Polymeric peptide pigments with sequence-encoded properties

Melanins are a family of heterogeneous polymeric pigments that provide ultraviolet (UV) light protection, structural support, coloration, and free radical scavenging. Formed by oxidative oligomerization of catecholic small molecules, the physical properties of melanins are influenced by covalent and noncovalent disorder. We report the use of tyrosine-containing tripeptides as tunable precursors for polymeric pigments. In these structures, phenols are presented in a (supra-)molecular context dictated by the positions of the amino acids in the peptide sequence. Oxidative polymerization can be tuned in a sequence-dependent manner, resulting in peptide sequence–encoded properties such as UV absorbance, morphology, coloration, and electrochemical properties over a considerable range. Short peptides have low barriers to application and can be easily scaled, suggesting near-term applications in cosmetics and biomedicine

Interactions of Intact Unfractionated Heparin with Its Client Proteins Can Be Probed Directly Using Native Electrospray Ionization Mass Spectrometry

Heparin and related members of the glycosaminoglycan (GAG) family are highly polyanionic linear saccharides that play important roles in a variety of physiological processes ranging from blood coagulation to embryo- and oncogenesis, tissue regeneration, and immune response regulation. These diverse functions are executed via a variety of mechanisms, including protein sequestration, activation, and facilitation of their interactions with cell-surface receptors, but deciphering the specific molecular mechanisms is frequently impossible due to the extremely high degree of GAG heterogeneity. As a result, the vast majority of studies of heparin (or related GAGs) interactions with its client proteins use synthetically produced heparin mimetics with defined structure or short heparin fragments. In this work we use native electrospray ionization mass spectrometry (ESI MS) in combination with limited charge reduction in the gas phase to obtain meaningful information on noncovalent complexes formed by intact unfractionated heparin and antithrombin-III, interaction which is central to preventing blood clotting. Complexes of different stoichiometries are observed ranging from 1:1 to 1:3 (heparin/protein ratio). In addition to binding stoichiometry, the measurements allow the range of heparin chain lengths to be obtained for each complex and the contribution of each complex to the total ionic signal to be calculated. Incorporation of ion mobility measurements in the experimental workflow allows the total analysis time to be shortened very significantly and the charge state assignment for the charge-reduced species to be verified. The possibility to study interactions of intact unfractionated heparin with a client protein carried out directly by native ESI MS without the need to use relatively homogeneous surrogates demonstrated in this work opens up a host of new exciting opportunities and goes a long way toward ameliorating the persistent but outdated view of the intractability of such interactions

A New Approach to Measuring Protein Backbone Protection with High Spatial Resolution Using H/D Exchange and Electron Capture Dissociation

Inadequate spatial resolution remains one of the most serious limitations of hydrogen/deuterium exchange-mass spectrometry (HDX-MS), especially when applied to larger proteins (over 30 kDa). Supplementing proteolytic fragmentation of the protein in solution with ion dissociation in the gas phase has been used successfully by several groups to obtain near-residue level resolution. However, the restrictions imposed by the LC–MS/MS mode of operation on the data acquisition time frame makes it difficult in many cases to obtain a signal-to-noise ratio adequate for reliable assignment of the backbone amide protection levels at individual residues. This restriction is lifted in the present work by eliminating the LC separation step from the workflow and taking advantage of the high resolving power and dynamic range of a Fourier transform ion cyclotron resonance-mass spectrometer (FTICR-MS). A residue-level resolution is demonstrated for a peptic fragment of a 37 kDa recombinant protein (N-lobe of human serum transferrin), using electron-capture dissociation as an ion fragmentation tool. The absence of hydrogen scrambling in the gas phase prior to ion dissociation is verified using redundant HDX-MS data generated by FTICR-MS. The backbone protection pattern generated by direct HDX-MS/MS is in excellent agreement with the known crystal structure of the protein but also provides information on conformational dynamics, which is not available from the static X-ray structure