Continuing their application of laser ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy and computational chemistry to biochemical molecules (see these previous posts), the Alonso group reports on the structure of the glycine-glycine dipeptide 1.¹ The microwave spectrum shows three different conformers. MP2/6-311++G(d,p) computations, the same method they have previously utilized for predicting geometries, revealed a number of different conformations. By matching the spectroscopic parameters obtained from the spectrum with those of the computed structures, they proposed the three conformations 1a, 1b, and 1c, shown in Figure 1.

1a

1b

1c

Figure 1. ωb97xd/6-31G(d) optimized structures of the three conformers of 1.
Note that the authors did not report their structures in their supporting materials(!) so I have optimized them.

The structures of conformers 1a and 1b are nearly planar. MP2 predicts a non-planar rotomer of 1a, which brings the carboxyl group out of plane, to be the lowest conformation in terms of electronic energy. With the M06-2x functional, this non-planar rotomer is about isoenergetic with 1a. With all computational levels 1a is the lowest in free energy. The barrier for rotation between the non-planar rotomer and 1a is very small, and this explains why it is not observed in the supersonic expansion.

References

1) Cabezas, C.; Varela, M.; Alonso, J. L., "The Structure of the Elusive Simplest Dipeptide Gly-Gly." Angew. Chem. Int. Ed. 2017, 56, 6420-6425, DOI: 10.1002/anie.201702425.

InChIs

1: InChI=1S/C4H8N2O3/c5-1-3(7)6-2-4(8)9/h1-2,5H2,(H,6,7)(H,8,9)
InChIKey=YMAWOPBAYDPSLA-UHFFFAOYSA-N

The Alonso group has yet again (see these posts) determined the gas-phase structure of an important, biologically significant molecule using a combination of exquisite microwave spectroscopy and quantum computations. This time they examine the structure of histidine.¹

They optimized four conformations of histidine, as its neutral tautomer, at MP2/6-311++G(d,p). These are schematically drawn in Figure 1. Conformer 1a is the lowest in free energy, likely due to the two internal hydrogen bonds. Its structure is shown in Figure 2.

Figure 1. The four conformers of histidine. The relative free energy (MP2/6-311++G(d,p)) in kcal mol^-1 are also indicated.

Figure 2. MP2/6-311++G(d,p) optimized geometry of 1a.

The initial experimental rotation constants were only able to eliminate 1b from consideration. So they then determined the quadrupole coupling constants for the ¹⁴N nuclei. These values strongly implicated 1a as the only structure in the gas phase. The agreement between the experimental values and the computed values at MP2/6-311++G(d,p) was a concern, so they rotated the amine group to try to match the experimental values. This lead to a change in the NHCC dihedral value of -16° to -23° Reoptimization of the structure at MP2/cc-pVTZ led to a dihedral of -21° and overall excellent agreement between the experimental spectral parameters and the computed values.

It is somewhat disappointing the supporting materials does not include the structures of the other three isomers, nor the optimized geometry at MP2/cc-pVTZ.

References

1) Bermúdez, C.; Mata, S.; Cabezas, C.; Alonso, J. L. "Tautomerism in Neutral Histidine," Angew. Chem. Int. Ed. 2014, 53, 11015-11018, DOI: 10.1002/anie.201405347.

InChIs

Histidine: InChI=1S/C6H9N3O2/c7-5(6(10)11)1-4-2-8-3-9-4/h2-3,5H,1,7H2,(H,8,9)(H,10,11)/t5-/m0/s1
InChIKey=HNDVDQJCIGZPNO-YFKPBYRVSA-N

Here are a couple of articles describing computational approaches to catalytic enantioselective reactions using variations upon the classic proline-catalyzed aldol reaction of List and Barbas¹ that started the whole parade. I have discussed the major computational papers on that system in my book (Chapter 5.3).

Yang and Wong² investigated the proline-catalyzed nitro-Michael reaction, looking at four examples, two with aldehydes and two with ketones (Reactions 1-4).

