Richard J. Payne

Glycine betaine and glycine betaine analogues in common foods

Abstract In this study we have surveyed the betaine content of a wide range of foods commonly found in the western diet. Glycine betaine, proline betaine (stachydrine), trigonelline and dimethylsulfoniopropionate (DMSP) were the only betaines to be found at ⩾150 μg/g. Glycine betaine was primarily found in shellfish, flour, and some vegetables, such as beetroot, spinach and silverbeet. Proline betaine was found in citrus fruit and alfalfa sprouts, while trigonelline was found in coffee, chick peas, lentils and rolled oats. Significant DMSP was only found in some shellfish. Different sources of individual foods showed variation in betaine content, and the way in which individual foods were cooked affected betaine content, with boiling causing the highest loss of betaine.

Solid‐Phase Synthesis of Peptide and Glycopeptide Thioesters through Side‐Chain‐Anchoring Strategies

An efficient new strategy for the synthesis of peptide and glycopeptide thioesters is described. The method relies on the side-chain immobilization of a variety of Fmoc-amino acids, protected at their C-termini, on solid supports. Once anchored, peptides were constructed using solid-phase peptide synthesis according to the Fmoc protocol. After unmasking the C-terminal carboxylate, either thiols or amino acid thioesters were coupled to afford, after cleavage, peptide and glycopeptide thioesters in high yields. Using this method a significant proportion of the proteinogenic amino acids could be incorporated as C-terminal amino acid residues, therefore providing access to a large number of potential targets that can serve as acyl donors in subsequent ligation reactions. The utility of this methodology was exemplified in the synthesis of a 28 amino acid glycopeptide thioester, which was further elaborated to an N-terminal fragment of the glycoprotein erythropoietin (EPO) by native chemical ligation.

Recent extensions to native chemical ligation for the chemical synthesis of peptides and proteins.

Native chemical ligation continues to play a pivotal role in the synthesis of increasingly complex peptide and protein targets twenty years after its initial report. This opinion article will highlight a number of recent, powerful extensions of the technology that have expanded the scope of the reaction, accelerated ligation rates, enabled chemoselective post-ligation modifications, and streamlined the ligation of multiple peptide fragments. These advances have facilitated the synthesis of a number of impressive protein targets to date and hold great promise for the continued application of native chemical ligation for the detailed study of protein structure and function.

Total Synthesis of Homogeneous Antifreeze Glycopeptides and Glycoproteins

Antifreeze glycoproteins (AFGPs) are a class of natural products found in deep sea teleost fish in Arctic and Antarctic waters. The physiological role of these biomolecules is to protect against cryoinjury in environments with subzero temperatures by preventing the growth of ice crystals in vivo. Structurally, AFGPs are polymeric, mucin-type glycoproteins that consist of a single glycotripeptide repeat (Ala-Thr-Ala/Pro) in which each secondary hydroxy group on threonine is linked to the disaccharide b-d-galactosyl-(1!3)a-N-acetyl-d-galactosamine (Scheme 1). AFGPs range in molecular weight from approximately 2.6 kDa (4 repeat units) to 33.7 kDa (50 repeat units).

Peptide Ligation–Desulfurization Chemistry at Arginine

The utility of a new β-thiol arginine building block in ligation-desulfurization chemistry has been demonstrated through reactions and kinetic studies with a range of peptide thioesters. Application of the method is highlighted by a one-pot, kinetically controlled, rapid ligation to generate a 7 kDa MUC1 glycopeptide.

Trifluoroethanethiol: An Additive for Efficient One-Pot Peptide Ligation−Desulfurization Chemistry

Native chemical ligation followed by desulfurization is a powerful strategy for the assembly of proteins. Here we describe the development of a high-yielding, one-pot ligation-desulfurization protocol that uses trifluoroethanethiol (TFET) as a novel thiol additive. The synthetic utility of this TFET-enabled methodology is demonstrated by the efficient multi-step one-pot syntheses of two tick-derived proteins, chimadanin and madanin-1, without the need for any intermediary purification.

