Exploring the Conformational Space of Cysteine by Matrix Isolation

Article pubs.acs.org/JPCB

Exploring the Conformational Space of Cysteine by Matrix Isolation Spectroscopy Combined with Near-Infrared Laser Induced Conformational Change Eszter E. Najbauer, Gábor Bazsó, Sándor Góbi, Gábor Magyarfalvi, and György Tarczay* Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, P.O. Box 32 H-1518, Budapest 112, Hungary S Supporting Information *

ABSTRACT: Six conformers of α-cysteine were identified by matrix isolation IR spectroscopy combined with NIR laser irradiation. Five of these conformers are identical with the five out of six conformers that have recently been identified by microwave spectroscopy. The sixth conformer observed in the present study is a short-lived conformer, which decays by H-atom tunneling; its half-life in a 12 K N2 matrix is (1.1 ± 0.5) × 103 s. This study proves that matrix isolation IR spectroscopy combined with NIR laser irradiation is a suitable method to identify conformers of a complex system for which computations predict several dozens of conformers, and that the reliability of this method for conformational assignment is comparable to that of microwave spectroscopy.

1. INTRODUCTION Although among small flexible molecules α-amino acids have probably the most thoroughly studied conformational space, the computationally predicted and the experimentally observed conformers are still not always completely consistent with each other. The conformational studies on these systems have a crucial importance, because their relatively small size allows the use of high-level, state-of-the art computational methods. In addition, because amino acids can be evaporated, they can be studied in the gas phase using high-resolution spectroscopic methods in molecular beams or in inert matrices. The conclusions drawn from the comparison of these computational and experimental investigations can then be utilized in structural studies of larger biomolecules. The conformers as well as the dependence of various molecular properties on the conformations of gaseous cysteine, the only natural amino acid with a reactive sulfhydryl group, have long been studied by quantum chemical computations.1−11 The number of computationally located conformers rapidly increased in the recent years. Among the most careful conformational searches is the work of Schäfer et al. in 1990 who located 10 conformers at the RHF/4−31G level;1 five years later Gronert and O’Hair identified 42 conformers at the MP2/6-31+G*//RHF/6-31G* level of theory.4 Dobrowolski and co-workers in 2007 have reported 51 conformers at the B3LYP/aug-cc-pVTZ and at the MP2/aug-cc-pVTZ levels of theory.9 The most recent conformational study was published by Császár, Allen and their co-workers.11 They have started conformational searches at the HF/3-21G level and have reoptimized the structures by the MP2(fc)/cc-pVTZ method. At this level, they have located as many as 71 conformers. © 2014 American Chemical Society

Eleven conformers were found to have energies within 10 kJ mol−1 range of the lowest-energy structure. For these 11 conformers, focal-point analysis (FPA)12 was performed to determine accurate energies. The high number of both the total and the low-energy conformers indicate that the solid experimental identification of gaseous cysteine conformers is very challenging. Alonso et al.13 investigated the cysteine conformers in the jet expansion by Fourier-transform microwave (FTMW) spectroscopy. They have identified six low-energy conformers. In addition, they have concluded that two conformers are absent in the jet expansion due to their relaxation to lower energy structures. Besides MW spectroscopy and high-resolution laser spectroscopic methods matrix isolation infrared (MI-IR) spectroscopy is also very often used for conformational studies of small and medium sized molecules. The conformational analysis based on MI-IR spectroscopy utilizes the fact that the gas-phase conformational ratio is maintained in the matrix. Exceptions are molecules with very low conformational barriers (98%) was evaporated into the vacuum chamber using a homebuilt Knudsen effusion cell. The evaporated sample was mixed with argon (Messer, 99.9999%), krypton (Messer, 99.998%), nitrogen (Messer, 99.999%) before deposition. The gas flow was kept at ∼0.07 mmol min−1; the evaporation temperature was ∼413 ± 5 K. The sample−rare gas mixture was deposited onto a cold (8−10 K for MIR, 12−14 K for NIR) CsI window, mounted on a Janis CCS-350R cold head cooled by a CTI Cryogenics 22 closed-cycle refrigerator unit. The temperature of the cold window was controlled by a Lake Shore 321 thermostat equipped with a silicon diode thermometer. The cold window was set at 45° to the optical path of the spectrometer and the irradiating laser beam was perpendicular to the optical path. All MI-IR spectra were recorded on a Bruker IFS 55 spectrometer using an MCT detector with a tungsten lamp for the 2500−8000 cm−1 (NIR) and with a Globar source for the 580−4000 cm−1 (MIR) spectral region. The spectra were recorded at 1 cm−1 instrumental resolution. For the measurement of overtones in the NIR region, at least 1000 scans were accumulated, while in the MIR spectral region spectra consisted of at least 256 scans. Conformational changes were selectively induced by an optical parametric oscillator (VersaScan MB 240 OPO, GWU/ Spectra Physics) pumped with the third harmonic (355 nm) of a pulsed (10 Hz, 2−3 ns) Quanta Ray Lab 150 Nd:YAG laser (Spectra Physics). The line width of the idler (NIR) output of the OPO was about 5 cm−1, pulse energies were 10−15 mJ. The laser beam was unfocused; its diameter was about 0.8 cm. The OPO was calibrated in preliminary experiments by optimizing for the shortest bleaching time of an irradiated species monitored by FT-IR measurements. A few hours of NIR irradiation was applied. This is comparable with and a 2094

