alpha helix

Examples of alpha helix in the following topics:

• Secondary & Tertiary Structure of Large Peptides and Proteins

  • The alpha-helix is right-handed, which means that it rotates clockwise as it spirals away from a viewer at either end.
  • Using this terminology, the alpha-helix is a 3.613 helix.
  • The alpha helix is the most stable of these, accounting for a third of the secondary structure found in most globular (non-fibrous) proteins.
  • Although most proteins and large peptides may have alpha-helix and beta-sheet segments, their tertiary structures may consist of less highly organized turns, strands and coils.
  • A large section of antiparallel beta-sheets is colored violet, and a short alpha-helix is green.

• RNA Bacteriophages

  • They adopt a secondary structure consisting of a six-stranded beta sheet and an alpha helix.

• Protein Structure

  • The most common forms of secondary structure are the α-helix and β-pleated sheet structures and they play an important structural role in most globular and fibrous proteins.
  • Every helical turn in an alpha helix has 3.6 amino acid residues.
  • The R groups (the side chains) of the polypeptide protrude out from the α-helix chain and are not involved in the H bonds that maintain the α-helix structure.
  • The α-helix and β-pleated sheet form because of hydrogen bonding between carbonyl and amino groups in the peptide backbone.
  • Certain amino acids have a propensity to form an α-helix, while others have a propensity to form a β-pleated sheet.

• Bovine Spongiform Encephalopathy

  • TSEs can arise in animals that carry an allele which causes previously normal protein molecules to contort by themselves from an alpha helix arrangement to a beta sheet, which is the disease-causing shape for the particular protein.

• The Secondary & Tertiary Structures of DNA

  • Indeed, the situation was similar to that occupied by the proteins a decade earlier, before the alpha helix and pleated sheet structures were proposed by Linus Pauling.
  • The double helix is further stabilized by hydrophobic attractions and pi-stacking of the bases.
  • The helix shown here has ten base pairs per turn, and rises 34 Å (3.4 nm) in each turn.
  • This right-handed helix is the favored conformation in aqueous systems, and has been termed the B-helix.
  • Separation of a portion of the double helix takes place at a site called the replication fork.

• The DNA Double Helix

  • The DNA double helix looks like a twisted staircase, with the sugar and phosphate backbone surrounding complementary nitrogen bases.
  • DNA has a double-helix structure, with sugar and phosphate on the outside of the helix, forming the sugar-phosphate backbone of the DNA.
  • The two strands of the helix run in opposite directions, so that the 5′ carbon end of one strand faces the 3′ carbon end of its matching strand.
  • During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
  • Native DNA is an antiparallel double helix.

• Alpha Decay

  • In alpha decay an atomic nucleus emits an alpha particle and transforms into an atom with smaller mass (by four) and atomic number (by two).
  • Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle that consists of two protons and two neutrons, as shown in .
  • Alpha decay is the most common cluster decay because of the combined extremely high binding energy and relatively small mass of the helium-4 product nucleus (the alpha particle).
  • Alpha decay typically occurs in the heaviest nuclides.
  • Alpha decay is one type of radioactive decay.

• Angle Addition and Subtraction Formulae

  • $\begin{aligned} \cos(\alpha + \beta) &= \cos \alpha \cos \beta - \sin \alpha \sin \beta \\ \cos(\alpha - \beta) &= \cos \alpha \cos \beta + \sin \alpha \sin \beta \end{aligned}$
  • $\begin{aligned} \sin(\alpha + \beta) &= \sin \alpha \cos \beta + \cos \alpha \sin \beta \\ \sin(\alpha - \beta) &= \sin \alpha \cos \beta - \cos \alpha \sin \beta \end{aligned}$
  • $\displaystyle{ \begin{aligned} \tan(\alpha + \beta) &= \frac{ \tan \alpha + \tan \beta}{1 - \tan \alpha \tan \beta} \\ \tan(\alpha - \beta) &= \frac{ \tan \alpha - \tan \beta}{1 + \tan \alpha \tan \beta} \end{aligned} }$
  • Apply the formula $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$:
  • We can thus apply the formula for sine of the difference of two angles: $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \cos \alpha \sin \beta$.

• Factoring General Quadratics

  • When $a$ is equal to one, $\alpha_1$ and $\alpha_2$ both equal one, and $\beta_1$ and $\beta_2$ are factors of the constant $c$ such that:
  • When $a$ is not equal to one and not equal to zero, you can FOIL the above expression for the factored form of the quadratic to find that  $\alpha_1$ and $\alpha_2$ are factors of $a$ such that:
  • In other words, the coefficient of the $x^2$ term is given by the product of the coefficients $\alpha_1$ and $\alpha_2$, and the coefficient of the $x$ term is given by the inner and outer parts of the FOIL process.
  • In some cases, it will be impossible to factor the quadratic such that $\alpha_1$ and $\alpha_2$ are integers.
  • Such that $b = \alpha_1 \beta_2 + \alpha_2 \beta_1$.  

• Reactions at the α-Carbon

  • Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue).
  • If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic.
  • First, these substitutions are limited to carbon atoms alpha to the carbonyl group.
  • Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens).
  • This is demonstrated convincingly by the third ketone, which is structurally similar to the second but has no alpha-hydrogen.