These four reactions were examined atMP2/311+G**//M06-2x/6-31G**, and PCM was also applied. The key element of this study is that they examined two different types of transition states: (a) based on the Houk-List model involving a hydrogen bond and (b) an electrostatic based model with no hydrogen bond. These are sketched in Scheme 1. For each of the reactions 1-4 there are 8 located transition states differing in the orientation of the attack on to the syn or anti enamine.

Scheme 1. TS models

The two lowest energy TS are shown in Figure 1. TS1-β1-RS is the lowest TS and it leads to the major enantiomer. The second lowest TS, TS1-β3-SR, lies 2.9 kJ mol^-1 above the other TS, and it leads to the minor enantiomer. This lowest TS is of the Houk-List type (Model A) while the other TS is of the Model B type. The enthalpies of activation suggest an ee of 54%, in reasonable
agreement with experiment.

TS1-β1-RS

TS1-β1-RS

Figure 1. M06-2x/6-31G** optimized geometries of TS1-β1-RS and TS1-β1-RS.

The computations of the other three reactions are equally good in terms of agreement with experiment, and importantly the computations indicate the reversal of stereoselection between the aldehydes and the ketones. These computations clearly implicate both the Houk-List and the non-hydrogen bonding TSs in the catalyzed Michael additions.

Houk in collaboration with Scheffler and Mahrwald investigate the use of histidine as a catalyst for the asymmetricaldol reaction.³ Examples of the histidine-catalyzed aldol are shown in Reactions 5-7.

	Reaction 5
	Reaction 6
	Reaction 7

The interesting twist here is whether the imidazole can also be involved in hydrogen bonding to the acceptor carbonyl group, serving the purpose of the carboxylic acid group in the Houk-List TS when proline is the catalyst (Model A). The transition states for these and a few other reactions were computed at M06-2x/6-31+G(d,p) including with the SMD continuum solvent model for water. The two lowest energy TSs for the reaction of isobutyraldehde and formaldehyde are shown in Figure 2; TS1 has the carboxylic acid group as the hydrogen donor while the imidazole is the donor in TS2. Of note is that these two TS are isoenergetic, indicating that both modes of stabilization are at play with histidine as the catalyst.

TS1

TS2

Figure 2. M06-2x/6-31+G(d,p) geometries of the two lowest energy TSs for the reaction of isobutyraldehde and formaldehyde catalyzed by histidine.

The possible TSs for Reactions 5-7 were also located. For example, with Reaction 5, the lowest energy TS involves the imidazole as the hydrogen donor and it leads to the major product. The lowest energy TS that leads to the minor product involves the carboxylic acid as the donor. The computed ee’s for Reactions 5-7 are in very good, if not excellent, agreement with the experimental values. The study should spur further activity in which one might tune the stereoselectivity by using catalysts with multiple binding opportunities.

References

(1) List, B.; Lerner, R. A.; Barbas, C. F., III; "Proline-Catalyzed Direct Asymmetric Aldol Reactions," J. Am. Chem. Soc., 2000, 122, 2395-2396, DOI: 10.1021/ja994280y.

(2) Yang, H.; Wong, M. W. "(S)-Proline-catalyzed nitro-Michael reactions: towards a better understanding of the catalytic mechanism and enantioselectivity," Org. Biomol. Chem.,
2012, 10, 3229-3235, DOI: 10.1039/C2OB06993H

(3) Lam, Y.-h.; Houk, K. N.; Scheffler, U.; Mahrwald, R. "Stereoselectivities of Histidine-Catalyzed Asymmetric Aldol Additions and Contrasts with Proline Catalysis: A Quantum Mechanical Analysis," J. Am. Chem. Soc. 2012, 134, 6286-6295, DOI: 10.1021/ja2118392

There are three generic conformations of α-amino acids in the gas phase: A-C. These are stabilized by intramolecular hydrogen bonds. While computations suggest that all three are close in energy, the very detailed laser ablation- molecular beam-Fourier transform microwave (LA-MB-FTMW) experiments of the Alonso group (mentioned in these previous posts: guanine, cysteine, ephedrine) have identified only the first two conformations. Cooling of the structures in the jet expansion appears to be the reason for the loss of the (slightly) higher energy conformer C.

