Journal of Electron Spectroscopy and Related Phenomena, 8 (1976) 161-164 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Short communication Application of photoelectron constituent parts III. Amino acids
L. KLASINC Znstitute “Ruder BoSkoviC” ( Yugoslavia)
and Faculty of Sciences and Mathemafics,
of Zagreb, Zagreb
4 June 1975)
Within a project dealing with properties of biologically important molecules we tried to determine gas phase ionization energies of amino acids by means of He1 photoelectron (PE) spectroscopy. The knowledge of these data would be of appreciable importance for the estimation of solvation and conformation energies in comparison with the results obtained by other methods in solutions. A success of this effort, at least concerning the photoelectron spectra measurements, seemed to be guaranteed by the recently published results for glycine and alanine2. Unfortunately, this was not always the case. Of the measured compounds, glycine (I), l-alanine (2), I-cc-amino-n-butyric acid (3), I-norvaline (4), I-valine (5), 1-norleucine (6), I-leucine (7), I-isoleucine (8), I-serine (9), 1-asparagine (lo), I-phenylalanine (1 l), I-histidine (12) and I-tryptophan (13), only (l)-(8), i.e., the pure aliphatic compounds without additional OH, NH, or SH groups yielded PE spectra. The others decomposed in the inlet system. Since (l)-(3) were the most promising compounds further measurements of pure amino acids have been abandoned. Now we intend to measure the PE spectra of amino acid methyl esters and we hope to be able to get spectra also for such esters whose corresponding pure amino acid decomposed. By finding out the relationship between the PE spectra of the pure and methyl ester form where spectra of both are accessible, we wish to deduct ionization energies of those amino acids which cannot be measured in the pure form. The PE spectra of (2)-(8) and the observed ionization energies (in eV, vertical energies are quoted above and the adiabatic below the spectrum) are given in Figs. 1 and 2. The values for (1) and (2) are in fairly good agreement with the three lowest energy ionizations quoted in ref. 2. Also in agreement is our assignment of the outer molecular orbital configuration of (1) and (2) as n&a”) < n,(a’) < n&a”) < xc0 <
162 I3 onset.However, this order seems to be changed in amino acids with a longeraliphatic chain. Amino acids (l)-(8) can be represented by the general formuIa
H,N - CH . COOH I
R with R = H, Me, Et, Pr, i-Pr, Bu, i-Bu and s-Bu, respectively. Thus, changes observed in the PE spectra show the effect of the alkyl chain. Such effects in other compounds have been extensively studied recently 3-7 . The correlation of the observed vertical (the lowest adiabatic energy is dotted) ionization energies of (l)-(8) as a “function” of R is represented in Fig. 3. In contrast to the first three systems, &a”), n,(a’) whose energy slightly decreases, nco and G systems exhibit a and n&a”) of (l)-(3) strong shift to lower energies. Consequently, it is to be expected that at least one of these two systems in (4)-(g) lies at lower energy than no(a”) as indicated in Fig. 3. As can be seen, there must be an additional system in the region from 11 eV to 12 eV ionization energy in (4)-(8), which was not observed because of overlapping. We hope that the planed measurements of amino acid methylesters will be also of help to solve this question.
/ 8 88 I
Figure 1. He1 photoelectron spectrum of I-alanine (2) at 200°C. Vertical and adiabatic ionization energies are indicated at the top and bottom, respectively. Doubtful value are given in parenthesis.
cj% CH ICH&
H>N-C-H CH iCH,),
C FOOH H+C-H
Figure 2. He1 photoelectron spectrum of (a) I-a-amino-n-butyric acid (3) at 2OOT. (b) 1-norvaline (4) at 200°C. (c) 1-valine (5) at 200°C. (d) 1-norleucine (6) at 200°C. (e) I-leucine (7) at 220°C. (f) I-isoleucine (8) at 210°C.
164 R HZN -CL - COOH
,_Pri_ “-By_ 5_
El IoV 8
__- _________ __.
n N WI
Figure 3. Correlation of vertical ionization energies of (l)-(8) with the alkyd function. The adiabatic ionization energy of the lowest energy system is indicated by dotts.
EXPERIMENTAL A Vacuum Generators photoelectron spectrometer UVG3 was used for spectra measurements at inlet temperatures of up to 230°C. The characteristics of the instrument have already been published elsewhere8. All the compounds have been of high purity (amino acids kit AS 10 of Serva-Heidelberg) and were used without further purification. The gases N,, Ar and Xe have been used in situ for calibration. In all spectra more or less amounts of CO2 originating from decarboxylated sample were observed, which could also be used for calibration_ The spectra were registrated at 160-200” (l), 200” (2)-(6), 220” (7) and 210” (8). REFERENCES 1. 2. 3. 4. 5. 6.
F. KajfeZ, L. Klasinc, V. SunjiC and T. T&h, J. Heterocycl. Chem., in press. T. P. Debies and J. W. Rabalais, J. Electron Specfrosc., 3 (1974) 315. H. Ogata, H. Onizuka, Y. Nihei and H. Kamada, Bull. Chem. Sot. Jap., 46 (1973) 3036. S. Katsumata, T. Iwai and K. Kimura, Bull. Chem. Sot. Jap., 46 (1973) 3391. S. F. Nelsen and J. M. Buschek, J. Amer. Chem. Sot., 96 (1974) 2392. K. Kimura, S. Katsumata, Y. Achiba, H. Matsumoto and S. Nagakura, Bull. Chem. Sot. Jap., 46 (1973) 373. 7. G. Wagner and H. Bock, Chem. Ber., 107 (1974) 68. 8. L. Klasinc, B. KovaZ and B. RuSEiC, Kern. Ind. (Zagreb) 23 (1974) 569.