Table 2.

Effect of the nature of residue Y on computed ¹³C′ shielding of A

Ac-GAY-NMea
Yb	Δ(φ = −60; ψ = −40)c	Δ(φ = −60; ψ = −60)c	Δ(φ = −140; ψ = +140)c
Thr	1.0	1.7	0.6
Asp	1.4	0.9	−0.3
Val	0.7	1.5	0.3
Met	0.7	1.1	0.1
Trp	0.4	0.7	0.1
Tyr	0.2	0.2	−0.2 (0.8)d
Gln	−1.0	−0.5	−0.3
Pro	−2.3	1.1	−1.1
Gly	0.6	1.2	−0.1

^aAll the listed results, in terms of Δ, were obtained assuming that the backbone torsional angles (φ,ψ)_Y of residue Y are fixed at a canonical α–helix conformation, namely φ = −60° and ψ = −40°. The Δ values were computed as: Δ = (¹³C′_A − ¹³C′_Y) where ¹³C′_A is the isotropic shielding value of residue A (Ala) in the tripeptide Ac-GAA-NMe, and ¹³C′_Y is the isotropic shielding value of residue A in the tripeptide Ac-GAY-NMe, with the identity of residue Y listed in column 1.

^bIdentity (by using a three letter code) of residue Y in the tripeptide Ac-GAY-NMe.

^cThe sub index of Δ represents the particular backbone (φ,ψ) torsional angles chosen for the residue A, among all possible ones from the Ramachandran map. Those values for which |Δ| > 0.5 ppm are highlighted in boldface and italics.

^dIn parentheses, an alternative value for Δ, (0.8), was computed for a slightly-shifted set of backbone torsional angles, namely (φ = −40°; ψ = +20°) rather than (φ = −40°; ψ = +40°); this result illustrates that the same absolute value of |Δ| computed for Tyr (0.2), in each column of this Table, is just a coincidence, and that the nature of residue Y, matters.