Protein Structure

Lecture 11/12 Video 5

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🧪 MS3 – Part 5: Mass Spectrometry Beyond Proteins

🔬 Big Idea

So far, mass spectrometry (MS) has been used mainly for proteins and peptides. But the core principle is broader:

👉 MS works best on polymers with repeating backbones + variable side groups

That means we can also study:

🧬 DNA / oligonucleotides
🍬 Carbohydrates
🧵 Synthetic polymers

🧬 1. Oligonucleotides (DNA) in Mass Spectrometry

🧩 Structure of DNA (important for MS)

DNA has a repeating backbone:

Phosphate group
Sugar
Base (A, T, G, C)

This regular structure is critical, because:

MS relies on predictable fragmentation
Similar to peptides (repeating amide bonds)

💥 Fragmentation of DNA

DNA fragmentation produces multiple ion types:

a, b, c ions
x, y, z ions
w ions

👉 Compared to proteins:

DNA has more labile bonds (breaks more easily)
This leads to:
- ⚠️ More fragmentation pathways
- ⚠️ More complex spectra

⚠️ Complication: Base Loss

Sometimes fragmentation removes:

The base (A, G, etc.)
Leaving only sugar + backbone

👉 Result:

Spectrum becomes messy
Interpretation becomes harder

🧠 Sequencing DNA with MS

You can sequence DNA by:

Tracking mass differences between fragments
Doing a sequential “walk”

Example:

G → A → A → A → G → C → G

👉 BUT:

MS is not the best method for DNA sequencing
Other methods are:
- Faster
- More accurate

🔗 Studying DNA Interactions (Important Use!)

MS is very useful for studying complex formation:

Example:

Single strand + double strand → triple helix

Using MS1 (not MS/MS):

Detect:
- Binding
- Complex formation
- Stability

👉 Applications:

Primer binding
DNA–DNA interactions

🍬 2. Carbohydrates in Mass Spectrometry

🧩 Structure of Carbohydrates

Carbohydrates are:

Built from monosaccharides (e.g., glucose)
Linked by condensation reactions (like peptides)

Example:

Glucose (180 Da)
Minus water (18 Da) → 162 Da per unit

📊 Fragmentation Pattern

In MS:

Peaks are spaced by ~162 Da

👉 This means:

Each peak = loss of one sugar unit

✔ Very useful for:

Determining chain length
Identifying repeating units

⚠️ Major Challenge: Isomers

Different sugars can have:

Same mass
Different structure

Examples:

Glucose
Mannose
Galactose

👉 MS cannot distinguish them directly

🧠 What CAN we identify?

We can still classify:

Pentose
Hexose
Deoxyhexose
Acetylated sugars

👉 Useful for:

Glycoprotein analysis

💥 Fragmentation Issues

Carbohydrates are:

Highly fragmentation-prone

Problems:

In-source fragmentation
Post-source decay (PSD)

👉 Result:

Very busy spectra
Hard to interpret manually

🤖 Solution

👉 Requires:

Computational tools
Advanced data analysis

🧵 3. Synthetic Polymers

🧠 Key Requirement for MS

For MS to work well:

Polymer must have:
- Regular backbone
- Labile bonds (easy to break)

❌ Pure alkane chains → NOT suitable ✔ Ester or amide bonds → GOOD

🧪 Example: Polyhydroxyalkanoates (PHA)

Contains ester bonds
These break predictably

📊 Fragmentation pattern:

Peaks separated by 86 Da

👉 This equals:

Mass of repeating unit

✔ Indicates:

Homopolymer (same unit repeated)

🔀 Heteropolymers (More Complex)

Example:

Backbone with:
- Amide bonds
- Ester bonds

👉 Now there are:

Multiple cleavage sites
More fragmentation types

💥 Fragmentation Behavior

You get:

Multiple ion series (called “1” and “2” here)

Examples:

Cleavage at ester → one fragment type
Cleavage at amide → another

👉 Result:

More complex, but still analyzable

⚖️ What determines which fragments appear?

Three main factors:

Ion stability
Ionization efficiency
Energy distribution

👉 Important for:

Fragmentation interpretation
Quantification (next lectures)

🧠 Key Takeaways

🔑 1. MS works best on polymers with:

Repeating backbone
Predictable fragmentation

🔑 2. Oligonucleotides:

Can be fragmented and sequenced
More complex than proteins
Great for studying interactions

🔑 3. Carbohydrates:

Show repeating mass differences (162 Da)
Hard due to isomers + fragmentation

🔑 4. Synthetic polymers:

Work if backbone has labile bonds
Regular spacing reveals structure

🔑 5. General Limitation

As complexity increases:

Spectra become messier
Interpretation requires:
- Experience
- Software

📌 Final Insight

Mass spectrometry is not limited to proteins:

👉 It is a general tool for analyzing structured polymers

But its success depends heavily on:

Backbone regularity
Fragmentation predictability
Chemical properties of the molecule

Quiz

Score: 0/30 (0%)

Q0. Why are proteins, peptides, oligonucleotides, carbohydrates, and some synthetic polymers all potentially suitable for mass spectrometry-based structural analysis?

Because they are all naturally fluorescent molecules

Because they all contain metal ions that improve ionization

Because they can have repeating backbone structures with variable units that create interpretable mass differences

Because they all fragment only once during MS/MS

Q1. In oligonucleotide mass spectrometry, why is the repeating phosphate-sugar backbone important?

