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Resolved pseudo-Raman band of PGA
Volume 45A, number 3
PHYSICS LEUERS
24 September 1973
RESOLVED PSEUDO-RAMAN BAND OF PGA J.P. BISCAR and N. KOLLIAS Department of Physics, the University of Wyoming, Laramie, Wyoming 82071, USA Received 8 August 1973 For the first time, the partially resolved pseudo-Raman band of Poly-L-Glutamic acid is reported.
We report here how the broad pseudo-Raman band of Poly-L-Glutamic Acid (PGA) has been resolved in a series of wide pseudo-Raman peaks. It has been previously shown [1] that the broad band displayed in the Raman spectra of dry PGA powder (MW 93 000), which is in excess of an order of magnitude larger than the the normal Raman lines, has itself a pseudo-Raman character. This also remains true for the presently investigated dry PGA of 103 000 molecular weight. It is not however the sum of individual Raman lines, and for that reason the present resolved spectrum is of great interest because it could lead to an explanation of this remarkable phenomenon. The optical technique used in the present experiments is the same as the one described in ref. [1L The present resolution of the pseudo-Raman broad band of dry PGA of molecular weight 103 000, is achieved not by increasing the resolving power of the instrumentation (which is easy) but by a more diffIcult task; “individualization” as much as possible of PGA molecules. Here we describe one of the techniques used for that end, namely, freeze drying. A small amount of PGA sample, of high purity, is first disolved in triply distilled water. An ultrasonic treatment of the solution dissociates the molecules from each other. A few drops of this low concentration solution are placed on the sample, holder on top of a thermoelectric cooler. The mixture is frozen by lowering the temperature down to —10°C. The temperature is measured with a thermolinear element followed by a bridge and a Digitéc 264 digital multimeter. The water is then sublimated, under clean vacuum, for several hours. The result is a powdery three dimensional white deposit of extremely low density. If one breaks the vacuum at this stage its structure is so fragile that the sample collapses. One has to pay extreme attention to the sample preparation as it is the main
6
-
5
-
i
~“~•i
i
PGA
o~o,
/
‘~
Laser 5145 3
I
A
0
~ 2
0 <.00
I
~ I
2
3
4x1000
WAVENLMBER (Cm-’) Fig. 1. Partially resolved pseudo-Raman band of dry Poly-LGlutamic acid (MW 103 000). The “wide peaks” at the bottom, after subtraction unresolved (dashed constitute a seriesofofthe harmonics of background the fundamental at line), 1. 860 cm
“resolving factor”. Molecules still bond to each other, some in long filaments, but enough of them hand in between quasi-isolated and they are the ones producing the resolved part of the spectrum. Until the technique is mastered one has to start the sample preparation over several times by varying the concentration, the cooling temperature, and sublimation rate until good spectra are obtained. The results are good when “wide peaks” are obtained, as shown in fig. 1, on top of the background. They grow in intensity with the number of individualized, or quasi-isolated, molecules. When those wide peaks appear, their position does not however depend on sample preparation technique, but is directly related 189
Volume 45A, number 3
PHYSICS LETTERS
to the chain length (thus to the molecular weight) of the polymer used. Interlinked molecules produce an underlying unresolved broad band, visualized by the dashed line in fig. 1. It is a characteristic of the pseudoRaman broad bands that their peak shifts down to 1000 cm~or below when long chains are connected in longer filaments. The position in Raman scale of the resolved wide peaks does not depend either on the shape or intensity of that underlying background. At the bottom of fig. I are shown five of the wide peaks, after subtraction of the unresolved background. Their center position is shown by the markers at the top of fig. 1. An immediate observation is that they constitute a series of harmonics. The fundamental peaks at about 860 cm~followed by four harmonics at 1 720, 2580, 2440 and 4300 dni’. The fact that the resolved wide peaks constitute a series of harmonics of the fundamental is the most remarkable feature of the pseudo.Raman bands of long chain polymers. This is the main thrust of this paper and has to be emphasized. By changing the molecular chain length the position of the fundamental is changed [2] but remains followed by the series of its harmonics. For a given chain length, secondary and tertiary structure also shift the fundamental. For a given chain length and conformation, water layers bonding to the chain molecules shift its frequency down in a precise way [3]. For that reason the resolved spectrum of PGA (103 000 MW) is here presented for a dry sample, with the appropriate technique to obtain it.
190
24 September 1973
The same wide resolved peaks remainted at the same crn~positions when analysed with different exciting lines, e.g. 4880 A and 4579 A. The data of fig. I are obtained with the 5145 A exciting line. Their pseudo-Raman behavior is established by this identical position in Raman scale at three different exciting lines. It is reasonable to think that the unresolved background (dashed line) is the sum of a great number of similar series of wide peaks. but produced by larger interlinked chains or filaments. They remain unresolvable since not many filaments have the same length. This is probable since a careful look at the wide peaks at the bottom of the fig. I shows that each one of them has a small dip at the middle. Such a 430 cn-i I series is the spectrum of two PGA molecules (2 X 103 000) linked end to end. For these reasons the subtraction of the unresolved background is more justifiable than trying to curve resolve the whole spectrum. The series of wide peaks extend in fact byond 5000 cm~with decreasing amplitude, explaining why the broad band when unresolved does the same. The next paper shows the polymer chain length dependence of the resolved pseudo-Raman wide peaks.
References [1] J.P. Biscar and N. Kollias, Phys. Lett. 44A (1973) 373.
121 J.P. Biscar and N. Kollias, [31 J.P. Biscar and N. Kollias,
Phys. Lett. 45A (1973) p. 191. to be published.
