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ABSTRACT:Paramagnetism-based constraints for the solution structure determination of metal-containing proteins are introduced and analyzed theoretically to emphasize the amount of information that they contain. They are the relaxation rates, contact shifts, pseudocontact shifts, self-orientation residual dipolar couplings and, cross correlations between Curie relaxation and dipolar relaxation. Computational strategies are discussed and examples are provided to show the effective impact of such constraints in real life.

The conformational space sampled by the two-domain protein calmodulin has been explored by an approach based on four sets of NMR observables obtained on Tb 3؉ -and Tm 3؉ -substituted proteins. The observables are the pseudocontact shifts and residual dipolar couplings of the C-terminal domain when lanthanide substitution is at the N-terminal domain. Each set of observables provides independent information on the conformations experienced by the molecule. It is found that not all sterically allowed conformations are equally populated. Taking the N-terminal domain as the reference, the C-terminal domain preferentially resides in a region of space inscribed in a wide elliptical cone. The axis of the cone is tilted by Ϸ30°with respect to the direction of the N-terminal part of the interdomain helix, which is known to have a flexible central part in solution. The C-terminal domain also undergoes rotation about the axis defined by the C-terminal part of the interdomain helix. Neither the extended helix conformation initially observed in the solid state for free calcium calmodulin nor the closed conformation(s) adopted by calcium calmodulin either alone or in its adduct(s) with target peptide(s) is among the most preferred ones. These findings are unique, both in terms of structural information obtained on a biomolecule that samples multiple conformations and in terms of the approach developed to achieve the results. The same approach is in principle applicable to other multidomain proteins, as well as to multiple interaction modes between two macromolecular partners. C almodulin (CaM) is a paradigm case in structural biology. The following brief survey of the history of the structural and dynamic studies on this protein serves the double purpose of putting the present findings in proper perspective and of acknowledging those pieces of previous information that were essential to allow the present approach to be developed and to yield novel structural information.CaMs are two-domain proteins belonging to the large family of EF-hand proteins (1-3). They contain Ϸ150 amino acid residues, organized into two domains of Ϸ70 aa each and connected by a short linker. Each domain is made up of two special helix-loop-helix motifs (EF-hand motifs) that can bind a calcium ion in the loop. The two loops are held close to one another by two short antiparallel ␤-strands forming a threehydrogen bond stretch of ␤-sheet. The function of CaM in cell cytoplasm is that of responding to sudden rises of calcium concentration by binding up to four calcium ions in the four EF-hand loops, by changing conformation because of metal binding, and by thus becoming able to recognize, bind to, and activate, a number of proteins and enzymes (1,(4)(5)(6)(7)(8). Early x-ray data (9) showed the four-calcium (Ca 2 ) N (Ca 2 ) C CaM form (subscripts N and C refer to the calcium atoms bound by the N-and C-terminal domains, respectively) to have a dumbbell shape, with helix 4, the last helix of the N-terminal domain, and helix 5, the first helix of the C-termi...

This article deals with the solution structure determination of paramagnetic metalloproteins by NMR spectroscopy. These proteins were believed not to be suitable for NMR investigations for structure determination until a decade ago, but eventually novel experiments and software protocols were developed, with the aim of making the approach suitable for the goal and as user-friendly and safe as possible. In the article, we also give hints for the optimization of experiments with respect to each particular metal ion, with the aim of also providing a handy tool for nonspecialists. Finally, a section is dedicated to the significant progress made on 13C direct detection, which reduces the negative effects of paramagnetism and may constitute a new chapter in the whole field of NMR spectroscopy.

Relatively large proteins in solution, spun in NMR rotors for solid samples at typical ultracentrifugation speeds, sediment at the rotor wall. The sedimented proteins provide high-quality solid-state-like NMR spectra suitable for structural investigation. The proteins fully revert to the native solution state when spinning is stopped, allowing one to study them in both conditions. Transiently sedimented proteins can be considered a novel phase as far as NMR is concerned. NMR of transiently sedimented molecules under fast magic angle spinning has the advantage of overcoming protein size limitations of solution NMR without the need of sample crystallization/precipitation required by solid-state NMR.ferritin | magic angle spinning NMR | sedimentation | gravity | high molecular weight "W e built a new ultracentrifuge which permitted the study of solutions in fields of force of up to about 100,000 times gravity…it is possible to cover the entire sector of the colloids down to the smallest particle sizes, and even…reach…the substances of high molecular weight, such as haemoglobin, protein, starch, etc.

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The concept of maximum occurrence (MO), i.e., the maximum percent of time that flexible proteins can spend in any given conformation, is introduced, and a rigorous method is developed to extensively sample the conformational space and to construct MO maps from experimental data. The method is tested in a case study, the flexible two-domain protein calmodulin (CaM), using SAXS and NMR data (i.e., pseudocontact shifts and self-orientation residual dipolar couplings arising from the presence of paramagnetic lanthanide ions), revealing that the "closed" and "fully extended" conformations trapped in the crystalline forms of CaM have MOs of only 5 and 15%, respectively. Compact conformations in general have small MOs, whereas some extended conformations have MO as high as 35%, strongly suggesting these conformations to be most abundant in solution. The method is universally applicable as it requires only standard SAXS data and specific NMR data on lanthanide derivatives of the protein (using native metal sites or lanthanide tagging). The computer program is publicly available using the grid computing infrastructure through the authors' Web portal.

In the Spring of 2013, NMR spectroscopists convened at the Weizmann Institute in Israel to brainstorm on approaches to improve the sensitivity of NMR experiments, particularly when applied in biomolecular settings. This multi-author interdisciplinary Review presents a state-of-the-art description of the primary approaches that were considered. Topics discussed included the future of ultrahigh-field NMR systems, emerging NMR detection technologies, new approaches to nuclear hyperpolarization, and progress in sample preparation. All of these are orthogonal efforts, whose gains could multiply and thereby enhance the sensitivity of solid- and liquid-state experiments. While substantial advances have been made in all these areas, numerous challenges remain in the quest of endowing NMR spectroscopy with the sensitivity that has characterized forms of spectroscopies based on electrical or optical measurements. These challenges, and the ways by which scientists and engineers are striving to solve them, are also addressed.