The power of combining sum frequency generation (SFG) vibrational spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy to quantify, with site specificity and atomic resolution, the orientation and dynamics of side chains in synthetic model peptides adsorbed onto polystyrene (PS) surfaces is demonstrated in this study. Although isotopic labeling has long been used in ssNMR studies to site-specifically probe the structure and dynamics of biomolecules, the potential of SFG to probe side chain orientation in isotopically labeled surface-adsorbed peptides and proteins remains largely unexplored. The 14 amino acid leucine-lysine peptide studied in this work is known to form an α-helical secondary structure at liquid-solid interfaces. Selective, individual deuteration of the isopropyl group in each leucine residue was used to probe the orientation and dynamics of each individual leucine side chain of LKα14 adsorbed onto PS. The selective isotopic labeling methods allowed SFG analysis to determine the orientations of individual side chains in adsorbed peptides. Side chain dynamics were obtained by fitting the deuterium ssNMR line shape to specific motional models. Through the combined use of SFG and ssNMR, the dynamic trends observed for individual side chains by ssNMR have been correlated with side chain orientation relative to the PS surface as determined by SFG. This combination provides a more complete and quantitative picture of the structure, orientation, and dynamics of these surface-adsorbed peptides than could be obtained if either technique were used separately.protein | surface | isotope labels | solid-liquid interface

The structure and orientation of amphiphilic α-helix and β-strand model peptide films on self-assembled monolayers (SAMs) have been studied with sum frequency generation (SFG) vibrational spectroscopy and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The α-helix peptide is a 14-mer and the β-strand is a 15-mer of hydrophilic lysine and hydrophobic leucine residues with hydrophobic periodicities of 3.5 and 2, respectively. These periodicities result the leucine side chains located on one side of the peptides and the lysine side chains on the other side. The SAMs were prepared from assembly of either carboxylic acid or methyl terminated alkyl thiols onto gold surfaces. For SFG studies the deuterated analog of the methyl SAM was used. SFG vibrational spectra in the C-H region of air dried peptides films on both SAMs exhibit strong peaks near 2965 cm−1, 2940 cm−1 and 2875 cm−1 related to ordered leucine side chains. The orientation of the leucine side chains was determined from the phase of these features relative to the non-resonant gold background. The relative phase for both the α-helix and β-strand peptides showed that the leucine side chains were oriented away from the carboxylic acid SAM surface and oriented towards the methyl SAM surface. Amide I peaks observed near 1656 cm−1 for the α-helix peptide confirm that the secondary structure is preserved on both SAMs. A strong linear dichroism related to the amide π* orbital at 400.8 eV was observed in the nitrogen K-edge NEXAFS spectra for the adsorbed β-strand peptides, suggesting that the peptide backbones are oriented parallel to the SAM surface with the side-chains pointing towards or away from the interface. For the α-helix the dichroism of the amide π* is significantly weaker, probably due to the broad distribution of amide bond orientations in the α-helix secondary structure.

Specialized bacteria trigger ice formation by controlling the molecular structure and energy transfer in interfacial water.

The thylakoid membrane of chloroplasts and cyanobacteria is a unique internal membrane system harbouring the complexes of the photosynthetic electron transfer chain. Despite their apparent importance, little is known about the biogenesis and maintenance of thylakoid membranes. Although membrane fusion events are essential for the formation of thylakoid membranes, proteins involved in membrane fusion have yet to be identified in photosynthetic cells or organelles. Here we show that IM30, a conserved chloroplast and cyanobacterial protein of approximately 30 kDa binds as an oligomeric ring in a well-defined geometry specifically to membranes containing anionic lipids. Triggered by Mg 2 þ , membrane binding causes destabilization and eventually results in membrane fusion. We propose that IM30 establishes contacts between internal membrane sites and promotes fusion to enable regulated exchange of proteins and/or lipids in cyanobacteria and chloroplasts.