The rubiscolins are a group of opioid peptide

Opioid peptide

Opioid peptides are short sequences of amino acids that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Opioid peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but they all resemble opiates...

s which are formed during digestion of the ribulose bisphosphate carboxylase/oxygenase (Rubisco

RuBisCO

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...

) protein from spinach

Spinach

Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...

 leaves. Rubiscolins mimic the effects of opiates and therefore influence the brain. These peptides have much in common with the better known gluten exorphins.

There are 2 known rubiscolins with known structure:

Rubiscolin-5

• Structure: H-Gly
  Glycine
  Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
  
  -Tyr
  Tyrosine
  Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
  
  -Tyr-Pro
  Proline
  Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
  
  -OH
• Chemical formula: C₃₀H₄₅N₅O₉
• Molecular weight: 619.70 g/mol

Rubiscolin-6

• Structure: H-Gly-Tyr-Tyr-Pro-Thr
  Threonine
  Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
  
  -OH
• Chemical formula: C₃₉H₅₄N₆O₁₀
• Molecular weight: 766.87 g/mol