrate, and acetate) were determined in the infrared (IR) region from 1300 to 1700 cm−1 under conditions that are appropriate for biological studies (i.e., in phosphate-buffered D2O solution). The strongest transition in this region is , with an extinction coefficient ∼1 mM−1 cm−1, and an emphasis was made to demonstrate use of this transition for enzymatic assays and to study proteins. To these ends, these relevant features were demonstrated. The value for is a function of the residue pK: the higher the frequency, the lower the pK of the carboxylic acid. The high extinction of permits detection of carboxyl groups in parvalbumin, a protein that is rich in Asp and Glu. The IR profiles for the amino acids and their metabolite products are sufficiently characteristic so that IR can be used to monitor enzymatic reactions involving amino acids. We show that transaminase reactions, which interconvert amino and keto acids, can be monitored by IR. © 1997 John Wiley & Sons, Inc. Biospectroscopy 3: 457–467, 1997