Determination of the protonation state of titratable protein residues is of critical importance for the interpretation of active site chemistry, as well as for understanding the role of electrostatic interactions in protein folding and stability. However, protein titration studies are limited by the fact that, at extreme pH values, increasing fractions of unfolded or partially unfolded structures may be present. This problem is particularly acute for lysine residues which have high pK values. In the present study, we point out that the use of the 13C resonance of lysine C-5 as a reporter for titration of the epsilon-amino group is preferable to the use of C-6 due to the 5-fold greater titration shift, so that reasonable results can be obtained using a two parameter fit of data obtained over a more limited pH range. A new synthetic procedure for [5-13C]lysine is described, and the pK value for Lys72 in the lyase domain of DNA polymerase beta has been determined using the [5-13C]lysine-labeled enzyme. The results agree well with recent studies of the Pol lambda lyase domain, demonstrating that the pK value for this residue is not optimized for Schiff base chemistry (Gao et al., Biochemistry 2006, 45, 1785-1794). We also have re-evaluated data for the pK of Lys73 in the TEM-1 beta-lactamase.

Surprisingly stable anti‐periplanar conformers of CFTA amides form the basis of a new and very reliable method for determining the absolute configuration of chiral primary amines by 1H NMR spectroscopy (see picture). CFTA=α‐cyano‐α‐fluoro‐p‐tolylacetic acid.

13C labeled (>M + 4) metabolites of nitrogen and sulfur‐based chemical warfare agent metabolites were prepared from readily available and 13C labeled commercial starting materials. The new chemical routes are efficient in the number of chemical steps, can be scaled to afford gram quantities, and occur in good yields on the basis of the 13C label. These labeled compounds are useful as internal standards in mass spectrometry for monitoring chemical warfare agents and their metabolites. Published 2012. This article is a US Government work and is in the public domain in the USA.