<P> In 1951 Johannes Martin Bijvoet for the first time used in X-ray crystallography the effect of anomalous dispersion, which is now referred to as resonant scattering, to determine absolute configuration . The compound investigated was (+) - sodium rubidium tartrate and from its configuration (R, R) it was deduced that the original guess for (+) - glyceraldehyde was correct . </P> <P> The R / S system is an important nomenclature system for denoting enantiomers . This approach labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn--Ingold--Prelog priority rules (CIP), based on atomic number . If the center is oriented so that the lowest - priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left). </P> <P> (R) (S) is written in italics and parentheses . If there are multiple chiral carbons, e.g. (1R, 4S), a number specifies the location of the carbon preceding each configuration . </P> <P> The R / S system also has no fixed relation to the D / L system . For example, the side - chain one of serine contains a hydroxyl group, - OH . If a thiol group, - SH, were swapped in for it, the D / L labeling would, by its definition, not be affected by the substitution . But this substitution would invert the molecule's R / S labeling, because the CIP priority of CH OH is lower than that for CO H but the CIP priority of CH SH is higher than that for CO H. For this reason, the D / L system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for the commonly occurring structures of a given type of structure in higher organisms . In the D / L system, they are nearly all consistent--naturally occurring amino acids are all L, while naturally occurring carbohydrates are nearly all D. In the R / S system, they are mostly S, but there are some common exceptions . </P>

What is the correct absolute configuration assignment at each of the indicated stereocenters