24G17
Nitrogen Radicals in Aminoacid Processing in Different Plasma Circumstances
種々のプラズマ条件下のアミノ酸プロセスにおける窒素ラジカルの役割
Iuliana Motrescu, Akihisa Ogino, Masaaki Nagatsu
モトレスク イウリアナ, 荻野 明久，永津 雅章
Graduate School of Science and Technology, Shizuoka University
3-5-1, Johoku Nakaku, Hamamatsu, Japan
静岡大学 創造科学技術大学院
静岡県浜松市中区城北3-5-1,〒432-8011
This study aims to reveal the role of nitrogen radicals in aminoacid processing. Low pressure nitrogenplasma was produced in a parallelepipedic stainless steel chamber using 2 kW microwave power at
different operating pressures. A grounded grid interposed between plasma and samples blocked thecharged radicals to reach the samples to investigate the effect of neutrals. Neutral molecular species seemto enhance the physical sputtering while N2+ ions increase the nitrogen content of aminoacids. The
processing proves to be dependent on the structure of the biomolecule.

1. Introduction
Recently, the field of plasma medicine applications
increased considerable. Despite these great outcomes ofplasma medical applications (such as facial rejuvenationand fast wound healing after nitrogen plasma skintreatment [1]), there is a lack of information on theinsights of how and why it is possible to obtain such
effects, and, of course, improve the methods for betterresults. It is also important to develop novel technologies
for the inactivation of biohazardous molecules (e.g.
prion). Thus the necessity of revealing the interactions
between plasma and such molecules arises, proteins
being an important component of human body.
Reactive plasmas bring interesting features to plasma
bio-processing because the abundance of radicals but UV
radiation as well, easily favoring chemical reactions withthe samples. This study aims to reveal the role ofnitrogen radicals in aminoacid processing.

ignited in the same conditions except the gas pressure.
This was varied from 1 to 7 Pa in order to study theeffects of different species with different concentrations
on aminoacid modification. Fig.2 (a) and (b) show theresults of plasma species charged and uncharged,respectively. The same trend was found in the maximumof the emitted lines for these plasmas [2]. When plasmais blocked by the grounded grid, the main species
reaching the aminoacid samples are plasma neutrals andsome VUV/UV radiation according to the opticaltransparency of the grid.

2. Experiment
Low pressure nitrogen plasma was produced in aparallelepipedic stainless steel chamber using 2 kW
microwave power at different operating pressures from 1
to 7 Pa. A grounded grid interposed between plasma andsamples blocked the charged radicals to reach thesamples to investigate the effect of neutrals. A schematicof the setup used in these experiments is presented inFig.1. The relative concentrations of ions and neutrals
were evaluated from emission measurements and ion
detection by quadrupole mass spectrometer.
In the conditions presented above, several aminoacid
samples were processed by plasma with and without
grounded grid, respectively, and then the results werecompared and discussed in relation with plasma
parameters.

3. Results and Discussions
Several conditions were studied, the discharge being

Fig.1. Schematic of the experimental setup
In all the processing conditions the temperature near
the sample place didn’t exceed 75C for direct plasma
treatment, and 50C for plasma neutral irradiation,
therefore we can consider that the effects were notcaused by heating but by plasma species.
Both plasma and only neutral exposure caused theincrease of the nitrogen content of the samples, though
the increase was higher in the case of direct plasma
treatment, as can be deduced form XPS measurements inFig.3 where the values for Lysine (C6H14N2O2, molecularmass 146.19 g/mol) are compared for plasma and neutral
exposure for 1 and 3 Pa discharge pressure, respectively.

The processing was monitored by mass spectrometry.
The main conclusion is that there is not a noticeable
difference between nitrogen plasma processing andnitrogen plasma neutrals processing regarding thevolatile by-products. Considering this and the fact thatthe common species for the two situations are the plasma
neutrals and the radiation, one can assume that neutrals
would be responsible for bond cleavage, with thecontribution of the radiation as well.
For higher pressure the process of bond breaking is
considerable reduced, probably due to the decreasing of
the concentrations of all plasma species but also to theVUV/UV radiation as well, for higher pressure some low
wavelength lines being absent and the whole spectrum
having a lower intensity.

Fig.2. Plasma species (a) ionic, and (b) neutrals,respectively, vs. discharge pressure.
In the same time, high performance liquid
chromatography (HPLC) results indicated the presenceof aminoacid fragments after processing. The
chromatogram for the untreated aminoacid has one peakcorresponding to a retention time of 35 minutes, all thechromatograms for the processed Lysine present somepeaks at lower retention periods. This means that there isa competition between bond cleavage and the formationof new bonds with nitrogen (as discussed based on XPSresults). The cleavage seems to be preferential since only
specific aminoacids are found in the processed samples,the same fragments in all treatment conditions.

Fig.3. Relative atomic composition of Lysine samples(from XPS measurements)
For a discharge pressure of 1 Pa the nitrogen addition
proved to be stronger than for example for 3 Pa. The
differences between the two situations can be analyzed
from Fig.2: while neutral concentrations seem to besimilar, there is a huge difference regarding the ionicspecies, which have higher concentrations for 1 Pa. Thus
one can affirm that the ionic species are responsible
mostly for nitrogen addition.

Fig.4. XPS spectra of (a) untreated Serine, and (b)exposed to nitrogen plasma produced at 3 Pa.
The processing under nitrogen plasma depends on theexposed biomolecule. In the case of another aminoacid,smaller and simpler, Serine (C3H7NO3, 105.096 g/mol)
no fragments were evidenced in the HPLC
chromatograms. The same increase of nitrogen content isfound as in the case of Lysine, this being a common
nominator for all the studied aminoacids exposed innitrogen plasma circumstances. The increased is shown
to be the result of nitro groups formation and also CN
as a result of aminoacid interaction with plasma species(Fig.4)More results will be presented in the conference.
References[1] J. D. Holcomb, K. J. Kent, D. E. Rousso, Arch.
Facial Plast. Surg. 11 (2009), pp. 184.
[2] I.Motrescu, A.Ogino, S.Tanaka, T.Fujiwara,S.Kodani, H.Kawagishi, G.Popa, M.Nagatsu, Jap. J.
Appl. Phys. 50 (2011) 08JF07.