Table 2.

Summary^a of results from X-ray crystallography and calculations^b

X-(C=O)- Hyp(Nbz)-OMe X=	X-ray crystallography				calculations. Flp
	molecule 1		molecule 2		α-helix		PPII
	ϕ, Ψ °	d, Å	ϕ, Ψ °	d, Å	d, Å	NBO Ec	d, Å
C(CH3)3	−56, +138	2.681	—	—	2.723	2.27d	2.726
CH(CH3)2	—45, —44	2.687	−47,−40	2.738	2.785	1.97	2.781
CH2CH3	−57,−38	2.828	−61,−34	2.883	2.807	1.87	2.816
CH3	−65,−34	2.991	−69, −29	3.049	2.812	1.85	2.816
CH2CI	−56, −36	2.847	−61,−32	2.879	2.837	1.68	2.834
CH2OCH3	−59, −41	2.893	−69, −44	3.046	2.841	1.70	2.844
CH2F	−69,−32	3.053	−83, −173	3.320	2.838	1.71	2.837
H	−76, +163	3.197	−84, +173	3.336	2.961	1.11	2.963
CHF2	−56, −36	2.858	−63,−30	2.921	2.861	1.40	2.864
CF3	−74, +167	3.099	—	—	2.854	1.41	2.859

^aAdditional details are in the Supporting Information. d = distance between the donor O (O_i) and the acceptor carbon (C_i+1).

^bCalculations on X-C(O)-Flp-OMe at the MP2 level of theory and 6–311++G(2d,2p) basis set in implicit water, on peptides with an exo ring pucker and the indicated general conformation. ϕ and ψ torsion angles are tabulated in the Supporting Information, as are calculations using the M06–2X DFT method and similar calculations on X-C(O)-Pro-OMe.

^cNBO energies for orbital overlap between the O_i p-like orbital and the C=O_i+1 π* orbital in kcal mol^–1.

^dAn interaction energy of 0.66 kcal mol^–1 was in addition calculated between the O_i s-like orbital and the C=O_i+1 π* orbital.