The reverse-sequence peptides CysAlaAla and AlaAlaCys may attach to gold nanoparticles through the thiol group, and they differ primarily by whether the charged amino or the carboxylate group is proximal to the sulfur. Alanine residues in these peptides are not expected to interact significantly with the gold surface and serve to place a large separation between the amino and carboxylate groups. Solid-state NMR experiments and DFT calculations were performed to explore the structural differences between CysAlaAla on gold nanoparticles and AlaAlaCys on gold nanoparticles. It is found that the relative positions between the thiol, amino, and carboxylate groups strongly influences the structures of the peptide-gold systems. CysAlaAla orients parallel to the gold surface in a monolayer fashion, whereas AlaAlaCys forms an interdigitating bilayer-like structure that is oriented upright relative to the gold surface.

This article communicates our study to elucidate the molecular determinants of weak Mg2+ interaction with the ribonuclease H (RNH) domain of HIV-1 reverse transcriptase in solution. Since the interaction is weak (a ligand-dissociation constant > 1 mM), non-specific Mg2+ interaction with the protein or interaction of the protein with other solutes that are present in the buffer solution can confound the observed Mg2+-titration data. To investigate these indirect effects, we monitored changes in the chemical shifts of backbone amides of RNH by recording NMR 1H-15N single-quantum coherence (HSQC) spectra upon titration of Mg2+ into an RNH solution. We performed the titration in three different conditions: (1) in the absence of NaCl, (2) in the presence of 50 mM NaCl, and (3) at a constant 160 mM Cl− concentration. Careful analysis of these three sets of titration data, along with molecular dynamics simulation data of RNH with Na+ and Cl− ions, demonstrates two characteristic phenomena distinct from the specific Mg2+ interaction with the active site: (a) weak interaction of Mg2+, as a salt, with the substrate-handle region of the protein, (b) overall apparent lower Mg2+-affinity in the absence of NaCl compared to that in the presence of 50 mM NaCl. A possible explanation may be that the titrated MgCl2 is consumed as a salt and interacts with RNH in the absence of NaCl. In addition, our data suggest that Na+ increases the kinetic rate of the specific Mg2+ interaction at the active site of RNH. Taken together, our study provides biophysical insight into the mechanism of weak metal interaction on a protein.

The measurement of heteronuclear dipolar couplings by solid-state NMR is an important tool for determining structures in solids. 1 H-observe 1 HÀ 2 H REDOR and rotary resonance recoupling experiments with fast magicangle spinning are presented. These dipolar recoupling ex-periments show promise for measuring heteronuclear dipolar couplings with 1 H nuclei. Potential limitations of the experiments include short transverse relaxation times of the 1 H magnetization and BlochÀSiegert effects.