Alonso now reports on the structure of 1-aminocyclopropanecarboxylic acid 1.¹ The three MP2/6-311+G(d,p) optimized conformation are shown in Figure 1. The interaction between the cyclopropyl orbitals and the carbonyl π-bond suggests that only two structures (where the carbonyl bisects thecyclopropyl plane) will exist and that rotation between them may require passage through a prohibitively high barrier. In fact, computations suggest a barrier of 2000 cm^-1 (5.7 kcal mol^-1). This is much larger than the typical rotation barrier of the amino acids that interconvert A with C, which are about 400 cm^-1 (1 kcal mol^-1).

1A

1B

1C

After careful examination of the microwave spectrum, all three conformations 1A-C were identified by comparing the experimental value of the rotational constants, and ¹⁴N nuclearquadrupole coupling constants with the computed values. Really excellent agreement is found, including in the ratio of the relative amounts of the three isomers. Once again, we have an exquisite example of the importance of computations and experiments being used in conjunction to solve interesting chemical problems.

References

(1) Jimenez, A. I.; Vaquero, V.; Cabezas, C.; Lopez, J. C.; Cativiela, C.; Alonso, J. L., "The Singular Gas-Phase Structure of 1-Aminocyclopropanecarboxylic Acid (Ac3c)," J. Am. Chem. Soc., 2011, 133, 10621-10628, DOI: 10.1021/ja2033603

InChIs

1: InChI=1/C4H7NO2/c5-4(1-2-4)3(6)7/h1-2,5H2,(H,6,7)/f/h6H
InChIKey=PAJPWUMXBYXFCZ-BRMMOCHJCR

Small energy differences pose a serious challenge for computation. The focal point analysis of Allen and Schaefer is one approach towards solving this problem, with energies extrapolated to the complete basis set limit at the HF and MP2 levels, and then corrections added on for higher-order effects.

These authors have applied 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 1. The HF/CBS estimate places 2 below 1, but this is reveres at MP2. With the correction for CCSD and CCSD(T), and core electrons, the energy gap is only 0.45 kJ mol^-1, favoring 1. Zero-point vibrational energy favors 1 by 1.66 kJ mol^-1, for a prediction that 1 is 2.11 kJ mol^-1 lower in energy than 2. It is interesting that most of this energy difference arises from differences in their ZPVE.

1

2

Figure 1. CCSD(T)/cc-pVTZ optimized geometries of the two lowest energy conformations of alanine.

The article also discusses the structures of these to conformers, obtained through a combination of theoretical treatment and revisiting the limited experimental measurements.

References

(1) Jaeger, H. M.; Schaefer, H. F.; Demaison, J.; Csaszar, A. G.; Allen, W. D., "Lowest-Lying Conformers of Alanine: Pushing Theory to Ascertain Precise Energetics and Semiexperimental R_e Structures," J. Chem. Theory Comput., 2010, 6, 3066-3078, DOI: 10.1021/ct1000236

InChIs

Alanine: InChI=1/C3H7NO2/c1-2(4)3(5)6/h2H,4H2,1H3,(H,5,6)/t2-/m0/s1/f/h5H
InChIKey=QNAYBMKLOCPYGJ-SNQCPAJUDI

Here’s a nice example of the productive interplay between experiment and computations.¹ The dipeptide N-Acyl-Ala-Ala-Benzyl was prepared and subjected to UV and IR/UV analysis. The IR showed two separate structures with distinctly different environments for the NH bonds: one structure showed intramolecular hydrogen bonding while the other did not.

B97/TZVPP computations revealed two structures. The first is a linear dipeptide with intramolecular hydrogen bonding occurring in a 5,5 relationship. (There are actually three conformers of this but all have similar energy, only one is shown in Figure 1.) The second structure displays a bent shape with a NH-π interaction, also shown in Figure 1. The computed vibrational spectra for each structure matches up well with the NH region of the experimental IR.

Figure 1. B97-D/TZVPP optimized structures of N-Acyl-Ala-Ala-Benzyl.