dx.doi.org/10.1021/jp412550q | J. Phys. Chem. B 2014, 118, 2093−2103

The Journal of Physical Chemistry B

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slightly longer than the irradiation time applied in our former experiment on alanine.26 For the lifetime measurements of short-lived conformers (in N2 matrix) 160 scans were recorded every 5 min. An LPW 3860 low pass filter was placed between the source and the cold window to prevent conformational conversion via the excitation of the overtone modes caused by the Globar source. In order to check if the MIR radiation of the source accelerates the decay, lifetime measurements were also carried out blocking the IR source between the measurements. 2.2. Computational Details. Quantum chemical calculations were performed using the PQS (Parallel Quantum Solutions) 3.272,73 and by the Gaussian0374 program packages. Initial geometries for geometry optimizations were roughly set to the previously reported structures and were optimized at the B3LYP75,76/6-31++G**77−80 level of theory. The optimizations were followed by second derivative calculations to determine whether the obtained stationary points correspond to minima. The barrier heights between conformers were computed at the B3LYP/6-31++G**79 and the MP2/6-311++G** levels of theory. Harmonic vibrational frequencies and intensities were calculated at the B3LYP/6-31++G** level of theory using the scaled quantum mechanical (SQM) force field scheme81,82 with scaling factors determined by Fábri et al.83

Figure 1. The 13 low-energy conformers (1−13) and conformer 14 of cysteine.

3. RESULTS AND DISCUSSION In the case of cysteine, there is no common notation for the conformers. In order to make the discussion easy we have correlated the lowest-energy structures with different notations of the most recent publications in Table 1. In the present paper, we use the notation of Dobrowolski et al.41 supplementing it with the notation of the backbone structure in brackets. For the latter the conformation labels of glycine introduced by Császár84 are applied. Because of interaction of the −SH group with the −NH2 or −COOH group the backbone of some cysteine conformers is significantly distorted compared to the glycine conformers. In these cases the apostrophe symbol after the backbone label is used, for example, I′ or III′. In addition to the lowest-energy conformers, a higher energy conformer with backbone VI, 14 (VI), is also included in Table 1. As it is discussed below, this is a short-lived conformer, which is expected to appear during NIR laser irradiation. The structures included in Table 1 are visualized in Figure 1. In Table 1 the computed zero-point vibrational relative energies (ΔG°0 K) at the B3LYP/6-31++G** and at the FPA levels, Gibbs free energies (ΔG°413 K) at the sample evaporation temperature, obtained as the sum of ΔG°0 K values and thermal corrections computed at the B3LYP/6-31++G** level, as well as the populations computed from ΔG°413 K are also listed. As it is seen from Table 1, 13 conformers are predicted with more than 2% population in the gas phase at 413 K. The thermal correction considerably decreases the relative Gibbs free energy of conformers with backbones I and III compared to the other conformers. Figure 2 presents the first O−H, N−H, and S−H stretching overtone region of the MI-NIR spectra of cysteine recorded in Ar, Kr, and N2 matrices. The wavenumbers of the NIR laser used to irradiate the matrix in different experiments are shown by arrows in Figure 2. Among these only the irradiations of the first overtone of the O−H stretching were effective enough, and these are discussed in detail. We will discuss the MIR spectrum recorded after deposition in Ar matrix (Figure 3), the difference

