What is "antiparallel"

Antiparallel \An`ti*par"al*lel\, a. Running in a contrary direction.
--Hammond.

a. 1 (context mathematics English) of vectors, parallel but of opposite direction 2 (context biochemistry English) describing the orientations of the two strands of DNA 3 (context biochemistry English) describing two beta strands comprising a protein's secondary structure that are aligned and hydrogen bonded but the vector describing the amino- and carboxy-terminal ends of each strand are oriented 180° relative to one another. n. (context geometry English) A line that forms equal angles with two other lines, but in opposite directions.

adj. (especially of vectors) parallel but oppositely directed; "antiparallel vectors"

The term antiparallel can refer to:

• Antiparallel (biochemistry), the orientation of adjacent molecules
• Antiparallel (mathematics), the placement of parallel lines in relation to an angle
• Antiparallel (electronics), the polarity of devices run in parallel
• Antiparallel (physics), vectors that lie along parallel lines but point in opposite directions
• Antiparallel (traffic), two parallel one-way streets on which traffic travels in opposing directions.

In geometry, anti- parallel lines can be defined with respect to either lines or angles.

In biochemistry, two biopolymers are antiparallel if they run parallel to each other but with opposite alignments. An example is the two complementary strands of a DNA double helix, which run in opposite directions alongside each other.

In electronics, two anti-parallel or inverse-parallel devices are connected in parallel but with their polarities reversed.

One example is the TRIAC, which is comparable to two thyristors connected back-to-back (in other words, reverse parallel), but on a single piece of silicon.

Two LEDs can be paired this way, so that each protects the other from reverse voltage. A series string of such pairs can be connected to AC or DC power, with an appropriate resistor. With AC, the LEDs in each pair take turns emitting light, on alternate half-cycles of supply power, greatly reducing the strobing effect to below the normal flicker fusion threshold of the human eye. Some two-color LEDs are constructed this way, with the two dies connected anti-parallel in one casing.