Selective Chemical Deuteration of Aromatic Amino Acids: A Retrospective

Abstract

In 1970 when we began post-doctoral work in the laboratory of Professor Oleg Jardetzky, selective deuteration of proteins to limit the number of protons present in the system for subsequent analysis was a newly developed and effective technique for NMR exploration of protein structure (Crespi et al., 1968; Markley et al., 1968). This approach allowed more facile assignment of specific resonances and generated the potential to follow the spectroscopic behavior of protons for a specific amino acid sidechain over a broad range of conditions. The primary method for labeling at that time involved growth of microorganisms (generally bacteria or algae) in D2O, followed by isolation of the deuteratedamino acids from a cellular protein hydrolysate. The amino acids isolated were, therefore, completely deuterated. Selective deuteration of a target protein was achieved by growing the producing organism on a mixture of completely deuterated and selected protonated amino acids under conditions that minimized metabolic interconversion of the amino acids. In one-dimensional spectra, aromatic amino acid resonances occur well downfield of the aliphatic resonances, and this region can therefore be examined somewhat independently by utilizing a single protonated aromatic amino acid to simplify the spectrum of the protein. However, the multiple spectral lines generated by aromatic amino acids can be complex and overlapping, precluding unequivocal interpretation. To address this complication, chemical methods were developed to both completely and selectively deuterate side chains of the aromatic amino acids, thereby avoiding the costly necessity of growing large volumes of microorganisms in D2O and subsequent tedious isolation procedures. In addition, selective deuteration of the amino acids simplified the resonance patterns and thereby facilitated assignment and interpretation of spectra. The methods employed were based on exchange phenomena reported in the literature and generated large quantities of material for use in growth of microorganisms for subsequent isolation of selectively labeled protein (Matthews et al., 1977a). The target protein for incorporation of the selectively deuterated aromatic amino acids generated by these chemical methods was the lactose repressor protein from Escherichia coli, and greatly simplified spectra of this 150,000 D protein were produced by this approach.