Figure 2. MI-NIR spectrum of cysteine in Ar matrix. The arrows show the wavenumber of the laser light in the different irradiation experiments. The red, blue, and the green arrows belong to the three irradiation experiments discussed in details in the text. (The same color code is used in Figure 4.)

Figure 3. MI-MIR spectrum of cysteine in Ar matrix.

spectra (i.e., the spectra obtained by subtracting the spectrum recorded before the NIR irradiation from the spectrum recorded after the irradiation) in Ar matrix (Figure 4), and 2095



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In the region of the MI-NIR spectra, where the first O−H stretching overtones for backbone I (and III) conformers of glycine and alanine were identified, not surprisingly only a wide, unresolved band was observed in the case of cysteine (Figure 2). Because of this, the completely selective irradiation of the individual conformers was not possible in this region. By shifting the wavelength of the NIR laser irradiation, however, significantly different changes in the amount of the conformers could be achieved, depending on the overlap of the bands of different conformers with the laser band. Comparing the difference MI-MIR spectra (Figure 4) obtained by irradiating the high- and the low-wavenumber side of the O−H overtone band at 6938 and 6953 cm−1, two groups of bands could be identified. Within the groups, the intensity of each band changed together approximately to the same extent. The intensity of the first group of bands decreased more when irradiating the low-wavelength side of the O−H overtone band, while the intensity of the second group of bands decreased more when irradiating the high-wavelength side of the O−H overtone band. Of the two groups, the first could be assigned to conformer 5 (I) (Table 2), and the second could be assigned to conformer 3 (I′) (Table 3). If the two band groups were tentatively assigned to other backbone I and I′ conformers, namely the low-energy conformers 3 and 7 for the first group of bands, and 5 and 7 for the second group, the RMS error was considerably larger, and in each case there were bands with very large deviations (see Tables in the Supporting Information). The consistency between the calculated and the measured intensities also supports the assignation. Thoroughly examining the two irradiation experiments, we found bands whose intensity was increased by the 6953 cm−1 and decreased by the 6938 cm−1 NIR laser irradiation (see Figure 4). On the basis of a comparison with the SQM wavenumbers these bands are assigned to conformer 9 (III). As it can be seen from Table 4, seven strong or medium intensity bands of this conformer could be identified. Although some of these bands are in the vicinity of stronger features of the difference spectra assigned to other conformers, some of them are very well visible and well resolved, for example, the bands at 1124.2 and 1093.9 cm−1. This observation is consistent with the findings for glycine, that is, the irradiation of conformer Ip in a site in which the O−H stretching overtone band appears at higher wavenumber increased the amount of IIIp; while irradiation in a site in which the same transition appears at a lower wavenumber bleaches the bands of conformer IIIp. The latter observation and the decreasing intensity of the bands of conformer 9 of cysteine due to the 6938 cm−1 irradiation is the result of the overlap between the laser light (with a bandwidth of a few cm−1) and the O−H stretching overtone of conformer 9. The identification of the conformers with backbone conformation II is more difficult. The main reason for this is that during the irradiations at the O−H stretching overtones of the backbone I conformers the irradiated conformers can convert into many, possibly even a few dozen, other conformers whose bands do not overlap with the irradiating light. Even spectrally overlapping conformers could show some increase if their original abundance was low. Conformers that were absent in the deposited matrix may also show up. The O−H stretching overtones of conformers with backbone conformation II are wide because of the strong intramolecular H-bonds. Consequently, the irradiation of these bands (Figure 4) is less effective compared to the irradiation of O−H stretching

