IUPAC-Condensed Glycan Notation

This article introduces IUPAC-condensed glycan notation, the main text format used by glyrepr for representing glycan structures. The goal is to show how a branched glycan structure can be written as a compact, readable string.

library(glyrepr)

Why Are There So Many Glycan Formats?

Different communities have developed different ways to describe glycans, each optimized for a specific use case. Some formats are easy for people to read, some are better for databases and software, and others preserve detailed chemical information.

Let’s use one N-glycan as an example:

The same molecule can be written in several formats:

IUPAC-condensed:

Neu5Ac(a2-3)Gal(b1-4)GlcNAc(b1-2)Man(a1-3)[Neu5Ac(a2-3)Gal(b1-4)GlcNAc(b1-2)Man(a1-6)]Man(b1-4)GlcNAc(b1-4)[Fuc(a1-6)]GlcNAc(b1-

IUPAC-extended:

α-D-Neup5Ac-(2→3)-β-D-Galp-(1→4)-β-D-GlcpNAc-(1→2)-α-D-Manp-(1→3)[α-D-Neup5Ac-(2→3)-β-D-Galp-(1→4)-β-D-GlcpNAc-(1→2)-α-D-Manp-(1→6)]-β-D-Manp-(1→4)-β-D-GlcpNAc-(1→4)[α-L-Fucp-(1→6)]-β-D-GlcpNAc-(1→

WURCS:

WURCS=2.0/6,12,11/[a2122h-1b_1-5_2*NCC/3=O][a1122h-1b_1-5][a1122h-1a_1-5][a2112h-1b_1-5][Aad21122h-2a_2-6_5*NCC/3=O][a1221m-1a_1-5]/1-1-2-3-1-4-5-3-1-4-5-6/a4-b1_a6-l1_b4-c1_c3-d1_c6-h1_d2-e1_e4-f1_f3-g2_h2-i1_i4-j1_j3-k2

InChI:

InChI=1S/C90H148N6O66/c1-21-47(116)59(128)62(131)81(142-21)140-20-40-69(55(124)43(77(135)143-40)93-24(4)108)152-78-44(94-25(5)109)56(125)66(36(16-103)148-78)153-82-63(132)72(156-86-76(61(130)51(120)33(13-100)147-86)158-80-46(96-27(7)111)58(127)68(38(18-105)150-80)155-84-65(134)74(53(122)35(15-102)145-84)162-90(88(138)139)9-29(113)42(92-23(3)107)71(160-90)49(118)31(115)11-98)54(123)39(151-82)19-141-85-75(60(129)50(119)32(12-99)146-85)157-79-45(95-26(6)110)57(126)67(37(17-104)149-79)154-83-64(133)73(52(121)34(14-101)144-83)161-89(87(136)137)8-28(112)41(91-22(2)106)70(159-89)48(117)30(114)10-97/h21,28-86,97-105,112-135H,8-20H2,1-7H3,(H,91,106)(H,92,107)(H,93,108)(H,94,109)(H,95,110)(H,96,111)(H,136,137)(H,138,139)/t21-,28-,29-,30+,31+,32+,33+,34+,35+,36+,37+,38+,39+,40+,41+,42+,43+,44+,45+,46+,47+,48+,49+,50+,51+,52-,53-,54+,55+,56+,57+,58+,59+,60-,61-,62-,63-,64+,65+,66+,67+,68+,69+,70+,71+,72-,73-,74-,75-,76-,77+,78-,79-,80-,81+,82-,83-,84-,85-,86+,89-,90-/m0/s1

Each format serves its purpose:

IUPAC formats: readable text formats for describing glycan structures.
WURCS/GlycoCT: structured formats designed for databases and software.
Semantic formats: formats designed for linking data across platforms.
Chemical formats: formats used for detailed chemical representation.

Why `glyrepr` Uses IUPAC-Condensed

When building glyrepr, we needed a native text format for glycan structures. IUPAC-condensed is a practical choice because it balances readability and information content.

Human-readable: structures can usually be interpreted without specialized software.
Information-rich: it captures the monosaccharides, branches, linkages, anomers, and substituents needed for many glycomics analyses.
Widely used: it is familiar to many researchers in the glycomics community.
Flexible: it can represent both simple and complex glycan structures.

For other formats such as WURCS or GlycoCT, the broader glycoverse workflow can use parser packages such as glyparse.

Reading IUPAC-Condensed Notation

Step 1: Monosaccharide Symbols

Every glycan is built from monosaccharide units, and IUPAC notation gives each one a short abbreviation:

Full Name	Symbol
Galactose	`Gal`
Glucose	`Glc`
Mannose	`Man`
N-Acetylglucosamine	`GlcNAc`
Fucose	`Fuc`

For a broader list of symbols, see the SNFG website or run available_monosaccharides().

Step 2: Substituents

Glycans can contain chemical modifications called substituents. In IUPAC-condensed notation, substituents are written after the monosaccharide name.

Neu5Ac9Ac = a sialic acid with an extra acetyl group at position 9.
Glc3Me = a glucose with a methyl group at position 3.
GlcNAc6Ac = an N-acetylglucosamine with acetylation at position 6.

Format rule: Position number + Modification type
Example: 6Ac = “acetyl group at position 6”

Step 3: Linkage Information

Linkages tell us how monosaccharides are connected to each other.

The anatomy of a linkage:

MonosaccharideA(anomeric_config + anomeric_position - target_position)MonosaccharideB

For example, Neu5Ac(a2-3)Gal means:

Neu5Ac is connected to Gal
The anomeric carbon of Neu5Ac is in alpha configuration (a)
The connection is from position 2 of Neu5Ac
To position 3 of Gal

Sometimes not all linkage details are known, so we use ? as a wildcard:

a2-? = the anomer and donor position are known, but the acceptor position is unknown.
??-3 = the acceptor position is known, but the anomer and donor position are unknown.

