We describe here a systematic study to determine the effect on secondary structure of d-amino acid substitutions in the nonpolar face of an amphipathic alpha-helical peptide. The helix-destabilizing ability of 19 d-amino acid residues in an amphipathic alpha-helical model peptide was evaluated by reversed-phase HPLC and CD spectroscopy. l-Amino acid and d-amino acid residues show a wide range of helix-destabilizing effects relative to Gly, as evidenced in melting temperatures (DeltaTm) ranging from -8.5 degrees C to 30.5 degrees C for the l-amino acids and -9.5 degrees C to 9.0 degrees C for the d-amino acids. Helix stereochemistry stability coefficients defined as the difference in Tm values for the l- and d-amino acid substitutions [(DeltaTm' = TmL and TmD)] ranging from 1 degrees C to 34.5 degrees C. HPLC retention times [DeltatR(XL-XD)] also had values ranging from -0.52 to 7.31 min at pH 7.0. The helix-destabilizing ability of a specific d-amino acid is highly dependent on its side-chain, with no clear relationship to the helical propensity of its corresponding l-enantiomers. In both CD and reversed-phase HPLC studies, d-amino acids with beta-branched side-chains destabilize alpha-helical structure to the greatest extent. A series of helix stability coefficients was subsequently determined, which should prove valuable both for protein structure-activity studies and de novo design of novel biologically active peptides.

Crustal low‐velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern‐central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200 km wide, over 10 km thick midcrustal LVZ is well resolved by shear velocity inversions using P‐to‐S receiver functions from more than 4600 earthquakes. It is generally overlain by a high‐velocity upper crust in the depth range of 8–15 km, especially in western‐central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.

Focal mechanisms of induced earthquakes reflect anthropogenic contributions to preexisting geological features and fault slippages. In this paper, we examine fault‐related (double‐couple (DC)) and possibly fluid‐related (non‐double‐couple (non‐DC)) mechanisms of induced earthquakes (M2–6) at regional scales. We systematically compare well‐resolved focal mechanisms of 33 events in the Western Canada Sedimentary Basin, among which 12 were induced by hydraulic fracturing and one by secondary recovery. Most of the seismicity is dominated by strike‐slip/thrust faulting regimes, whereas limited (but consistent) non‐DC components are obtained from injection‐induced seismicity in central Alberta. We interpret the persistent compensated‐linear‐vector‐dipole components (M2.1–3.8) as reflecting fracture growth and/or noncoplanar faults slippages during hydraulic‐fracturing stimulations. We further expand the moment tensor decomposition analysis to four representative classes of induced seismicity globally and find that the overall contribution of non‐DC components is comparable between induced and tectonic earthquakes.

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