Schaefer, Csaszar, and Allen have applied the focal point method towards predicting the energies and structures of cysteine.1 This very high level method refines the structures that can be used to compare against those observed by Alonso2 in his laser ablation molecular beam Fourier transform microwave spectroscopy experiment (see this post). They performed a broad conformation search, initially examining some 66,664 structures. These reduced to 71 unique conformations at MP2/cc-pvTZ. The lowest 11 energy structures were further optimized at MP2(FC)/aug-cc-pV(T+d)Z. The four lowest energy conformations are shown in Figure 1 along with their relative energies.
 I |
 II |
 III |
 IV |
Figure 1. MP2(FC)/aug-cc-pV(T+d)Z optimized geometries and focal point relative energies (kJ mol-1) of the four lowest energy conformers of cysteine.1
The three lowest energy structures found here match up with the lowest two structures found by Alonso and the energy differences are also quite comparable: 4.79 kJ and 5.81 mol-1 with the focal point method 3.89 and 5.38 kJ mol-1 with MP4/6-311++G(d,p)// MP2/6-311++G(d,p). So the identification of the cysteine conformers made by Alonso remains on firm ground.
References
(1) Wilke, J. J.; Lind, M. C.; Schaefer, H. F.; Csaszar, A. G.; Allen, W. D., "Conformers of Gaseous Cysteine," J. Chem. Theory Comput. 2009, DOI: 10.1021/ct900005c.
(2) Sanz, M. E.; Blanco, S.; López, J. C.; Alonso, J. L., "Rotational Probes of Six Conformers of Neutral Cysteine," Angew. Chem. Int. Ed. 2008, 4, 6216-6220, DOI: 10.1002/anie.200801337
InChIs
Cysteine:
InChI=1/C3H7NO2S/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1
InChIKey: XUJNEKJLAYXESH-REOHCLBHBU
Computational Organic Chemistry » Conformers of Alanine responded on 11 Jan 2011 at 5:59 pm #
[…] the method to the conformations of alanine (similar to their previous study on cysteine – see this post).1 There are two low energy conformers 1 and 2. The CCSD(T)/cc-pVTZ structures are shown in Figure […]