Effect of protonation on the ground state properties of retinal analogs: an ab-initio study

Abstract

. Can. J. Chem. 65, 892 (1987). The ground state properties (bond lengths, bond orders, net atomic charge distribution) of various cis and trans isomers of retinal analogs have been studied at ab initio SCF and correlated levels. The effect of protonation on the properties of the retinal Schiff base analogs has also been studied. Convergence in various properties has been observed with increasing chain length justifying the use of smaller analogs mimicking retinal. Convergence is, however, slower for the protonated retinal Schiff base analogs. The protonated retinal Schiff base analogs show an increased conjugation reflected in the bond orders, bond lengths, and in the decrease in the HOMO-LUMO gap. The nodal characteristics of the HOMO of the protonated retinal Schiff base analog also indicate a strong conjugation in the vicinity of the protonated nitrogen. Based on the results of calculations at the correlated level, the sudden polarization charge transfer mechanism is shown to be an artifact of HF-based methods used in the calculations. Utilisant des mCthodes ab initio en champ auto-cohhent et a des niveaux permettant d'Ctablir une corrtlation, on a CtudiC les propriCtCs a 1'Ctat fondamental (longueurs des liaisons, ordres des liaisons, distributions des charges atomiques nettes) des divers isomkres cis et trans d'analogues du retinal. On a aussi CtudiC l'effet de la protonation sur les propriCtts d'analogues des bases de Schiff du rCtinal. On a observk une convergence des diverses proprittts avec une augmentation de la longueur de la chaine et cette observation justifie l'utilisation d'analogues plus petits qui reproduisent les proprittCs du rCtinal; toutefois, la convergence est plus lente pour les analogues protonCs des bases de Schiff du retinal. Les analogues protones des bases de Schiff du rCtinal prksentent une plus grande conjugaison qui se rkflkte dans les ordres et les longueurs des liaisons et par une diminution de la diffkrence entre la haute occupCe et la basse vacante. Les caracttristiques nodales de la haute occupte de I'analogue proton6 de la base de Schiff du rttinal suggkrent qu'il existe une forte conjugaison au voisinage de I'azote protont. En se basant sur les rCsultats de ces calculs aux niveaux oh l'on peut ttablir une corrklation, on dCmontre que la polarisation soudaine dans le mkcanisme de transfert de charge n'a pas de signification physique et qu'elle est inhCrente aux mkthodes basCes sur HF qui sont utilistes dans les calculs.[Traduit par la revue]

Factors That Influence the CN Stretching Frequency in Imines

“…Compared with the early HF/STO-3G results by Poirier et al., the GVB(6/12)/6-31G*-optimized geometries for the polyimines have much shorter bond lengths, especially in the CN bonding region. Protonation causes a similar increase in conjugation along the chain, which is reflected by the equalization of the single and double C−C bond lengths.…”

“…For the unprotonated imines with n = 1 to n = 6, the CN bond length is essentially constant. On the other hand, for the protonated case, the CN bond length increases from n = 0 to n = 3, in agreement with previous results . Therefore, the change in the CN bond length upon protonation increases with chain length, from −0.001 to +0.022 Å, for n = 0 to n = 3, and remains constant from thereon.…”

Atomic and molecular analysis highlights the biophysics of unprotonated and protonated retinal in UV and scotopic vision

“…31,32 Delocalization depends strongly on the bond order, bond length, and πand σ-bond energy, and therefore, the UR is less delocalized and has a shorter bond length and higher bond order. Moreover, the PR increases the electron delocalization and exhibits a bathochromic shift.…”

“…Moreover, the PR increases the electron delocalization and exhibits a bathochromic shift. 31,32 Delocalization depends strongly on the bond order, bond length, and πand σ-bond energy, and therefore, the UR is less delocalized and has a shorter bond length and higher bond order. The additional proton makes the PR acidic in nature; nevertheless, because of its higher polarizability and electronegativity as well as its lower hardness and large size, it can be considered to be a "soft acid".…”

Carotenoids and polyterpenoids

“…448 The properties of the ground states of various (Z)-and (@-isomers of retinal analogues, and the effect of protonation of the retinal Schiff bases, have been studied at ab initio SCF and correlated levels. 449 The differential-pulse-polarographic behaviours of (all-E)-retinol, retinal, retinoic acid, and retinyl acetate have been A range of physicochemical properties of retinyl palmitate have been For the oxidation of retinyl acetate, a mechanism has been proposed in which intramolecular and intermolecular chain propagation occur with migration of the reaction centre from one end of the polyene system to the other, and Kinetic parameters have been determined for the autoxidation of retinal in the solid state at 25 "C and 20 kPa in the presence of agents such as 2-t-b~tyl-4-methoxyphenol.~~~ Thermal degradation of retinyl palmitate in hexane at 60 "C gave a product that was not identified. 454 The mechanism of fluorescence quenching of indole by retinal and its Schiff bases has been studied by luminescence and pulse f l u o r ~r n e t r y .…”