Proteins directly control the nucleation and growth of biominerals, but the details of molecular recognition at the protein-biomineral interface remain poorly understood. The elucidation of recognition mechanisms at this interface may provide design principles for advanced materials development in medical and ceramic composites technologies. Here, we describe both the theory and practice of double-quantum solid-state NMR (ssNMR) structure-determination techniques, as they are used to determine the secondary structures of surface-adsorbed peptides and proteins. In particular, we have used ssNMR dipolar techniques to provide the first high-resolution structural and dynamic characterization of a hydrated biomineralization protein, salivary statherin, adsorbed to its biologically relevant hydroxyapatite (HAP) surface. Here, we also review NMR data on peptides designed to adsorb from aqueous solutions onto highly porous hydrophobic surfaces with specific helical secondary structures. The adsorption or covalent attachment of biological macromolecules onto polymer materials to improve their biocompatibility has been pursued using a variety of approaches, but key to understanding their efficacy is the verification of the structure and dynamics of the immobilized biomolecules using double-quantum ssNMR spectroscopy.
We present a two-dimensional NMR technique for the
measurement of dipolar couplings in polycrystalline
solids. This experiment is fully transverse and uses a windowless
dipolar recoupling pulse sequence (DRAWS,
described in Gregory, D. M.; et al. Chem. Phys.
Lett.
1995, 246, 654−663) to effect coherence
transfer.
Direct, internuclear coherence transfer produces negative
cross-peaks in the 2D spectrum. Cross-peak
development and experimental requirements for obtaining distances from
the two-dimensional solid-state
NMR spectra of two- and three-spin systems are discussed, and
demonstrations are shown for thymidine-2,4-13
C
2 and
l-alanine-13
C
3.
Internuclear distances are derived by comparison of experimental
cross-peak
buildup curves with numerical simulations. In the three-spin
system, indirect coherence-transfer mechanisms
prohibit the interpretation of buildup curves as due to isolated spin
pair interactions and limit the accuracy of
some distance measurements. This 2D technique can also be used for
spectral assignment, as demonstrated
by an application to
l-arginine·HCl-U-13
C,
N.
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