Figure 4. Difference MI-MIR spectra of cysteine in Ar matrix as obtained by subtracting the spectrum recorded before irradiation from the spectrum recorded after irradiation for 240 min at 6953 cm−1 (a), for 135 min at 6938 cm−1 (b), and for 180 min at 6588 cm−1 (c). In the case of weak bands, the conformational assignment is not shown. Two medium intensity unassigned bands are marked by asterisks, see text.

the difference spectra used in the identification of a short-lived conformer in N2 matrix (Figure 5). Further spectra are presented in the Supporting Information. Except for small traces of ethanol (coming from the sample and most visible in Ar at 3573 cm−1), the MI-MIR spectrum recorded after the deposition in Ar matrix (Figure 3) is almost identical to the spectrum published by Dobrowolski et al.41

Figure 5. Difference MI-MIR spectra of cysteine in N2 matrix as obtained by subtracting the spectrum recorded right after irradiation at 6925 cm−1 from the spectrum recorded after 15 min in the dark after the end of irradiation. The negative peaks are assigned to conformer 14, positive bands belong to conformer 3. 2096



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Table 2. Computed (SQM B3LYP) Fundamental Frequencies (ν̃ in cm−1), Intensities (I in km mol−1) and Experimental Vibrational Transitions (ν̃ in cm−1) of the Conformer 3 of Cysteinea computed ν̃/cm

−1

Ar matrix −1

I/(km·mol )

3565.9 3424.4 3339.8 3006.2 2963.3 2937.7 2588.4 1752.2

66 10 4 4 6 13 5 316

1633.4 1442.8 1394.1 1320.2 1296.1 1276.6 1225.0 1139.9 1122.1 1097.9 991.6 918.4 897.5 834.5 786.0 756.3 688.6 642.1 584.8 504.6 380.6 321.3 297.9 235.5 211.2 167.7 98.1 47.4

36 6 7 52 11 1 7 75 53 157 28 5 35 116 30 16 39 12 93 13 20 36 10 7 13 1 3 2

ν̃/cm 3562.5vs 3405.3w

Table 3. Computed (SQM B3LYP) Fundamental Frequencies (ν̃ in cm−1), Intensities (I in km mol−1) and Experimental Vibrational Transitions (ν̃ in cm−1) of the Conformer 5 of Cysteinea

Kr matrix

−1

ν̃/cm

computed

−1

ν̃/cm

3547.4vs 3413.7w

3566.8 3417.9 3335.0 3032.2 2972.7 2888.8 2591.5 1760.4 1629.6 1425.2 1364.7 1337.0 1296.8 1272.1 1238.0 1159.8 1121.6 1100.5 1025.8 924.6 888.0 832.2 772.0 721.8 674.0 626.4 571.2 489.1 448.4 340.6 291.6 243.2 227.4 184.1 85.6 40.6

2955.8w 2961.44w 2915.4w 2583.3w 2603.8w 1769.0vs, 1779.1mc 1763.3vs, 1767,7m, 1773.0wc

1335.1wb

1235.4w 1147.5m 1121.2mb 1107.6vs 996.7w

1234.9w 1146.8m, 1143.3mc 1125.5m, 1134.9wc 1006s, 1090.6mc 977.9m

897.9w 819.6s 788.6s-m

902.6w 819.6s 799.3m 767.7m 679.9w

679.8m

−1

592.6m

Ar matrix −1

Kr matrix

−1

ν̃/cm−1

I/(km·mol )

ν̃/cm

61 12 3 2 9 13 7 296 42 9 2 13 17 8 1 16 21 201 71 5 120 40 5 24 2 102 25 10 11 27 33 21 22 0 1 2

