a broad, rather flat minimum for angles from 400 to 1200. .... 19 ± 1. 16i 2. Bl-ionone. CH2CI2. -500C. 15 + 3. 8 i 1I all-trans. CDC13 amb. 15. 12. 0. -3. 5CHs.
Proc. Nat. Acad. Sci. USA Vol. 68, No. 6, pp. 1289-1293, June 1971
Ring Orientation in 3-Ionone and Retinals (theoretical/NMR/semi-empirical approach/Overhauser effect/s-cis conformation
B. HONIG, B. HUDSON, B. D. SYKES, AND M. KARPLUS Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138
Communicated April 12, 1971 ABSTRACT The ring orientation in i-ionone, alltrans retinal, and li-cis retinal, relative to that of the polyene chain, has been determined by means of semiempirical calculations and magnetic resonance measurements of the nuclear Overhauser effect and long-range coupling constants. The experimental results yield a distorted s-cis conformation about the C6-C7 "single bond", with the torsional angle in the range 300 to 700. This agrees well with the semi-empirical potential function, which has a broad, rather flat minimum for angles from 400 to 1200. The temperature dependence of the NMR results provide confirmation for the form of the torsional potential.
The structure of retinal and related compounds is of considerable interest because of the importance of these compounds in living systems (1). Primary attention has been given to two aspects (2) of the retinal structure that might play a direct role in its function as the chromophore of the vertebrate visual pigment. One of these is the nature of the molecular distortion induced by the strong nonbonded repulsions present in the 1 1-cis isomer; the other is the orientation of the f3-ionone ring relative to the hydrocarbon chain. We determine the ring orientation in fl-ionone, all-trans retinal, and 1 1-cs retinal in this paper by means of semi-empirical calculations and nuclear magnetic resonance (NMR) measurements. In a subsequent paper we will report a similar NMR determination of the distortion in the polyene chain of 11-cis retinal (3). Earlier NMR studies of the retinals include the detailed analysis of the 220-MHz spectrum by Patel (4), as well as partial analyses of 60-MHz spectra by Barber et al. (5), Planta et al. (6), and Karver et al. (7). Theoretical calculations on the torsional potential of the o-ionone ring have been made by Pullman, Langlet, and Berthod (8) with the extended Huckel method, and by Langlet, Pullman, and Berthod (9) with the PCILO method. METHOD The semi-empirical approach for determining the ground-state energy is similar to that of others (10). The total energy, E, of the molecule relative to the energy of a standard geometry (e.g., the planar all-trans compound) is written in the form E = Et + Enb, (1) where Et is the 7r-electron energy obtained from a Huckel calculation and Enb is the sum over the nonbonded interactions, which are expressed as pairwise terms dependent only on the distance between atoms or groups of atoms. Since there
is considerable uncertainty about the value of both the Huckel and the nonbonded parameters, a number of sets were tried; it was found that the essential features of the potentialenergy surface are independent of the parameter values. To avoid the complication of having to minimize the energy as a function of all the bond lengths and angles for each ring orientation, we assumed that the only variable was the torsional angle j about the C6-C7 bond (see Fig. 1); that is, the ring and the polyene chain were taken to have an invariant idealized structure with trigonal angles equal to 1200, tetrahedral angles equal to 109028', conjugated single bonds equal to 1.46 AO, conjugated double bonds equal to 1.35 A, sp2-spI single bonds equal to 1.52 A, sp'-sp3 single bonds equal to 1.54 Ai, and C-H bonds equal to 1.082 AO. Specifically, the nonbonded interactions 1,1 '(CH3)-7(CH), 1,1 '(CHa)-8(CH), 5(CHj)-7(CH), 5(CH3)-8(CH) were included; other interactions are independent of orientation and/or make negligible contributions. Nuclear magnetic double resonance methods were used to determine long-range coupling constants and nuclear Overhauser enhancements (11, 12). All measurements were made on a Varian HA-100 spectrometer* operating in the frequencysweep mode with either a capillary of hexamethyldisiloxane (HMDS) or H2SO4, or internal HMDS (about 1-2% v/v), serving as the lock signal. Samples were made up in dez
7,-
A, 2'H
H
/
x
FIG. 1. Torsional angle s in fl-ionone; o is defined as the angle between the C4-C5==C6--C1 portion of thef#-ionone ring and the C7==Cs-C,9=O plane. * A variable receiver gain modification was added to the V4311 rf unit to allow attenuation of strong signals and prevent receiver saturation.
