SME
Biochemistry · Problem type
Length of an α-helix or a β-strand
Use this for “How long is a stretch of 18 residues as an α-helix, and how many turns does it make? How long would it be as a β-strand?”
Diagram
18 residues in every view: 9 in the helix (pink), 8 in the sheet (gold), 1 in the turn.
Secondary: a helix and a pleated sheet, held by C=O···H–N hydrogen bonds (dotted).
This is an example. Type your own values to start a new problem.
L (nm)
=
n
×
d (nm)
t
Helix turns
calculated
Formulas
L
=
n
×
d
2.7 = 18 × 0.15
t
=
n
3.6
when
d
is
the
α-helix
rise,
0.15
nm
5
=
18
3.6
when
0.15
is
the
α-helix
rise,
0.15
nm
Assumptions
•
An α-helix rises 0.15 nm along its axis for each residue and makes one turn every 3.6 residues.
•
A β-strand is almost fully stretched: 0.34 nm for each residue, with no turns.
•
The stretch keeps one shape from end to end; real chains switch between helix, strand and loops.
Step-by-step
WE KNOW
n = 18, d = 0.15 nm
FIND
length (L), helix turns (t)
Step 1 · Find length (L)
Use L = n × d
Each residue adds its rise along the axis.
L
=
18
×
0.15
L
=
2.7
nm
Step 2 · Find helix turns (t)
Use t = n ÷ 3.6 when d is the α-helix rise, 0.15 nm
An α-helix makes one turn every 3.6 residues.
t
=
n
3.6
t
=
18
3.6
t
=
5
Check
Put the numbers back in. Both sides should match.
2.7 = 18 × 0.15 ✓
5
=
18
3.6
✓
Topic check
See what you know: 5 questions on this topic
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