3555.3vs 3420.4w

3545.0vs 3398.0w

2894.4w 2612.3w 1766.1vs

2590.8w 1777.1s, 1773.4w, 1768.7sc

1344.0wb

1148.8w 1137.5mb 1107.3vs 1023.1m

1147.8m 1134.0m 1106.0vs 1028.2m

876.0m 832.1m 769.5m 733.4m

873.3m 829.9m, 827.7m

618.1m

608.1m

733.0w

a See footnote to Table 2. bUncertain wavenumber and intensity due to nearby bands of other conformers. cSite split band.

a

Table lists only those bands that are visible in the difference spectra. Low-intensity bands of the as-deposited spectra that could tentatively be assigned to this conformer are not included. vs, very strong; s, strong; m, medium; w, weak; br, broad. The assignment of weak bands should be considered to be tentative. bUncertain wavenumber and intensity due to nearby bands of other conformers. cSite split band.

Irradiation at 6588 cm−1 also proved the assignment of the intensive bands of conformers 5 and 9, which appear clearly as positive bands in the corresponding difference spectra. This does not hold for the bands for conformer 3, whose bands mostly show positive−negative couplets. These couplets most probably indicate that conformer 3 is produced as a result of this irradiation in a site slightly different from the one in the deposited matrix. (Because this irradiation is less effective than the other two, the effect of the slow sublimation of the matrix, which uniformly decreases the amount of each conformer, is less negligible in the difference spectra. As a result of this, the increase of the bands of conformers 5, 9 and especially that of 3 is somewhat less prominent compared to the decrease of the bands of conformers 1 and 2.) As it is mentioned above, further experiments were also carried out in which the first overtone of the N−H and the S− H stretchings were irradiated. In these experiments, no

overtones of conformers with backbone conformation I or III. (The same observation was found in the case of glycine and alanine.) Therefore in the case of identification of conformers 1 (II) and 2 (II′) (see Tables 5 and 6) the results of both type of experiments, that is, irradiation of conformers with backbone I at (6953 and 6938 cm −1) and II (6588 cm −1) were used. As can be seen in Figure 4, irradiation at 6953 cm −1 mostly increased the concentration of conformer 2 and less prominently that of conformers 1 and 9. In contrast to this, irradiation at 6938 cm−1 increased the amount of conformer 1 more than that of conformer 2. Irradiation at 6588 cm−1 depleted conformers 1 and 2 to approximately the same extent. 2097



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Table 4. Computed (SQM B3LYP) Fundamental Frequencies (ν̃ in cm−1), Intensities (I in km mol−1) and Experimental Vibrational Transitions (ν̃ in cm−1) of the Conformer 9 of Cysteinea computed −1

ν̃/cm

3574.5 3436.1 3349.8 3001.7 2980.9 2940.3 2589.0 1755.7 1628.3 1443.4 1380.0 1320.7 1305.0 1275.1 1243.7 1139.6 1121.2 1098.0 979.7 919.7 874.2 820.2 780.3 765.9 696.3 618.8 585.2 532.0 373.3 318.2 288.4 249.2 230.3 163.3 101.7 27.1

Ar matrix −1

I/(km·mol ) 76 10 4 5 3 14 5 333 41 6 10 21 32 10 17 113 114 16 21 5 64 76 49 22 27 54 102 4 15 46 1 9 5 0 1 1

ν̃/cm

−1

c

1764.1b

1128.9s 1124.2s 1093.9m 980.5m

Table 5. Computed (SQM B3LYP) Fundamental Frequencies (ν̃ in cm−1), Intensities (I in km mol−1) and Experimental Vibrational Transitions (ν̃ in cm−1) of the Conformer 1 of Cysteinea

Kr matrix ν̃/cm

computed

−1

ν̃/cm

c

−1

3428.1 3319.0 3288.5 3023.3 2959.7 2912.8 2596.8 1779.1 1615.1 1422.7 1386.2 1366.7 1302.9 1261.5 1218.2 1185.0 1141.0 1082.6 1026.3 948.1 887.8 842.1 822.6 772.6 743.7 673.7 561.7 525.7 494.3 363.8 353.3 297.1 265.1 196.8 88.3 68.1

1740.4sb

1122.5m 1088.6m

862.9m c c

769.8m 618.2m d

a See footnote to Table 2. bUncertain wavenumber and intensity due to nearby bands of other conformers. cAlthough computations predicts medium intensity for these bands, they cannot be identified due to overlap with strong bands of conformer 3. dOutside of the investigated spectral region.

