Hi Weixing
Below are the answers from Copilot.
Best regards,
Guangxin
Key mechanisms that legitimately produce multiple ¹³C‑frequency
components in HSQC/HSQC‑COSY
1. Strong homonuclear ¹H–¹H coupling + E.COSY‑type effects
Glucose protons sit in a strong‑coupling regime, especially H3–H4–H5.
This produces:
* Tilted cross‑peak multiplets
* Apparent splitting in the heteronuclear dimension, even though the ¹³C
nucleus is not directly split by the other protons.
In HSQC‑COSY–style experiments, mixing transfers coherence between
strongly coupled proton partners, which carries with it the frequency shifts of
the strong‑coupling eigenstates.
Why this produces extra ¹³C components:
During mixing, magnetization is relayed from H4 via H3/H5. The effective frequencies
of these protons are perturbed by strong coupling. This “tags” the
transferred coherence so that the evolution during t₁ reflects different
effective proton eigenstates, generating multiple correlated carbon frequencies.
This produces a pattern very similar to what your contour plot shows: a main
peak with several satellite components spread along the ¹³C (F1 axis.
________________________________
2. Residual ¹³C–¹H long‑range couplings (2J,3J)
surviving the pulse sequence
In most HSQC experiments, long‑range ²J/³J(CH) couplings are suppressed, but:
* In HSQC‑COSY variants, some long‑range coherences can survive.
* Glucose contains multiple small but non‑negligible ²J(CH) couplings (~2–5 Hz).
* These can create extra carbon‑dimension components displaced
by several hertz.
This cannot normally explain a full 42 Hz span, but can contribute
satellite features on top of a larger effect (e.g., strong coupling).
________________________________
3. ¹³C–¹³C coupling from natural‑abundance
doublets (rare but visible in carbohydrates)
Even at natural abundance (1.1% ¹³C):
* A small fraction (~0.01%) of glucose molecules contain adjacent ¹³C–¹³C
pairs (e.g., C4–C3 or C4–C5).
* ¹J(CC) couplings can be 30–50 Hz in carbohydrates.
This is extremely consistent with your observed ~42 Hz carbon‑dimension separation.
You would see:
* A main peak (12C–13C majority species),
* And weak additional peaks corresponding to ¹³C–¹³C doublets.
Because you’re viewing a COSY‑type cross peak, these weak components can become
disproportionately visible due to the pathway transfer.
This is one of the best matches to your observation.
________________________________
4. Multiple conformers / mutarotation microstates (β‑glucose)
Although you said the peaks look non‑artifactual:
* β‑glucose exists in multiple rapidly interconverting hydroxyl rotamer states.
* These can generate very small ¹³C shifts (<1 ppm),
but occasionally can appear as partially separated peaks in high‑resolution
settings, especially in indirect carbon dimensions of 2D experiments.
However, the ~42 Hz spread is larger than what rotamers normally generate.
This is probably a secondary contributor at most.
________________________________
5. Z‑Q–type pathways (accidental zero‑quantum coherence in mixing)
In some HSQC‑COSY implementations, the COSY transfer can allow:
* Zero‑quantum artifacts, especially in strongly coupled spin systems.
ZQ‑coherence can generate replicas symmetrically offset in the carbon dimension.
These are often subtle, but if your mixing time is long, their presence becomes more prominent.
> From: main_at_ammrl.groups.io On Behalf Of Zhang, Weixing via groups.io
> Sent: Monday, March 9, 2026 1:35 PM
> To: main_at_ammrl.groups.io
> Subject: Re: [AMMRL] : Puzzle of glucose HSQC
Dear AMMRL Members,
In addition to the expected COSY cross‑peak, I’m observing
several additional signals associated with glucose C4 in the 13C dimension.
These peaks appear consistently and do not resemble common artifacts, so I
am interested in what underlying mechanisms might generate them.
If anyone has encountered similar features or has suggestions on possible
origins, I would greatly appreciate your insights.
Best regards,
Weixing
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Received on Mon Mar 09 2026 - 13:17:17 MST