The authors spend a great deal of time noting that the 0 K energies predict that the second structure, being 4 kcal mol^-1 more stable, should be the only one observed. However, since the jet cooling will likely trap the structures at their 300 K distribution, this could account for the existence of two structures. However, when the computations include entropy corrections, so now we’re looking at ΔG(200 K), B97-D and MO6-2x suggest that the two structures are very close in energy. But they caution that MP2 predicts a large energy gap unless atomic counterpoise corrections are used to account for intramolecular basis set superposition (see this post), a problem that appears to be much less severe with the DFT methods.

References

(1) Gloaguen, E.; de Courcy, B.; Piquemal, J. P.; Pilme, J.; Parisel, O.; Pollet, R.; Biswal, H. S.; Piuzzi, F.; Tardivel, B.; Broquier, M.; Mons, M. J. Am. Chem. Soc, 2010, 132, 11860-11863, DOI: 10.1021/ja103996q

InChIs

InChI=1/C15H20N2O4/c1-10(16-12(3)18)14(19)17-11(2)15(20)21-9-13-7-5-4-6-8-13/h4-8,10-11H,9H2,1-3H3,(H,16,18)(H,17,19)/t10-,11-/m0/s1/f/h16-17H
InChIKey=KRIKKPGWLXOEAS-VFIKCTIADD

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 (0.0)	II (4.79)
III (5.81)	IV (5.95)

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

Molecular structures can differ depending on phase, particularly between the gas and solution phase. Kass has looked at the protonation of 4-aminobenzoic acid. In water, the amino is its most basic site, but what is it in the gas phase? The computed relative energies of the protonation sites are listed in Table 1. If one corrects the B3LYP values for their errors in predicting the proton affinity of aniline and benzoic acid, the carbonyl oxygen is predicted to be the most basic site by 5.0 kcal mol^-1, in nice accord with the G3 prediction of 4.1 kcal mol^-1. Clearly, the structure depends on the medium.

Table 1. Computed relative proton affinities (kcal mol^-1) of 4-aminobenzoic acid.

Electrospray of 4-aminobenzoic acid from 3:1 methanol/water and 1:1 acetonitrile/water solutions gave different CID spectra. H/D exchange confirmed that electrospray from the emthanol/water solution gave the oxygen protonated species while that from the acetonitrile/water solution gave the ammonium species.

References

(1) Tian, Z.; Kass, S. R., “Gas-Phase versus Liquid-Phase Structures by Electrospray Ionization Mass Spectrometry,” Angew. Chem. Int. Ed., 2009, 48, 1321-1323, DOI: 10.1002/anie.200805392.

InChIs

4-aminobenzoic acid: InChI=1/C7H7NO2/c8-6-3-1-5(2-4-6)7(9)10/h1-4H,8H2,(H,9,10)/f/h9H
InChIKey=ALYNCZNDIQEVRV-BGGKNDAXCD

Following on their prediction that the thiol of cysteine¹ is more acidic than the carboxylic acid group (see this post), Kass has examined the acidity of tyrosine 1.² Which is more acidic: the hydroxyl (leading to the phenoxide 2) or the carboxyl (leading to the carboxylate 3) proton?

Kass optimized the structures of tyrosine and its two possible conjugate bases at B3LYP/aug-cc-pVDZ, shown in Figure 1, and also computed their energies at G3B3. 2 is predicted to be 0.2 kcal mol^-1 lower in energy than 3 at B3LYP and slightly more stable at G3B3 (0.5 kcal mol^-1). However, both computational methods underestimate the acidity of acetic acid more than that of phenol. When the deprotonation energies are corrected for this error, the phenolic proton is predicted to be 0.4 kcal mol^-1 more acidic than the carboxylate proton at B3LYP and 0.9 kcal mol^-1 more acidic at G3B3.

1
2	3

Figure 1. B3LYP/aug-cc-pVDZ optimized structures of tyrosine 1 and its two conjugate bases 2 and 3.²

Gas phase experiments indicate that deprotonation of tyrosine leads to a 70:30 mixture of the phenoxide to carboxylate anions. The computations are in nice agreement with this experiment. (A Boltzmann weighting of the computed lowest energy conformers makes only a small difference to the distribution relative to using simply the single lowest energy conformer.) This demonstrates once again the important role of solvent, since only the carboxylate anion is seen in aqueous solution.