Ar matrix −1

I/(km·mol ) 19 23 252 1 15 9 1 293 39 17 405 10 34 7 2 12 9 18 34 37 70 144 19 1 7 1 3 2 7 12 2 22 26 2 4 3

ν̃/cm

−1

Kr matrix ν̃/cm−1

3286.1br

1785.0vs

1383.9vs 1367.3s 1294.7m 1267.3m

1103.8w 1031.5m 952.2m 880mb 852mb 805mb

1781.4s, 1787.4mb,c 1412.9w, 1417.6wc 1381s 1364.8s 1292.4w, 1287.7wc

1030.7w 881.8wb

743.1m 676.6m

a See footnote to Table 2. bUncertain wavenumber and intensity due to nearby bands of conformer 2. cSite split band.

conformer 7 to conformer 5. As a result of this low barrier, the higher energy conformer can likely convert (classically or by Htunneling) along the C−C−S−H torsion angle to the lower energy conformer faster than the timescale limit of our experiments (∼1 s). In correspondence with this, all the identified conformers have lower energy, than the other, experimentally unidentified, conformers differing only in the C−C−S−H torsion angle (e.g., conformer 5 vs 7). As it follows from the assignments and the discussion above, the irradiation of the O−H stretching overtones most efficiently promoted the 5 → 9, 5 → 1, and the 3 → 2 conformer conversions, while in the case of irradiation of conformers 1 and 2 most probably the reverse processes occur. It is interesting to note that the C−C−C−S torsion angle does not vary during these processes (see Figure 1). One possible explanation for this fact is that the energy pumped into the O− H stretching vibrations does not dissipate into vibrational

considerable changes in the spectrum could be observed, except for the slow sublimation of the matrix, that is, the slow decrease in the intensity of all bands of each conformer. This can most likely be explained by the lower absorption of these overtones and the lower laser power in this spectral region. As was the case with conformers of glycine and alanine differing only in the torsion angle C−C−N−H, we could not observe a conformational change restricted only to the torsion angle C−C−S−H either. The reason for this is the low barrier. As an example only 336 and 346 cm−1 barrier heights from conformer 6 to conformer 4 computed at the B3LYP/6-31+ +G** and the MP2/6-31++G** levels of theory, respectively. Somewhat larger but still low barrier heights, 692 and 697 cm−1, were computed at the same levels of theory from 2098