Step 4: Topological Structure

The main challenge in IUPAC-condensed notation is writing a branched tree as a linear string. Branches are placed in square brackets and inserted before the residue they attach to.

The basic rules are:

Find the longest backbone
Treat everything else as a branch
Branches go in square brackets []
Write branches just before the monosaccharide they connect to
Apply the same rules recursively inside each branch

Example 1: A Simple O-Glycan

Step-by-step construction:

Identify the main chain: Gal -> GlcNAc -> GalNAc
Add linkage info: Gal(b1-4)GlcNAc(b1-6)GalNAc(a1-
Spot the branch: The bottom Gal connects to GalNAc
Insert the branch: Gal(b1-4)GlcNAc(b1-6)[Gal(b1-3)]GalNAc(a1-
The branch is a single “Gal(b1-3)” unit, no need for step 5.

Final result:

Gal(b1-4)GlcNAc(b1-6)[Gal(b1-3)]GalNAc(a1-

Example 2: The N-Glycan Core

This example has two chains of equal length. The tie-breaker rule decides which one becomes the main chain.

IUPAC’s tie-breaker rule: When chains are equal, choose the one that creates branches with lower position numbers.

Analysis: - Option A: Man(a1-6) branch -> position 6 - Option B: Man(a1-3) branch -> position 3

Option B is chosen because 3 is lower than 6.

Final result:

Man(a1-3)[Man(a1-6)]Man(b1-4)GlcNAc(b1-4)GlcNAc(b1-

Example 3:

This example is more complex: it has three branches with the same length, branching on different residues. In this case, we look for the first breaking point from right to left: the b4 Man. Two mannoses are connected to that Man, one with an a1-3 linkage and the other with an a1-6 linkage. According to the tie-breaker rule, we choose the a1-3 branch as the main chain. First, write the main chain:

Man(a1-2)Man(a1-2)Man(a1-3)[BRANCH]Man(b1-4)GlcNAc(b1-4)GlcNAc(b1-

Now look at the branch, which also contains its own branch.

Using the tie-breaker rule again, the branch can be written as:

BRANCH = Man(a1-2)Man(a1-3)[Man(a1-2)Man(a1-6)]Man(a1-6)

Combining the main chain and the branch gives:

Man(a1-2)Man(a1-2)Man(a1-3)[Man(a1-2)Man(a1-3)[Man(a1-2)Man(a1-6)]Man(a1-6)]Man(b1-4)GlcNAc(b1-4)GlcNAc(b1-

Step 5: Reducing-End Anomeric Information

You might wonder: “Why does the last monosaccharide end with (b1- instead of a complete linkage?”

The root monosaccharide (rightmost) doesn’t connect to anything further, so its anomeric carbon is “free.” The format (xy- tells us about its anomeric state without a target.

Practice

Challenge: Look at the complex N-glycan at the beginning of this article and try to write its IUPAC-condensed string yourself.

Hint: Start by identifying the main chain, then work on the branches one by one.

Test your answer:

# Try your hand-written string here!
my_attempt <- "Your_IUPAC_string_here"

# This will tell you if it's valid
tryCatch({
  result <- as_glycan_structure(my_attempt)
  cat("Your IUPAC string is valid.\n")
  print(result)
}, error = function(e) {
  cat("The string could not be parsed.\n")
  cat("Error:", e$message, "\n")
})
#> The string could not be parsed.
#> Error: In index: 1.

Summary

IUPAC-condensed notation is compact enough for routine analysis while still preserving the structural details that glyrepr needs. In this article, you saw:

why different glycan formats exist and when IUPAC-condensed is useful.
how to read monosaccharide symbols and substituents.
how linkage notation encodes anomeric configuration and positions.
how branched tree structures are converted into linear strings.
how main chains and branches are selected with tie-breaker rules.

Next steps:

Practice with more complex structures
Explore the glyrepr package functions
Use as_glycan_structure() to validate and work with IUPAC-condensed strings

Session Information

sessionInfo()
#> R version 4.6.0 (2026-04-24)
#> Platform: x86_64-pc-linux-gnu
#> Running under: Ubuntu 24.04.4 LTS
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 
#> LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0
#> 
#> locale:
#>  [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8       
#>  [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8   
#>  [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C          
#> [10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   
#> 
#> time zone: UTC
#> tzcode source: system (glibc)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] glyrepr_0.11.0
#> 
#> loaded via a namespace (and not attached):
#>  [1] vctrs_0.7.3       cli_3.6.6         knitr_1.51        rlang_1.2.0      
#>  [5] xfun_0.57         stringi_1.8.7     purrr_1.2.2       generics_0.1.4   
#>  [9] textshaping_1.0.5 jsonlite_2.0.0    glue_1.8.1        htmltools_0.5.9  
#> [13] ragg_1.5.2        sass_0.4.10       rmarkdown_2.31    tibble_3.3.1     
#> [17] evaluate_1.0.5    jquerylib_0.1.4   fastmap_1.2.0     yaml_2.3.12      
#> [21] lifecycle_1.0.5   stringr_1.6.0     compiler_4.6.0    dplyr_1.2.1      
#> [25] fs_2.1.0          pkgconfig_2.0.3   systemfonts_1.3.2 digest_0.6.39    
#> [29] R6_2.6.1          tidyselect_1.2.1  pillar_1.11.1     magrittr_2.0.5   
#> [33] bslib_0.10.0      tools_4.6.0       pkgdown_2.2.0     cachem_1.1.0     
#> [37] desc_1.4.3