References

(1) Tian, Z.; Pawlow, A.; Poutsma, J. C.; Kass, S. R., "Are Carboxyl Groups the Most Acidic Sites in Amino Acids? Gas-Phase Acidity, H/D Exchange Experiments, and Computations on Cysteine and Its Conjugate Base," J. Am. Chem. Soc., 2007, 129, 5403-5407, DOI: 10.1021/ja0666194.

(2) Tian, Z.; Wang, X.-B.; Wang, L.-S.; Kass, S. R., "Are Carboxyl Groups the Most Acidic Sites in Amino Acids? Gas-Phase Acidities, Photoelectron Spectra, and Computations on Tyrosine, p-Hydroxybenzoic Acid, and Their Conjugate Bases," J. Am. Chem. Soc., 2009, 131, 1174-1181, DOI: 10.1021/ja807982k.

InChIs

1: InChI=1/C9H11NO3/c10-8(9(12)13)5-6-1-3-7(11)4-2-6/h1-4,8,11H,5,10H2,(H,12,13)/t8-/m0/s1/f/h12H
InChIKey=OUYCCCASQSFEME-QAXLLPJCDY

2: InChI=1/C9H11NO3/c10-8(9(12)13)5-6-1-3-7(11)4-2-6/h1-4,8,11H,5,10H2,(H,12,13)/p-1/t8-/m0/s1/fC9H10NO3/q-1
InChIKey=OUYCCCASQSFEME-HVHKCMLZDU

3: InChI=1/C9H11NO3/c10-8(9(12)13)5-6-1-3-7(11)4-2-6/h1-4,8,11H,5,10H2,(H,12,13)/p-1/t8-/m0/s1/fC9H10NO3/h11h,12H/q-1
InChIKey=OUYCCCASQSFEME-XGYCJDCADS

The gas phase structure of the amino acids is in their canonical or neutral form, while their aqueous solution phase structure is zwitterionic. An interesting question is how many water molecules are needed to make the zwitterionic structure more energetically favorable than the neutral form. For glycine, it appears that seven water molecules are needed to make the zwitterion the favorable tautomer.^1,2

Arginine, on the other hand, appears to require only one water molecule to make the zwitterion lower in energy than the neutral form.³ The B3LYP/6-311++G** structures of the lowest energy neutral (1N) and zwitterion (1Z) cluster with one water are shown in Figure 1. The zwitterion is 1.68 kcal mol^-1 lower in energy. What makes this zwitterion so favorable is that the protonation occurs on the guanidine group, not on the amine group. The guanidine group is more basic than the amine. Further, the water can accept a proton from both nitrogens of the guanidine and donate a proton to the carboxylate group.

1N (1.68)

1Z (0.0)

Figure 1. B3LYP/6-311++G** structures and relative energies (kcal mol^-1) of the lowest energy arginine neutral (1N) and zwitterion (1Z) cluster with one water.³

References

(1) Aikens, C. M.; Gordon, M. S., "Incremental Solvation of Nonionized and Zwitterionic Glycine," J. Am. Chem. Soc., 2006, 128, 12835-12850, DOI: 10.1021/ja062842p.

(2) Bachrach, S. M., "Microsolvation of Glycine: A DFT Study," j. Phys. Chem. A, 2008, 112, 3722-3730, DOI: 10.1021/jp711048c.

(3) Im, S.; Jang, S.-W.; Lee, S.; Lee, Y.; Kim, B., "Arginine Zwitterion is More Stable than the Canonical Form when Solvated by a Water Molecule," J. Phys. Chem. A, 2008, 112, 9767-9770, DOI: 10.1021/jp801933y.

InChIs

1: InChI=1/C6H14N4O2/c7-4(5(11)12)2-1-3-10-6(8)9/h4H,1-3,7H2,(H,11,12)(H4,8,9,10)/f/h8,10-11H,9H2
InChIKey=ODKSFYDXXFIFQN-MYOKTFMPCK