Amino acid
Amino acids are organic compounds containing amine (-NH) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid. The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of certain amino acids. About 500 naturally occurring amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways. They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.
Synephrine
In a survey of over 50 citrus fruit juices, either commercially-prepared or hand-squeezed from fresh fruit, obtained on the US market, Avula and co-workers found synephrine levels ranging from ~4–60 mg/L; no synephrine was detected in juices from grapefruit, lime, or lemon.
An analysis of the synephrine levels in a range of different citrus fruits, carried out on juices that had been extracted from fresh, peeled fruit, was reported by Uckoo and co-workers, with the following results:
Marrs sweet orange ("C. sinensis" Tan.): ~85 mg/L; Nova tangerine ("C. reticulata" Tan.): ~78 mg/L; clementine ("C. clementina" Tan.): ~115 mg/L; Meyer lemon ("C. limon" Tan.) ~3 mg/kg; Ugli tangelo ("C. reticulata" × "C. paradisi") ~47 mg/kg. No synephrine was detected in: Rio Red grapefruit ("C. paradisi" Macf.); Red-fleshed pummelo ("C. grandis" Tan.); or Wekiwa tangelo ("C. reticulata" × "C. paradisi").