Rh blood types A blood group that involves 45 dif-
ferent antigens on the surface of red cells that are con-
trolled by two closely linked genes on chromosome 1.
Can lead to serious medical complications between a
mother and her developing fetus if they are different (+
or –)—mother is Rh– and the father is Rh+.
See alsoKARLLANDSTEINER.
rhodopsin The red photosensitive pigment found in
the retina’s rod cells; contains the protein opsin linked
to retinene (or retinal), a light-absorbing chemical
derived from vitamin A and formed only in the dark.
Light striking the rhodopsin molecule forces it to
change shape and generate signals while the retinene
splits from the opsin, which is then reattached in the
dark, and completes the visual cycle.
rho-sigma (ρσ) equation SeeHAMMETT EQUATION;
RHO(ρ) VALUE; SIGMA(σ) CONSTANT; TAFT EQUATION.
rho (ρ) value A measure of the susceptibility to the
influence of SUBSTITUENT groups on the RATE CON-
STANTor equilibrium constant of a particular organic
reaction involving a family of related SUBSTRATEs.
Defined by Hammett for the effect of ring substituents
in meta- and para-positions of aromatic side-chain
reactions by the empirical “ρσ-equation” of the general
form
lg(kx/kH) = ρσXin which σx is a constant characteristic of the sub-
stituent X and of its position in the reactant molecule.
More generally (and not only for aromatic series),
ρ-values (modified with appropriate subscripts and
superscripts) are used to designate the susceptibility of
reaction series for families of various organic com-
pounds to any substituent effects, as given by the modi-
fied set of σ-constants in an empirical ρσ-correlation.
Reactions with a positive ρ-value are accelerated
(or the equilibrium constants of analogous equilibria
are increased) by substituents with positive σ-constants.
Since the sign of σ was defined so that substituents
with a positive σincrease the acidity of benzoic acid,
such substituents are generally described as attracting
electrons away from the aromatic ring. It follows that
reactions with a positive ρ-value are considered to
involve a TRANSITION STATE(or reaction product) so
that the difference in energy between this state and the
reactants is decreased by a reduction in electron density
at the reactive site of the substrate.
See also HAMMETT EQUATION; SIGMA (σ) CON-
STANT; STABILITY CONSTANT; TAFT EQUATION.ribonucleic acid(RNA) Linear polymer molecules
composed of a chain of ribose units linked between
positions 3 and 5 by phosphodiester groups. The bases
adenine or guanine (via atom N-9) or uracil or cytosine
(via atom N-1), respectively, are attached to ribose at
its atom C-1 by β-N-glycosidic bonds (seeNUCLEO-
TIDE). The three most important types of RNA in the
cell are: MESSENGER RNA (mRNA), transfer RNA
(tRNA), and RIBOSOMALRNA (rRNA).ribonucleotide reductases ENZYMES that catalyze
the reduction of ribonucleotide diphosphates or
triphosphates to the corresponding deoxyribonu-
cleotides by a RADICAL-dependent reaction. The
enzyme of animal, yeast, and AEROBICE. colicells con-
tains an oxo-bridged DINUCLEAR iron center and a
tyrosyl radical cation, and it uses thioredoxin, a thiol-
containing protein, as a reductant. At least three other
ribonucleotide reductases are known from bacteria,
containing, respectively, an IRON-SULFUR CLUSTERwith
a glycyl radical, adenosyl COBALAMIN, and a dinuclear
manganese CLUSTER.ribose The sugar component of RNA; a five-carbon
(pentose) aldose.ribosomal RNA(rRNA) The most common form
of RNA. When combined with certain proteins, it
forms ribosomes that are responsible for translation of
MESSENGERRNA (mRNA) into protein chains.ribosomes A subcellular unit composed of specific
RIBOSOMALRNA (rRNA) and proteins that are respon-
sible for the TRANSLATIONof MESSENGERRNA (mRNA)
into protein synthesis.238 Rh blood types