signal transduction pathway Signal transduction
refers to the movement of signals from outside the cell
to the inside and is a mechanism connecting the stimu-
lus to a cellular response.
silica, silicon dioxide(SiO 2 ) A crystalline or amor-
phous material that occurs naturally in impure forms
such as quartz and sand.
silicate A mineral composed of silicon and oxygen.
Silicates comprise the most abundant mineral group in
the Earth’s crust.
silicones Organic polymers with a backbone of alter-
nating silicon and oxygen atoms.
silylene (1) Generic name for H 2 Si: and substitution
derivatives thereof, containing an electrically neutral
bivalent silicon atom with two nonbonding electrons.
(The definition is analogous to that given for CAR-
BENE.)
(2) The silanediyl group (H 2 Si), analogous to the
methylene group (H 2 C).
single covalent bond The sharing of an electron pair
by two atoms.
single-electron transfer mechanism(SET) A reac-
tion MECHANISMcharacterized by the transfer of a sin-
gle electron between the species occurring on the
REACTION COORDINATEof one of the elementary steps.
single-step reaction A reaction that proceeds
through a single TRANSITION STATE.
siroheme A HEME-like PROSTHETIC GROUPfound in a
class of ENZYMEs that catalyze the six-electron reduc-
tion of sulfite and nitrite to sulfide and ammonia,
respectively.
See alsoNITRITE REDUCTASE; SULFITE REDUCTASE.
site-directed mutagenesis SeeMUTAGENESIS.site-specific delivery An approach to target a DRUG
to a specific tissue, using PRODRUGs or antibody recog-
nition systems.Slater-type orbital An approximate atomic orbital
that attempts to allow for electron-electron repulsion
by scaling the nuclear charge for each orbital.smectic phase A phase type found in LIQUID
CRYSTALs, a phase of matter whose order is intermedi-
ate between a liquid and a crystal. The molecules are
typically rod-shaped. The nematic phase is character-
ized by the control of the orientational order and opti-
cal properties of the constituent molecules with applied
electric fields. Nematics are the most commonly used
phase in liquid crystal displays (LCDs). The smectic
phase, found at lower temperatures than the nematic,
forms well-defined layers that can slide over one
another like soap. Smectics are positionally ordered
along one direction and are characterized by a layered
structure. As many as 12 smectic phases have been
identified. Also used to describe clays in which water
can be present between the aluminosilicate layers and
which show a large change in volume upon wetting or
drying.smectic phase 247