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1997 (1)
1995 (1)
1Author    Amelia Thereza, Roberto Soares3, Ricardo Dias Lins3, RichardG. Longo3, Ricardo Arrattb, Ferreira3Requires cookie*
 Title    Plural Origins of Molecular Homochirality in Our Biota Part II. The Relative Stabilities of Homochiral and Mixed Oligoribotides and Peptides  
 Abstract    By computer simulations -molecular mechanics and molecular dynamics with the amber force field (Weiner et al., (1986), J. Comp. Chem. 7, 2 3 0 -2 5 2) -we have determined the stabilities of oligoribotide strands built with d -and L-riboses, and of peptide chains with d -and L-amino acid residues. In particular, complementary double-chains of oligoribotides were studied, since they are an important feature of the growing mechanism o f modern nucleic acids. Peptide chains on the other hand, grow without need of a template. We found that mixed oligoribotides are less stable than homochiral ones, and that this chiral effect is less noticeable in peptide chains. The results support the interpretation that L-riboses act as termi­ nators to the template-assisted growth of oligo-r-GD (enantiomeric cross-inhibition; Joyce et al., (1987), Proc. Natl. Acad. Sei. U S A 84, 4 398-4402). Based on this effect, a chemical pathway is proposed which could, under assumed prebiotic conditions, bypass the hindrance of homochiral growth. 
  Reference    Z. Naturforsch. 52c, 89 (1997); received July 10/September 4 1996 
  Published    1997 
  Keywords    Chirality, Peptides, Oligoribotides, Computer Simulations 
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 TEI-XML for    default:Reihe_C/52/ZNC-1997-52c-0089.pdf 
 Identifier    ZNC-1997-52c-0089 
 Volume    52 
2Author    Ricardo FerreiraRequires cookie*
 Title    Why 43 Codons?  
 Abstract    An attempt is made to answer the question posed by the title of this paper. First we show that in primitive self-replicating oligoribotide systems, selection depended from the very start on the existence of 4 kinds of ribotides, forming 2 complementary pairs. Further selection required that the condensation reactions involving the two last positions o f the 3'-end of a growing oligoribotide fragment and the first position of the 5 '-end of another fragment were catalyzed by randomly synthesized peptides. This established a codon -amino acid concen­ tration correlation and clinched triplet segments as the basis o f the translation process. Finally, physical arguments are given to show that the monochirality of the ribotides arose from stereochemical reasons, as firstly described by Wald, but that of the amino acids is the result of natural selection acting during the peptide-assisted stage o f oligoribotide growth. 
  Reference    Z. Naturforsch. 50c, 148 (1995); received August 8 1994 
  Published    1995 
  Keywords    G enetic Code, Codons, Oligoribotides, Amino Acids, Monochirality 
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 TEI-XML for    default:Reihe_C/50/ZNC-1995-50c-0148.pdf 
 Identifier    ZNC-1995-50c-0148 
 Volume    50