Photo-tautomerization of Acetaldehyde to Vinyl Alcohol: A Potential Route to Tropospheric Acids

Enols in the Atmosphere? Keto/enol tautomerization (HC−C=O→C=C−OH) plays a central role in the chemistry of carbonyl compounds in a solution in which solvent and catalytic acids or bases can facilitate the proton transfer from C to O and back again. In contrast, analyses of atmospheric chemistry tend to exclude enol structure, on the assumption that tautomerization does not proceed regularly in gas phase. Andrews et al. (p. 1203, published online 16 August) used isotopic labeling to probe the photoisomerization pathway of gaseous acetaldehyde in the lab and discovered evidence for an enol. Subsequent modeling indicates that photogenerated enols could build up sufficiently in the troposphere to account for previously puzzling observations of organic acids in the atmosphere. Enol tautomers may play a bigger role in atmospheric chemistry than previously suspected. Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers). The model quantitatively reproduces the experiments and shows unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the crucial first step. When collisions at atmospheric pressure are included, the model quantitatively reproduces previously reported quantum yields for photodissociation at all pressures and wavelengths. The model also predicts that 21 ± 4% of the initially excited acetaldehyde forms stable vinyl alcohol, a known precursor to organic acid formation, which may help to account for the production of organic acids in the troposphere.

Polysialylation controls dendritic cell trafficking by regulating chemokine recognition.

A chemokines sugary release As immune cells survey the body for pathogens, they circulate through the blood and migrate through the lymphatic system. The latter route allows for tissues and lymph nodes—the central hubs of the immune system—to communicate. Kiermaier et al. reveal the importance of the monosaccharide sialic acid in keeping immune cells in motion. Multiple sialic acids decorate the surface CCR7 on immune cells. CCR7 recognizes proteins called chemokines, which direct where cells move in the body. Sialic acids on CCR7 release one such chemokine present on lymph node endothelial cells from an inhibited state, allowing immune cells to enter lymph nodes. Science, this issue p. 186 For dendritic cells to find their way to lymph nodes, the chemokine receptor CCR7 needs to have polysialic acid on it. The addition of polysialic acid to N- and/or O-linked glycans, referred to as polysialylation, is a rare posttranslational modification that is mainly known to control the developmental plasticity of the nervous system. Here we show that CCR7, the central chemokine receptor controlling immune cell trafficking to secondary lymphatic organs, carries polysialic acid. This modification is essential for the recognition of the CCR7 ligand CCL21. As a consequence, dendritic cell trafficking is abrogated in polysialyltransferase-deficient mice, manifesting as disturbed lymph node homeostasis and unresponsiveness to inflammatory stimuli. Structure-function analysis of chemokine-receptor interactions reveals that CCL21 adopts an autoinhibited conformation, which is released upon interaction with polysialic acid. Thus, we describe a glycosylation-mediated immune cell trafficking disorder and its mechanistic basis.

One-Pot Peptide Ligation–Desulfurization at Glutamate

An efficient methodology for ligation at glutamate (Glu) is described. A γ-thiol-Glu building block was accessed in only three steps from protected glutamic acid and could be incorporated at the N-terminus of peptides. The application of these peptides in one-pot ligation-desulfurization chemistry is demonstrated with a range of peptide thioesters, and the utility of this methodology is highlighted through the synthesis of the osteoporosis peptide drug teriparatide (Forteo).

Rapid additive-free selenocystine-selenoester peptide ligation.

We describe an unprecedented reaction between peptide selenoesters and peptide dimers bearing N-terminal selenocystine that proceeds in aqueous buffer to afford native amide bonds without the use of additives. The selenocystine-selenoester ligations are complete in minutes, even at sterically hindered junctions, and can be used in concert with one-pot deselenization chemistry. Various pathways for the transformation are proposed and probed through a combination of experimental and computational studies. Our new reaction manifold is also showcased in the total synthesis of two proteins.