It prevents all fragmentation from occurring

It allows predictable backbone cleavage patterns that can be used for sequence interpretation

It makes all bases have different charges

It removes the need for ionization

Q2. Compared with peptide backbones, oligonucleotide backbones often produce more complex fragmentation spectra because:

DNA bases are always radioactive

Oligonucleotides have more labile bonds, especially near oxygen atoms

DNA cannot be ionized in mass spectrometry

Oligonucleotides only form one fragment ion type

Q3. Which set of fragment ion labels is associated with oligonucleotide fragmentation in the lecture?

Only b and y ions

Only a and z ions

a, b, c, d, w, x, y, and z ions

Only precursor and reporter ions

Q4. What additional complication can occur during oligonucleotide fragmentation besides backbone cleavage?

The sugar can convert into an amino acid

The phosphate group can become a peptide bond

The base can be cleaved off, leaving sugar-backbone fragments

The DNA strand becomes double-stranded automatically

Q5. What does a sequential walk mean in the context of oligonucleotide sequencing by MS?

Measuring the walking speed of DNA through a gel

Following a series of fragment mass differences to infer the nucleotide sequence

Separating DNA strands by electrophoresis before MS

Using only the intact molecular mass to identify a sequence

Q6. Why is mass spectrometry not usually the preferred method for full DNA sequencing?

Because DNA cannot fragment

Because DNA has no mass

Because other sequencing methods are usually faster, easier, and more complete

Because MS can only detect proteins

Q7. How can MS1 be useful for studying DNA strands?

By detecting whether single strands, double strands, or triple-strand complexes form

By forcing DNA to replicate

By converting DNA into protein

By removing all phosphate groups from DNA

Q8. In carbohydrate analysis, why does a glucose-based polymer often show mass differences of approximately 162 Da between adjacent peaks?

Because glucose has a mass of 162 Da before polymerization

Because glucose has a mass of 180 Da, and formation of a glycosidic linkage removes water with mass 18 Da

Because every carbohydrate loses phosphate groups of 162 Da

Because MALDI always adds 162 Da to every peak

Q9. Why is maltose considered a relatively regular carbohydrate example in MS?

It contains only amino acids

It has repeating glucose-derived units that can give predictable mass spacing

It cannot fragment at all

It contains a peptide backbone

Q10. What is a major limitation of MS when distinguishing glucose, mannose, and galactose?

They have identical masses because they are isomers

They cannot form ions

They all contain nitrogen

They always fragment into amino acids

Q11. Even if MS cannot distinguish exact carbohydrate isomers, what can it often still determine?

The exact 3D position of every hydroxyl group

Whether a unit belongs to a class such as pentose, hexose, deoxyhexose, or acetylated amino hexose

The DNA sequence attached to the sugar

The organism that produced the carbohydrate with certainty

Q12. Why can carbohydrate spectra become especially busy and difficult to interpret?

Carbohydrates are prone to post-source decay and in-source fragmentation

Carbohydrates never fragment

Carbohydrates have no labile bonds

Carbohydrates always produce only one peak

Q13. For synthetic polymers, which structural feature makes MS/MS interpretation more feasible?

A pure alkane backbone with no labile bonds

A completely random backbone with no repeating pattern

A regular backbone containing labile bonds such as ester or amide bonds

A polymer with no repeating units

Q14. In the polyhydroxyalkanoate example, what does an equal interpeak distance of approximately 86 units indicate?

The polymer contains only DNA bases

The polymer has a repeating building block with mass around 86 units

The polymer cannot be a homopolymer

The polymer contains only glucose

Q15. A pure alkane backbone is generally ideal for MS/MS polymer sequencing.

True

False

Q16. Oligonucleotides are similar to peptides in that both can be treated as polymers with a repeating backbone and variable side units.

True

False

Q17. Oligonucleotide fragmentation is usually simpler than peptide fragmentation because DNA has fewer labile bonds.

True

False

Q18. Base loss during oligonucleotide fragmentation can make spectra harder to interpret.

True

False

Q19. MS can be used to study whether DNA strands interact and form complexes, not only to fragment and sequence them.

True

False

Q20. MS/MS is required in the lecture example where DNA single strands and double strands are checked for complex formation.

True

False

Q21. A glucose unit has a mass of about 180 Da, but a glucose residue in a polymer contributes about 162 Da because water is lost during linkage formation.

True

False

Q22. Glucose, mannose, and galactose can always be distinguished by MS because they have different molecular masses.

True

False

Q23. Carbohydrate MS can still provide useful information even when exact isomer identity is unclear.

True

False

Q24. Post-source decay and in-source fragmentation can limit the applicability of carbohydrate MS unless the experimental conditions are carefully controlled.

True

False

Q25. Computational tools can help interpret complex carbohydrate fragmentation spectra.

True

False

Q26. Synthetic polymers can be analyzed by MS only if they are biological molecules.

True

False

Q27. In the PHA example, regular ester bond cleavage helps reveal the mass of the building block between adjacent ester bonds.

True

False

Q28. A heteropolymer with both amide and ester bonds may generate multiple fragment ion series because there are multiple labile cleavage positions.

True

False

Q29. Which fragments appear most strongly in MS depends only on the polymer sequence and never on ionization efficiency or ion stability.

True

False