title: Changes of secondary structure detected by laser Raman spectroscopy in model peptides of human alphaA-crystallin due to substitution of D-aspartyl residues for L-aspartyl residues.
abstract: OBJECTIVE
  We have found that two aspartyl (Asp-151 and Asp-58) residues of alphaA-crystallin are inverted and isomerized to the biologically uncommon D-beta-Asp residues during aging. In order to elucidate the correlation between the formation of the D-beta-Asp isomer and the environment surrounding the Asp in the protein, we performed a Raman spectroscopic study using two synthetic peptides: T6 peptide containing Asp-58, and T18 peptide, containing Asp-151, which correspond to the tryptic peptides of human alphaA-crystallin.
  
  
  METHODS
  Both T6 (Thr-Val-Leu-Asp(58)-Ser-Gly-Ile-Ser-Glu-Val-Arg) and T18 (Ile-Gln-Thr-Gly-Leu-Asp(151)-ala-thr-his-ala-Glu-Arg) peptides were synthesized with four optical isomers which have L-alpha-, D-alpha, L-beta and D-beta-aspartyl residues. These peptides were subjected to Raman measurement.
  
  
  RESULTS
  The Raman spectrum of the L-alpha-Asp T18 peptide measured as dry powder revealed that the secondary structure of this peptide is mainly anti-parallel beta-sheet. The main structure of the D-beta-Asp T18 peptide when in dry powder form was altered to an alpha-helix and/or random structure. The main structure of L-alpha-Asp T18 peptide when measured in aqueous solution also converted to an alpha-helix and/or random structure. The conversion of L-alpha-to D-beta-Asp in T6 peptides when in dry powder form revealed no alteration of secondary beta-sheeted structure.
  
  
  CONCLUSIONS
  Raman spectroscopy clearly revealed a large conformational change in the secondary structure of T18 peptide caused by substitution of normal L-alpha-Asp to biologically uncommon Asp-isomers. This result indicates that the inversion of an amino acid in a protein greatly affects the secondary structure of the protein.
authors.affiliation: Department of Educational Health, Faculty of Education, Ibaraki University, Mito, Ibaragi, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Mizuno
authors.forename: N
authors.initials: N
authors.lastname: Fujii
authors.forename: M
authors.initials: M
authors.lastname: Akaboshi
authors.forename: Y
authors.initials: Y
authors.lastname: Momose
authors.forename: T
authors.initials: T
authors.lastname: Shimo-oka
mesh_terms: D001224:Aspartic Acid / Q000032:analysis*; D003459:Crystallins / Q000737:chemistry*; D006801:Humans; D008956:Models, Chemical; D015394:Molecular Structure; D013059:Spectrum Analysis, Raman / Q000379:methods*
chemical_list: D003459:Crystallins; D001224:Aspartic Acid