Carbon nitride films deposited by three different methods have been analyzed using in situ Auger electron spectroscopy and ex situ x-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry.The XPS data for all 27 samples indicate that these films have a similar composition consisting of two phases. One phase has a stoichiometry near C3N4 and is identified as a tetrahedral component.The other phase has a variable stoichiometry from C5N to C2N and is identified as predominantly an sp' bonded structure. For a film composition of [N]/ [C] ( 1, the tetrahedrally bonded component grows only moderately as the nitrogen content of the films is increased. PACS numbers: 68.55.Nq, 81.15. -z, 82.80.PvIn 1990, Liu and Cohen presented a pseudopotential study [1] of the structural and electronic properties of P-C&N4, a hypothetical compound, and of P-Si3N4, a compound with well-known properties. The good agreement between calculated and experimental data for the latter compound lends credibility to the findings of this study concerning the exciting properties of the unknown C-N compound. The calculated bulk modulus of P-C3N4 was found to be comparable to that of diamond. In addition, the velocity of sound in the C-N compound was predicted to be about 1.1 X 106 cm/s, suggesting a high thermal conductivity. It was suggested that P-C,N4 may be metastable because of its moderately large cohesive energy. Synthesis of P-C3N4 has recently been claimed [2] on the basis of electron diffraction data but the overall composition of the films in this, as in most other experiments [3 -6], is not stoichiometric: the ¹o-Cconcentration ratio attained in most cases is about [N]/[C] = 0.7.We propose that the reason for this discrepancy, which may raise questions about the results of Niu, Lu, and Lieber [2], is that for the deposition methods investigated, P-CsN4 forms only in very small crystallites that are embedded in amorphous sp2 bonded C"N, where y /x is typically between 0.2 and 0.5, depending on deposition conditions. Furthermore, we present x-ray photoelectron spectroscopic (XPS) data which shows that these two carbon nitride phases can be distinguished by their binding energies. Such a distinction provides researchers with a tool to assess quickly and effectively the quality of their films not simply on the basis of the overall nitrogen content, but rather based on the nitrogen and carbon that

Mitochondrial metabolic function is affected by the morphology and protein organization of the mitochondrial inner membrane. Cardiolipin (CL) is a unique tetra-acyl lipid that is involved in the maintenance of the highly curved shape of the mitochondrial inner membrane as well as spatial organization of the proteins necessary for respiration and oxidative phosphorylation. Cardiolipin has been suggested to self-organize into lipid domains due to its inverted conical molecular geometry, though the driving forces for this organization are not fully understood. In this work, we use coarse-grained molecular dynamics simulations to study the mechanical properties and lipid dynamics in heterogeneous bilayers both with and without CL, as a function of membrane curvature. We find that incorporation of CL increases bilayer deformability and that CL becomes highly enriched in regions of high negative curvature. We further show that another mitochondrial inverted conical lipid, phosphatidylethanolamine (PE), does not partition or increase the deformability of the membrane in a significant manner. Therefore, CL appears to possess some unique characteristics that cannot be inferred simply from molecular geometry considerations.

Mitochondrial dysfunction underlies many heritable diseases, acquired pathologies, and aging-related declines in health. Szeto–Schiller (SS) peptides comprise a class of amphipathic tetrapeptides that are efficacious toward a wide array of mitochondrial disorders and are believed to target mitochondrial membranes because they are enriched in the anionic phospholipid cardiolipin (CL). However, little is known regarding how SS peptides interact with or alter the physical properties of lipid bilayers. In this study, using biophysical and computational approaches, we have analyzed the interactions of the lead compound SS-31 (elamipretide) with model and mitochondrial membranes. Our results show that this polybasic peptide partitions into the membrane interfacial region with an affinity and a lipid binding density that are directly related to surface charge. We found that SS-31 binding does not destabilize lamellar bilayers even at the highest binding concentrations; however, it did cause saturable alterations in lipid packing. Most notably, SS-31 modulated the surface electrostatics of both model and mitochondrial membranes. We propose nonexclusive mechanisms by which the tuning of surface charge could underpin the mitoprotective properties of SS-31, including alteration of the distribution of ions and basic proteins at the interface, and/or modulation of bilayer physical properties. As a proof of concept, we show that SS-31 alters divalent cation (calcium) distribution within the interfacial region and reduces the energetic burden of calcium stress in mitochondria. The mechanistic details of SS-31 revealed in this study will help inform the development of future compound variants with enhanced efficacy and bioavailability.

Thin carbon–nitrogen films have been formed by direct impingement of 5–100 eV C+ and N+ or N+2 ions upon solid surfaces, as well as by 5–350 eV N+ bombardment of graphite surfaces. The influences of ion energy, N+/C+ arrival rate, and type of substrate have been studied. The films deposited in this manner are found to be essentially amorphous, with some graphitic regions on the scale of a few nm. Two distinct types of C–N bonding, one attributed to graphitelike local structure (C–N π bonds) and one attributed to C3N4-like local structure (C–N σ bonds), have been detected by x-ray photoelectron spectroscopy. Films deposited by dual-beam deposition and single-beam nitridation at 75 eV or less exhibit differences in the single-bonded structure. Total nitrogen concentrations of up to 47 at. % have been measured by Auger electron spectroscopy (AES) and Rutherford backscattering spectrometry. The C KVV Auger line shapes of the two phases have been determined by factor analysis. These line shapes are consistent with the expected band structures for the two phases. Film growth is consistent with a combined surface deposition/subplantation model, with high incident energies resulting preferentially in damage to the C3N4-like phase. A significant amount of disorder is present in all of the films, as indicated by AES line shapes and transmission electron microscopy analysis. Preferential sputtering of N is observed during AES depth profiling with a 1 keV Ar+ beam. Implications of this work for deposition of C–N films by energetic particle bombardment are discussed.

The mitochondrial lipid cardiolipin (CL) contributes to the spatial protein organization and morphological character of the inner mitochondrial membrane. Monolysocardiolipin (MLCL), an intermediate species in the CL remodeling pathway, is enriched in the multisystem disease Barth syndrome. Despite the medical relevance of MLCL, a detailed molecular description that elucidates the structural and dynamic differences between CL and MLCL has not been conducted. To this end, we performed comparative atomistic molecular dynamics studies on bilayers consisting of pure CL or MLCL to elucidate similarities and differences in their molecular and bulk bilayer properties. We describe differential headgroup dynamics and hydrogen bonding patterns between the CL variants and show an increased cohesiveness of MLCL's solvent interfacial region, which may have implications for protein interactions. Finally, using the coarse-grained Martini model, we show that substitution of MLCL for CL in bilayers mimicking mitochondrial composition induces drastic differences in bilayer mechanical properties and curvature-dependent partitioning behavior. Together, the results of this work reveal differences between CL and MLCL at the molecular and mesoscopic levels that may underpin the pathomechanisms of defects in cardiolipin remodeling.