Enantiodifferent proton exchange in alanine and asparagine in the presence of H(2)(17)O.
Using Time Domain (1)H Nuclear Magnetic Resonance with H (2) (17) O (H (2) (17) O-TD-(1)HNMR), we found [H (2) (17) O]- and pH-controlled chiral differences in proton exchange properties in alanine (Ala) and asparagine (Asn). To minimize and equalize chemical impurities, Asn enantiomers were purified by crystallization from racemic solution. At <0.1 M H (2) (17) O, a shift in isoelectric pH (pI) occurred, approximately 1.14 kJ mol(-1) L: -D: -Asn DeltaDeltaG (o)' in the 5.91-6.42 pH range. One potential source for this asymmetry is the enantio-different magnetic moments (L: mu upward arrow not equal D: mu downward arrow) produced by neutral ring currents in the chiral center, leading to enantio-different nuclear spin organization and charge distribution in the amino group. At >or=pI, dissimilar interactions may occur in the hydration of the amino group with H (2) (17) O (NH(2)/H (2) (17) O not equal NH(2)/H (2) (16) O; NH(3) (+)/H (2) (17) O not equal NH(2)/H (2) (17) O; L: -*C-NH(2)/H (2) (17) O not equal D: -*C-NH(2)/H (2) (17) O). As L: mu upward arrow not equal D: mu downward arrow, the L: -*C-amino and the D: -*C-amino groups are diastereo spin-isomers. The nuclear spin of (17)O may be parallel or antiparallel with the ortho-(1)H(1)H pair; hence two ortho-H (2) (17) O molecules exist, also diastereo spin-isomers. As the pK of H (2) (17) O is different from H (2) (16) O, dissimilarities between L: -*C- and D: -*C-amino groups are converted into proton exchange differences. During H (2) (17) O-TD-(1)HNMR, the H (2) (17) O molecule is a "probe" of the state of the amino group. Regarding prebiotic evolution: prebiotic chirality may not require stochastic symmetry breaking or preexisting chiral conditions; chemical chiral effects due to L: mu upward arrow not equal D: mu downward arrow are small and need chiral amplification to generate an enantiomeric excess significant for prebiotic evolution; and prebiotic symmetry breaking was homochiral because the effect of L: mu upward arrow and D: mu downward arrow on the amino group should be similar in all alpha amino acids.
Cimpoiaşu
Vily Marius
VM
Department of Biochemistry, University of Craiova, Craiova, Romania.
Scorei
Romulus Ion
RI
Popa
Radu
R
D000409:Alanine / Q000737:chemistry*; D001216:Asparagine / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D010103:Oxygen Isotopes / Q000737:chemistry*; D013237:Stereoisomerism
D010103:Oxygen Isotopes; D001216:Asparagine; D017666:Deuterium Oxide; D000409:Alanine