Abbreviation: PCILO, perturbative configuration interaction with localized orbitals.
1289
Chemistry: Honig et al.
1290
Proc. Nat. Acad. Sci. USA 68 (1971)
(a)
-150) E 0 "I
(
(b) Hz -'-1k-
10
LU
z >
(c) f 5s,- s-cis
s-trans I
00
FIG.
400
1600 800 1200 TORSIONAL ANGLE
I~~
2000
2600
2. Semi-empirical potential for the ring torsional angle +' calculated as outlined in text.
uterated solvents (about 0.5 M), except when CH2Cl2 was used, and were thoroughly degassed. The ambient temperature in the spectrometer was 32 4 10°C. All enhancements were determined from the relative peak areas of the observed signal with the irradiating field H2 first off-resonance and then on-resonance for the irradiated line. The amplitude of H2 was set to give maximum enhancement and then held fixed throughout the experiment. Since all the coupling constants are smaller than any of the chemical-shift differences between the irradiated and observed resonances, the observed enhancements result entirely from intramolecular dipole-dipole interactions (11), and total peak areas could be used for the resonances that appear as multiplets. Where resonances overlapped (only in all-trans retinal), the contributions to the composite resonance could be calculated from other peaks in the spectrum. All spectra were first-order for the protons considered here, with the exception of the AB character of the 7H,8H pair in all three compounds, and the approximate ABX character of 10H, 11H, and 12H in 11-cis retinal. The spectral assignments for the retinal resonances agree with those of Patel (4). The ,3-ionone and all-trans retinal were obtained from the Eastman Chemical Co.; the 11-cis retinal was a generous gift from Mr. P. K. Brown.
(f)
FIG. 3. NMR resonances for 6-ionone: (a) upfield peak of 7H doublet, (b) downfield peak of 8H doublet, (c) 7H doublet with simultaneous irradiation of 5HC,, (d) 5CH3 resonance, (e) 7H doublet with simultaneous irradiation of 9CHa, (f) 5CH3 resonance with simultaneous irradiation of 7H.
functions are obtained for all-trans and 11-cis retinal. It is clear that two minima, which differ somewhat in energy, are predicted, although the accuracy of the calculations is not sufficient to be certain that the relative values are correct. One of the minima (400 < 0 120°) is rather broad and flat, while the other (q _ 180°) is narrow and sharp. They correspond, respectively, to an s-cis conformation around the C6-C7 "single" bond that minimizes the 5(CHs)-8(CH) repulsion, and an s-trans conformation with strong 1,1'(CH3)8(CH) interactions that are smallest when the 8(CH) bond RESULTS bisects the 1 (CH3)-C,-1'(CH2) angle. The pertinent NMR results for fl-ionone, all-trans retinal, Fig. 2 shows the semi-empirical potential-energy function for and 11-cis retinal can be divided into two complementary the ring torsional angle, +, in #-ionone; essentially identical TABLE 1. Coupling-costant values
7H-4,4'CH2
1.60 i 1.35 + 1.40 0.85 + 0.75 i 0.90 0.85
7H-5CHa
5CH34,4'CH2
7H-9CHg
Compound
Temperature
0.05 0.05
f-ionone 6-ionone 11-cis
0.05 0.05
fl-ionone
amb* -51 OC amb amb amb amb amb amb amb
J(Hz)
Groups
0.80
