1. one singlet
2. L amino acid is used in living thing
all naturally occuring amino acids are L amino acid, just by convention, when we draw amino acid on fisher projection, we put amino group (NH2) on left side, so called L amino acid (L means simply left)
3. One more thing to remember is D sugar, all sugars digested by human is D sugar.
-the L means "levorotation" (left-rotating) - early last century, chemists discovered that solutions of natural amino acids rotate polarized light to the left (counter-clockwise). On the other hand, solutions of natural sugars rotated polarized light to the right ("dextrarotatory", or "right-rotating" in latin) - hence the D.
A handy thing to remember is that 19 of the 20 natural amino acids are (S) by the Cahn-Ingold-Prelog rules - the exception is cysteine, where the sulfur takes priority over the carbonyl (but is still an L-amino acid because it rotates light to the left).