#peptide structure

NMRSpectroscopy

A 30-residue helical peptide in NMR shows:

Hydrogen bonds in alpha helix link carbonyl oxygen of residue i to amide proton of residue i+4, shielding amides from solvent and reducing exchange with deuterium. In a 30-residue stable helix, many NH groups exhibit protection, exchanging slowly over hours versus unstructured coils exchanging within seconds to minutes. Observing slow NH to ND exchange after transferring peptide into D2O, evidenced by persistent amide resonances in successive proton spectra, indicates hydrogen-bonded, folded structure. This slow exchange corroborates NOE ladder and coupling constant data for helix stabilization and core packing.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Ramachandran plot

Why are all the theoretical combinations of phi (ϕ) and psi (ψ) not possible?

Two atoms cannot occupy the same space is the correct choice because it does not accurately describe or belong to the category addressed in this question. In the context of Peptide and Ramachandran plo, the other options (Peptide bond restricts angles, β-sheets and α-helices limit space, and Tertiary structure folding restricts movement) are all valid and well-established concepts. Two atoms cannot occupy the same space is either unrelated to the topic, describes a different biological process, or represents a common misconception. Questions framed as 'which is NOT' require students to identify the exception among otherwise correct statements, demanding comprehensive knowledge of the topic rather than recognition of a single fact.

Ref: Campbell Biology, Urry et al., 12th Ed.

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Ramachandran plot

Which of the following peptides can coexist in both cis- and trans-conformation?

Val-Pro-CONH2 is the scientifically accurate answer to this question. Within the study of Peptide and Ramachandran plo, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of Val-Pro-CONH2 directly address what is being asked. Among the other options, Ala-Ala-CONH2, Pro-Gly-CONH2, and Asn-Gly-CONH2 do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Campbell Biology, Urry et al., 12th Ed.

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Ramachandran plot

What prevents the complete randomization of phi (ϕ) and psi (ψ) angles in peptides?

Steric hindrance is the scientifically accurate answer to this question. Within the study of Peptide and Ramachandran plo, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of Steric hindrance directly address what is being asked. Among the other options, Hydrogen bonding, Hydrophobic interactions, and Peptide bond hydrolysis do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Campbell Biology, Urry et al., 12th Ed.

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Ramachandran plot

Which amino acid has the least allowed regions in the Ramachandran plot?

Proline correctly identifies the graphical representation, mathematical relationship, or plot parameter described in this question. In Peptide and Ramachandran plo, graphical analysis transforms complex kinetic or biological data into linear relationships that allow precise determination of key parameters. The specific feature described by Proline is derived from the mathematical transformation of the underlying equation and has a defined physical meaning. The other options (Glycine, Alanine, and Serine) represent different parameters, intercepts, or slopes from either the same or different analytical methods.

Ref: Campbell Biology, Urry et al., 12th Ed.

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Ramachandran plot

The psi (ψ) angle in peptides refers to:

Rotation around Cα-C bond accurately defines or describes the concept asked in this question. Within Peptide and Ramachandran plo, precise definitions and terminology are essential for clear scientific communication. The other options (Rotation around N-Cα bond, Rotation around C=O bond, and Rotation around C-N bond) either describe related but distinct concepts, use incorrect terminology, or confuse similar-sounding terms that have different scientific meanings. A thorough understanding of exact definitions helps distinguish between closely related biological concepts and is crucial for competitive examinations.

Ref: Campbell Biology, Urry et al., 12th Ed.

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