title: Mirrors in the PDB: left-handed alpha-turns guide design with D-amino acids.
abstract: BACKGROUND
  Incorporating variable amino acid stereochemistry in molecular design has the potential to improve existing protein stability and create new topologies inaccessible to homochiral molecules. The Protein Data Bank has been a reliable, rich source of information on molecular interactions and their role in protein stability and structure. D-amino acids rarely occur naturally, making it difficult to infer general rules for how they would be tolerated in proteins through an analysis of existing protein structures. However, protein elements containing short left-handed turns and helices turn out to contain useful information. Molecular mechanisms used in proteins to stabilize left-handed elements by L-amino acids are structurally enantiomeric to potential synthetic strategies for stabilizing right-handed elements with D-amino acids.
  
  
  RESULTS
  Propensities for amino acids to occur in contiguous alpha(L) helices correlate with published thermodynamic scales for incorporation of D-amino acids into alpha(R) helices. Two backbone rules for terminating a left-handed helix are found: an alpha(R) conformation is disfavored at the amino terminus, and a beta(R) conformation is disfavored at the carboxy terminus. Helix capping sidechain-backbone interactions are found which are unique to alpha(L) helices including an elevated propensity for L-Asn, and L-Thr at the amino terminus and L-Gln, L-Thr and L-Ser at the carboxy terminus.
  
  
  CONCLUSIONS
  By examining left-handed alpha-turns containing L-amino acids, new interaction motifs for incorporating D-amino acids into right-handed alpha-helices are identified. These will provide a basis for de novo design of novel heterochiral protein folds.
authors.affiliation: Department of Biochemistry, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway, NJ 08854, USA. annavara@cabm.rutgers.edu
authors.forename: Srinivas
authors.initials: S
authors.lastname: Annavarapu
authors.forename: Vikas
authors.initials: V
authors.lastname: Nanda
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry*; D030562:Databases, Protein; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000596:Amino Acids; D011506:Proteins