Due to the inherent complexity of the natural biological environment, most studies on polypeptides, proteins and nucleic acids have so far been performed in vitro, away from physiologically relevant conditions. Nuclear magnetic resonance is an ideal technique to extend the in vitro analysis of simple model systems to the more complex biological context. This work shows how diffusion-based spectroscopic selection can be combined with isotopic labeling to tackle and optimize the NMR analysis of specific macromolecules in multicomponent mixtures. Typical media include cell-free systems containing overexpressed proteins, lysates and proteolytic mixtures. We present a few variants of diffusion-edited HSQC pulse sequences for the selective spectroscopic detection of protein and polypeptide resonances within complex mixtures containing undesired species of smaller molecular weight. Due to diffusion-based filtering, peak intensities of fast diffusing small molecules are attenuated more than peaks due to large molecules. The basic sequence, denoted as PFGSTE-HSQC, combines translational diffusion-ordering with two dimensional heteronuclear single quantum correlation spectroscopy. The GCSTE-HSQC and BPPSTE-HSQC sequences include bipolar gradients and are therefore suitable for both diffusion-based filtering and determination of diffusion coefficients of individual mixture components. Practical applications range from protein stability/folding investigations in physiologically relevant contexts to prescreening of tertiary fold and resonance assignments in structural genomics studies. A few applications of diffusion-edited HSQC to an E. coli cell lysate containing the 15N-labeled B domain of streptococcal protein G (GB1), and to a 15N-labeled N-acetylglycine/apomyoglobin mixture are presented. In addition, we provide specific guidelines for experimental setup and parameter optimization.