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slightly increased, while its intensity did not change upon the other two irradiations. According to former experiments, the water−ethanol complex has an intensive band at 3574 cm−1,85 which in experimental uncertainty coincides with the position of the band observed in our experiment. The closest computationally predicted bands of the five identified conformers have been assigned to other nearby peaks, but it cannot be excluded that these two bands belong to these conformers and appear due to a relatively large site splitting. Although according to the computations conformers 4, 6 and 8 also have a band close to 700 cm−1, no other band can exclusively be assigned to these conformers. Moreover, these conformers are expected to be absent in the matrix because, except for a small distortion of the backbone, they differ from conformer 1 or 2 only in the C−C−S−H or the C−C−N−H torsion angle. Except for the bands in the very congested C−H stretching region all the strong to medium bands of the original MI-MIR spectrum recorded in Ar matrix after deposition (Figure 3) are accounted for in Tables 2−6. Inspecting the spectra recorded in Kr matrix (see spectra in the Supporting Information, assignments in Tables 2−6) no other stable conformers could be unambiguously assigned. In addition to these five conformers Alonso et al. have also identified conformer 13.13 In our investigated spectral region, all the computationally predicted intensive bands and almost all the medium intensive bands of this conformer are very close to the bands of conformer 3 (see Supporting Information), so the presence of conformer 13 in the matrix cannot be experimentally proved or excluded. Unlike in the case of glycine and alanine21,22,26,27 it was not possible to identify short-lived conformers of cysteine in Ar and Kr matrices. When using a N2 matrix, as a result of the irradiation at 6925 cm−1 (i.e., at the O−H stretching overtone of conformers with backbone I) some bands appeared, which were not present in the spectrum recorded before the irradiation, and that decreased after switching off the laser (Figure 5). On the basis of the experiments carried out on glycine and alanine,21,22,26,27 these bands can be assigned to a short-lived conformer most probably with a backbone conformation VI. According to our SQM calculations, the most intensive bands of all conformers with a backbone configuration VI are close to these experimentally observed bands (see Supporting Information). Because the growth of these bands correlates with the decrease of the bands of conformer 3, we can conclude that the short-lived conformer was formed from that conformer. Supposing that no conformational change occurs along the C−C−C−S coordinate, we can identify the observed short-lived conformer as conformer 14 (Figure 1) as the one with the lowest energy (see the computed ΔG°0K of these conformers in the Supporting Information), among the conformers differing only in the C−C−N−H or in the C−C−S−H torsion angle. The excellent correspondence between the computed and the experimental wavenumbers of conformer 14 can be seen in Table 7. It was shown for glycine and alanine that the conformer with backbone VI converts to a conformer with backbone I by Hatom tunneling. The half-life of this conformer of glycine at 12 K was determined to be 4.4 ± 1 s in Ar matrix,22 while for alanine 5.7 ± 1 s, 2.8 ± 1 s half-lives were measured in two different sites of solid Ar.26 The half-lives are 4 orders of magnitude larger (6.69 × 103 s and 1.38 × 104 s in two different sites for glycine, and 2.8 × 103 s for alanine) in N2 matrix.22,26,27 The decay of conformer 14 (VI) of cysteine in the dark 12 K


−1

3410.6 3332.2 3297.0 3022.8 2953.5 2915.8 2588.7 1783.6 1629.5 1436.2 1378.9 1346.0 1314.2 1274.1 1252.1 1184.8 1153.0 1075.7 1005.2 934.2 881.8 855.9 839.3 792.4 762.0 660.5 647.6 542.7 403.8 346.0 322.6 267.0 248.1 169.5 108.5 75.2

Ar matrix −1

I/(km·mol ) 24 7 246 1 7 16 7 345 41 7 412 36 8 8 10 10 15 16 56 82 44 74 18 2 16 11 7 2 5 12 31 12 5 11 7 3

ν̃/cm

−1

Kr matrix ν̃/cm−1 3402w, 3388wc

3303br

3275br

1789.9vs 1784.7mc

2595.0w 1783.0vs, 1785.0sc

1382.0vs 1362.0vs 1360.3vs

1376.2s 1357.6m 1306.1w

1255.6w

1254.0w 1180.0w

1003.0m 931.0w 880mb 852sb 840.1m 798wb 753.7s 667.4m

950.7w 849.5m 823.0m

a See footnote to Table 2. bUncertain wavenumber and intensity due to nearby bands of conformer 1. cSite split band.

modes that could lead to such conformer conversion. A more likely explanation, however, is that the conversion along the C− C−C−S torsion angle requires a larger volume than available in the stiff noble gas matrix cages. As seen in Figure 4, all the strong and medium intensity features of the difference spectra could be assigned to the above five conformers except for two bands centered at 3573 and 700 cm−1. The identification of these two bands remains uncertain; at this stage, only some possible carriers of these bands can be mentioned. Because the band at 700 cm−1 has a negative sign in the difference spectrum obtained by irradiation at 6588 cm−1 and it appears as positive bands in the other two difference spectra of Figure 4, it very likely belongs either to a conformer with a strong intramolecular H-bond, or to a H-bonded complex, that is, dimer or complex of cysteine with the water or ethanol contaminant. The latter possibility is suggested by the fact that upon the 6588 cm−1 irradiation the 3573 cm−1 band is 2099



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−1

I/(km·mol )

