# Double Bond Positional Isomer Generation

> Use when when constructing a comprehensive lipid spectral reference library that must disambiguate lipids differing only in carbon-carbon double-bond position (e.

- Skill: `holobiomicslab/double-bond-positional-isomer-generation` (Agent Skill)
- Install (CLI): `npx skillmds@latest add holobiomicslab/double-bond-positional-isomer-generation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/holobiomicslab/double-bond-positional-isomer-generation/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: CC-BY-4.0
- Author: HolobiomicsLab (https://skillmd.com/u/holobiomicslab)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/holobiomicslab/double-bond-positional-isomer-generation

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# double-bond-positional-isomer-generation

## Summary

Systematically enumerate all regioisomeric double-bond position variants for acyl chain compositions within defined carbon count and unsaturation constraints, generating theoretical m/z fragment ions for each positional isomer to populate a hierarchical lipid fragmentation library.

## When to use

When constructing a comprehensive lipid spectral reference library that must disambiguate lipids differing only in carbon-carbon double-bond position (e.g., OA 18:1 Δ9 vs. OA 18:1 Δ11), and you need to precompute all valid positional isomers for a given lipid class, carbon count range, and unsaturation level before matching unknown MS/MS spectra.

## When NOT to use

- Input spectra are already matched to a curated spectral database with known positional assignments — use spectral matching directly rather than regenerating isomer variants.
- Lipid standard reference materials are unavailable for validation — library completeness cannot be verified without spot-checking against known standards.
- Analysis requires only broad lipid class annotation without positional isomer discrimination (e.g., 'phosphatidylcholine' is sufficient) — the full positional enumeration adds computational overhead without benefit.

## Inputs

- Lipid class taxonomy definition (list of lipid classes and their nomenclature rules)
- Acyl chain composition rules (carbon count range, unsaturation degree constraints per class)
- Lipid fragmentation rules (neutral loss patterns, characteristic ions for each class)

## Outputs

- Hierarchical lipid fragmentation library indexed by (lipid class, chain composition, positional isomer, fragment m/z)
- Complete enumeration of all valid double-bond positional variants for each chain composition
- Theoretical m/z values for all fragment ions derived from positional isomers

## How to apply

First, define the lipid class taxonomy and acyl chain composition rules by specifying valid carbon count ranges and degree-of-unsaturation constraints for each lipid class. Second, enumerate all valid acyl chain structures by iterating over all permissible carbon counts and unsaturation levels; for each composition, systematically generate all regioisomeric double-bond position variants (e.g., for C18:1, generate Δ1, Δ2, ..., Δ17). Third, apply lipid fragmentation rules (neutral loss patterns, characteristic ion generation) to compute theoretical m/z values for each double-bond variant. Fourth, organize the resulting entries into a hierarchical index structure (lipid class → chain composition → positional isomer → fragment m/z) for efficient O(log n) lookup during spectral querying. Fifth, validate library completeness by spot-checking fragment patterns of known standards and verifying the entry count matches the target (e.g., 168.6 million entries).

## Related tools

- **XCMS** (Peak alignment, matching, and preprocessing of mzML mass spectrometry data prior to spectral querying against positional isomer library)
- **CAMERA** (Compound spectra extraction and annotation from LC/MS data sets to group isotopologues and adducts before matching against fragmentation library)
- **LipidIN** (Integration framework housing the 168.6 million entry hierarchical library and expeditious querying module for matching preprocessed spectra against enumerated positional isomers) — https://github.com/LinShuhaiLAB/LipidIN

## Evaluation signals

- Library entry count matches target (e.g., 168.6 million entries); subset counts match combinatorial expectation (e.g., number of C18:1 Δn variants = 17 for each lipid class).
- Spot-checked fragment ion m/z values for known lipid standards align with measured fragmentation patterns within ppm tolerance (e.g., ±5 ppm MS1, ±10 ppm MS2).
- Hierarchical index structure supports efficient querying (~70 billion spectral comparisons in <1 second per the README), implying correct indexing and no orphaned entries.
- False discovery rate on annotated lipids is consistent with expected background (e.g., 5.7% FDR across 8923 lipids in the LipidIN application).
- Positional isomers within a composition produce distinguishable fragment profiles; cosine similarity between different Δ positions shows statistically significant separation.

## Limitations

- Enumeration scales combinatorially with carbon count range and unsaturation limits; very large ranges may exceed practical memory/storage (e.g., C0–C50 with up to 15 double bonds).
- Double-bond regioisomerism alone cannot distinguish stereoisomerism (cis vs. trans); library does not enumerate or disambiguate E/Z geometry variants.
- Theoretical m/z fragments assume standard lipid fragmentation rules; non-standard or oxidized lipids may require manual rule extension (README mentions OxPC, OxPG, OxPI, OxPS added separately).
- Positional assignment accuracy depends on MS/MS spectral resolution and fragment ion intensity ratios; low-resolution or noisy spectra may not support definitive isomer discrimination.

## Evidence

- [other] Enumerate all valid acyl chain structures by iterating over carbon counts and unsaturation levels, generating all regioisomeric double-bond position variants for each composition.: "Enumerate all valid acyl chain structures by iterating over carbon counts and unsaturation levels, generating all regioisomeric double-bond position variants for each composition."
- [other] Generate fragment ion theoretical m/z values for each chain composition using lipid fragmentation rules (neutral loss, characteristic ion generation).: "Generate fragment ion theoretical m/z values for each chain composition using lipid fragmentation rules (neutral loss, characteristic ion generation)."
- [other] Organize the 168.6 million entries into a hierarchical index structure (lipid class → chain composition → positional isomer) for efficient lookup.: "Organize the 168.6 million entries into a hierarchical index structure (lipid class → chain composition → positional isomer) for efficient lookup."
- [readme] 168.6 million lipid fragmentation hierarchical library that encompass all potential chain compositions and carbon-carbon double bond locations: "168.6 million lipid fragmentation hierarchical library that encompass all potential chain compositions and carbon-carbon double bond locations"
- [other] Validate the library completeness and correctness by spot-checking fragment patterns against known lipid standards and verifying entry count matches the target 168.6 million.: "Validate the library completeness and correctness by spot-checking fragment patterns against known lipid standards and verifying entry count matches the target 168.6 million."