Isoleucine (Ile, I)
Isoleucine is a hydrophobic, beta-branched amino acid with two stereocentres. It favours beta-sheet structure, packs in hydrophobic cores and is isobaric with leucine.
Structure and properties
Isoleucine has a four-carbon side chain branched at the beta carbon, giving it a second stereocentre. Natural L-isoleucine has a defined configuration at both centres; inverting the beta carbon gives L-allo-isoleucine. The side chain is entirely aliphatic and strongly hydrophobic, so isoleucine is usually buried in folded proteins or faces the hydrophobic side of amphipathic helices. Beta branching restricts the side chain near the backbone and favours extended beta-strand conformations, although isoleucine is also accommodated in helices. Isoleucine and leucine have the same elemental composition and differ only in where the side chain branches.
Role in peptides
Isoleucine contributes hydrophobic contact surfaces in receptor-binding and membrane-active peptides. In GLP-1 analogues, the C-terminal helical region includes a Phe-Ile-Ala-Trp-Leu stretch that forms part of the receptor-binding surface, retained in semaglutide. LL-37 contains several isoleucines on the non-polar face of its amphipathic helix. Isoleucine is also common in beta-hairpin and amyloid-forming sequences. Swapping leucine for isoleucine, or the reverse, is a subtle probe of side-chain shape, because the two residues have similar hydrophobicity but different branching. Leucine-to-isoleucine swaps can therefore reveal how precisely a binding pocket recognises side-chain shape.
Stability and analysis
Isoleucine is chemically stable and has no oxidisable or cyclising side chain. The main analytical issues are identity related. Isoleucine and leucine are isobaric, so intact mass and many fragmentation methods cannot tell them apart; amino acid analysis, specialised fragmentation or reference-standard comparison is needed. Epimerisation at the alpha carbon during synthesis gives D-allo-isoleucine, a diastereomer that may separate on HPLC. Beta branching makes couplings slower and can promote on-resin aggregation.
In PeptideWiki compounds
Compounds whose sequence contains isoleucine, with the number of occurrences.
- LL-37 (3)
- CJC-1295 (2)
- Sermorelin (2)
- Tesamorelin (2)
- Thymosin β4 (2)
- AOD-9604 (1)
- Abaloparatide (1)
- Exenatide (1)
- Humanin (1)
- IGF-1 LR3 (1)
- MOTS-c (1)
- Oxytocin (1)
- Pramlintide (1)
- Teriparatide (1)
- Thymosin α1 (1)
- VIP (vasoactive intestinal peptide) (1)
Frequently asked questions
Can mass spectrometry distinguish isoleucine from leucine?
Not from intact mass, because they have identical elemental formulas. Standard collision-induced fragmentation also gives the same backbone fragments. Distinguishing them requires side-chain-specific fragmentation, such as w-ions from electron-based methods, amino acid analysis with chromatographic separation, or comparison with a synthetic reference of known sequence. For most catalogue peptides the sequence is set by synthesis, so identity rests on the synthesis record and reference comparison.
What is allo-isoleucine?
Isoleucine has two stereocentres, at the alpha and beta carbons. Allo-isoleucine has the opposite configuration at the beta carbon relative to the alpha carbon. In synthetic peptides, epimerisation of L-isoleucine at the alpha carbon produces D-allo-isoleucine, a diastereomeric impurity. Because diastereomers have different shapes, they often separate by reversed-phase HPLC and can be detected as a closely eluting peak.
Cite this page