3612.1 3419.5 3334.3 2994.1 2942.9 2923.7 2588.7 1784.3 1633.4 1443.0 1396.4 1304.0 1285.0 1266.6 1224.6 1134.0 1118.4 1093.5 990.4 918.1 895.9 839.2 791.5 751.3 670.1 659.5 508.6 473.1 378.0 327.7 302.6 235.7 223.5 170.4 94.9 52.4 a

N2 matrix −1

42 11 7 7 14 12 5 263 39 6 2 38 87 225 87 13 7 25 43 7 75 68 3 7 3 17 3 93 25 28 27 14 11 8 6 8

ν̃/cm−1 3587.1s

1794.0vs, 1785.7ma

Figure 6. Decay of conformer 14 in the dark N2 matrix at 12.5 K. Single exponential fit is shown by red, dashed line, dispersive kinetic fit is plotted by solid, blue line.

1306.5m 1289m 1276.9m 1246.5m, 1235.5ma

uncertainty and the difference between the half-lives of the two decay kinetics, we estimate the error bar to be 5 × 102 s.) These values are 3 to 14 times smaller than the corresponding values measured for glycine and alanine. The shorter lifetime and the lower conversion efficiency compared to glycine and alanine explains the lack of observation of conformer 14 (VI) in Ar and Kr matrices.

4. CONCLUSIONS In the present paper, the conformers of α-cysteine were investigated by matrix isolation IR spectroscopy combined with selective NIR irradiation. Five of the six conformers previously observed by MW spectroscopy were also observed by our method. In addition, a formerly unobserved short-lived conformer produced by the irradiation could be identified. This shows that MI-IR spectroscopy combined with NIR laser irradiation is similarly informative as MW spectroscopy. It is also important to emphasize that conformers differing only in the C−C−N−H or in the C−C−S−H torsions can convert into each other on timescales faster than a second. Out of these conformers only the lowest energy one can be observed by (nonultrafast) conventional spectroscopic methods in low-temperature matrices. The most important conclusion of the present study is that even when selective NIR irradiation is not possible due to the overlapping bands of different conformers, the careful analysis of the irradiation experiments carried out at various wavelengths makes the unambiguous conformational assignment possible. It was found that the NIR irradiation in the rigid Ar, Kr, and N2 matrices could induce only conformational changes having relatively small spatial requirements, that is, changes of the C−C−CO and in the C−C−O−H coordinates. No detectable conformational change occurred along the C−C− C−S coordinate. Although in the irradiation experiments described only backbone conformation changes were observed, this method aided by reliable quantum chemical computations of IR spectra allows us to distinguish even between conformers differing only in side chain conformation for cysteine and for molecular systems with similar size. The method could be further improved by replacing the above matrices by quantum hosts, for example, by para-H2 matrix or by He nanodroplets. In these hosts, the energy dissipation, which is necessary for the

Site split band.

N2 matrix is shown in Figure 6. A simple single exponential decay (eqs 1 and 2) and a dispersive kinetic65,68,86 decay (eqs 3 and 4) curve was also fitted to the experimental data. A(t ) = A 0e−kt t1/2 =

ln 2 k

A(t ) = A 0e−Bt t1/2 =

(1)

α

(2) α

(3)

α ln 2 B

(4) −1

Here A is the absorbance of the 3587.1 cm band of conformer 14, and k, B, and α are the fitted parameters. The latter measures the inhomogeneity of the medium. The two fits resulted in almost the same half-lives, 1.5 × 103 s and 1.1 × 103 s, respectively. (On the basis of the experimental 2100



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conformational conversion, can be similarly effective but conformational motions are not hindered by the rigid lattice.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometries and SQM-scaled vibrational wavenumbers for 16 conformers at the B3LYP/6-31++G** level of theory, further MI-IR spectra, and alternative assignments of the bands showing intensity decrease in the 6953 cm−1 and the 6938 cm−1 irradiation experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +36-1-372-2500/6587. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by the Hungarian Scientific Research Fund (OTKA K75877) and by the European Union (KMOP4.2.1/B-10-2011-0002).

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REFERENCES

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Exploring the Conformational Space of Cysteine by Matrix Isolation

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