Biochemistry

A central α-carbon bonded to four groups: an amino group (-NH3+), a carboxyl group (-COO-), a hydrogen atom, and a variable R-group (side chain).

L-configuration (S in the CIP/R-S system, except L-cysteine, which is R because sulfur raises the side chain's priority).

At pH ~7 the carboxyl group is deprotonated (-COO-) and the amino group is protonated (-NH3+), so the molecule carries both a negative and a positive charge but is net neutral.

Glycine, alanine, valine, leucine, isoleucine, methionine, and proline (proline's side chain forms a ring with the backbone N).

Phenylalanine, tyrosine, and tryptophan. (Histidine has an aromatic imidazole ring but is usually classed as basic/positively charged.)

Serine, threonine, cysteine, asparagine, glutamine, and tyrosine. They contain groups (-OH, -SH, amide) that hydrogen-bond but are not ionized at pH 7.