Commonly, adenine in its amino form (non-tautomeric) binds to thymine by means of two hydrogen bonds, and guanine in its non-tautomeric keto form binds to cytosine by means of three hydrogen bonds between them.

An enol structure occurs when the hydrogen atom bonds to an nearby oxygen atom that sticks out from the ring. These two types of structures are known as tautomers. Both guanine and thymine can switch easily from one tautomer to another. The change in shape affects the three-dimensional shape of the molecule.

Answer: C has the most stable enol tautomer.

three tautomeric forms
However, recent theoretical and experimental investigations indicate that adenine in the dimethylsulfoxide solution exists in three tautomeric forms (N9H, N7H, and N3H), where the N9H form is the major tautomer, whereas the other two are minor tautomers.

In the case of thymine, a number of possible tautomeric forms are possible, and an equilibrium exists between the keto (or “lactam”) form and the alternative enol (or “lactim”) form. Transformation between the two forms involves the movement of a hydrogen atom and the transfer of bonding electrons.

In the third tautomer we have conjugated double bonds which give extra stability so III is the most stable. Among the first and second tautomers, the first tautomer is the enol form and the second is the keto form. We know that keto is more stable than enol tautomer so structure II is more stable than structure I.

In organic chemistry, keto–enol tautomerism refers to a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (an alcohol). The keto and enol forms are said to be tautomers of each other.

Tautomers are isomers of a compound which differ only in the position of the protons and electrons. The carbon skeleton of the compound is unchanged. A reaction which involves simple proton transfer in an intramolecular fashion is called a tautomerism.

In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine. Uracil is a demethylated form of thymine. Uracil is a common and naturally occurring pyrimidine derivative….CHEBI:17568.

The enol form pairs with guanine instead of adenine, so if a thymine happens to be in the enol form at the moment it takes a partner during replication, a guanine will be inserted in place of adenine. (figure14.

An enol structure occurs when the hydrogen atom bonds to an nearby oxygen atom that sticks out from the ring. These two types of structures are known as tautomers.

Tautomers of Adenine, Cytosine, Guanine, and Thymine. The four bases of DNA can exist in at least two tautomeric forms as shown below. amino or imino forms, and guanine, thymine, and uracil (which are cyclic amides) can exist in either lactam (keto) or lactim (enol) forms.

In thymine, the groups at C-4 and C-2 are hydrogen acceptors, and N-3 is a hydrogen donor. (Only two of these sites, C-4 and N–3, are used to form base pairs in DNA.) The hydrogen-bonding patterns of bases have important consequences for the three-dimensional structure of nucleic acids.

Guanine and thymine can have alternate molecular structures based on different locations of a particular hydrogen atom. A keto structure occurs when the hydrogen atom bonds to a nitrogen atom within the ring.