PHYSICS LEUERS
24 September 1973
RESOLVED PSEUDO-RAMAN BAND OF PGA J.P. BISCAR and N. KOLLIAS Department of Physics, the University of Wyoming, Laramie, Wyoming 82071, USA Received 8 August 1973 For the first time, the partially resolved pseudo-Raman band of Poly-L-Glutamic acid is reported.
We report here how the broad pseudo-Raman band of Poly-L-Glutamic Acid (PGA) has been resolved in a series of wide pseudo-Raman peaks. It has been previously shown [1] that the broad band displayed in the Raman spectra of dry PGA powder (MW 93 000), which is in excess of an order of magnitude larger than the the normal Raman lines, has itself a pseudo-Raman character. This also remains true for the presently investigated dry PGA of 103 000 molecular weight. It is not however the sum of individual Raman lines, and for that reason the present resolved spectrum is of great interest because it could lead to an explanation of this remarkable phenomenon. The optical technique used in the present experiments is the same as the one described in ref. [1L The present resolution of the pseudo-Raman broad band of dry PGA of molecular weight 103 000, is achieved not by increasing the resolving power of the instrumentation (which is easy) but by a more diffIcult task; “individualization” as much as possible of PGA molecules. Here we describe one of the techniques used for that end, namely, freeze drying. A small amount of PGA sample, of high purity, is first disolved in triply distilled water. An ultrasonic treatment of the solution dissociates the molecules from each other. A few drops of this low concentration solution are placed on the sample, holder on top of a thermoelectric cooler. The mixture is frozen by lowering the temperature down to —10°C. The temperature is measured with a thermolinear element followed by a bridge and a Digitéc 264 digital multimeter. The water is then sublimated, under clean vacuum, for several hours. The result is a powdery three dimensional white deposit of extremely low density. If one breaks the vacuum at this stage its structure is so fragile that the sample collapses. One has to pay extreme attention to the sample preparation as it is the main
6
-
5
-
i
~“~•i
i
PGA
o~o,
/
‘~
Laser 5145 3
I
A
0
~ 2
0 <.00
I
~ I
2
3
4x1000
WAVENLMBER (Cm-’) Fig. 1. Partially resolved pseudo-Raman band of dry Poly-LGlutamic acid (MW 103 000). The “wide peaks” at the bottom, after subtraction unresolved (dashed constitute a seriesofofthe harmonics of background the fundamental at line), 1. 860 cm
“resolving factor”. Molecules still bond to each other, some in long filaments, but enough of them hand in between quasi-isolated and they are the ones producing the resolved part of the spectrum. Until the technique is mastered one has to start the sample preparation over several times by varying the concentration, the cooling temperature, and sublimation rate until good spectra are obtained. The results are good when “wide peaks” are obtained, as shown in fig. 1, on top of the background. They grow in intensity with the number of individualized, or quasi-isolated, molecules. When those wide peaks appear, their position does not however depend on sample preparation technique, but is directly related 189
Volume 45A, number 3
PHYSICS LETTERS
to the chain length (thus to the molecular weight) of the polymer used. Interlinked molecules produce an underlying unresolved broad band, visualized by the dashed line in fig. 1. It is a characteristic of the pseudoRaman broad bands that their peak shifts down to 1000 cm~or below when long chains are connected in longer filaments. The position in Raman scale of the resolved wide peaks does not depend either on the shape or intensity of that underlying background. At the bottom of fig. I are shown five of the wide peaks, after subtraction of the unresolved background. Their center position is shown by the markers at the top of fig. 1. An immediate observation is that they constitute a series of harmonics. The fundamental peaks at about 860 cm~followed by four harmonics at 1 720, 2580, 2440 and 4300 dni’. The fact that the resolved wide peaks constitute a series of harmonics of the fundamental is the most remarkable feature of the pseudo.Raman bands of long chain polymers. This is the main thrust of this paper and has to be emphasized. By changing the molecular chain length the position of the fundamental is changed [2] but remains followed by the series of its harmonics. For a given chain length, secondary and tertiary structure also shift the fundamental. For a given chain length and conformation, water layers bonding to the chain molecules shift its frequency down in a precise way [3]. For that reason the resolved spectrum of PGA (103 000 MW) is here presented for a dry sample, with the appropriate technique to obtain it.
190
24 September 1973
The same wide resolved peaks remainted at the same crn~positions when analysed with different exciting lines, e.g. 4880 A and 4579 A. The data of fig. I are obtained with the 5145 A exciting line. Their pseudo-Raman behavior is established by this identical position in Raman scale at three different exciting lines. It is reasonable to think that the unresolved background (dashed line) is the sum of a great number of similar series of wide peaks. but produced by larger interlinked chains or filaments. They remain unresolvable since not many filaments have the same length. This is probable since a careful look at the wide peaks at the bottom of the fig. I shows that each one of them has a small dip at the middle. Such a 430 cn-i I series is the spectrum of two PGA molecules (2 X 103 000) linked end to end. For these reasons the subtraction of the unresolved background is more justifiable than trying to curve resolve the whole spectrum. The series of wide peaks extend in fact byond 5000 cm~with decreasing amplitude, explaining why the broad band when unresolved does the same. The next paper shows the polymer chain length dependence of the resolved pseudo-Raman wide peaks.
References [1] J.P. Biscar and N. Kollias, Phys. Lett. 44A (1973) 373.
121 J.P. Biscar and N. Kollias, [31 J.P. Biscar and N. Kollias,
Phys. Lett. 45A (1973) p. 191. to be published.