11 March 1954

Dr. Kenneth Pitzer

University of California

Berkeley 4, California

Dear Kenneth:

Some time ago I noticed that a letter from you had arrived during my absence from Pasadena. I have been uncertain until recently as to whether or not I could make a trip to Berkeley in the near future, as you have suggested. Now, however, I have had to decide that it will not be possible for me to take time for a visit to Berkeley this spring - or at any rate before May. I expect, moreover, that I shall be so busy during May as to make it unwise for me to plan on the Berkeley trip.

There is a question that I should like to ask you. There seems to be some chance that one of our juniors will apply for a graduate assistantship and for admission as a graduate student, without having received his bachelor's degree. I judge that he would then begin his graduate work with training at least equivalent to that of the average graduate student that we have in chemistry. One question that he has asked is what the attitude of other universities would be, in case that he decided to apply for admission as a graduate student elsewhere, after one year as a graduate student here. What do you think the University of California would decide?

You may remember that Alex Kosiakoff was admitted as a graduate student at Johns Hopkins at the end of his junior year here. He was, in fact, given a bachelor's degree by the Institute after he had been a graduate student at Hopkins for one year.

Sincerely yours,

Linus Pauling:W

To: Professor pauling

From: R. A. Pasternak

Subject: The Average Number of Amino Acid Residues Contained in the Unit Cell of Collagen

I have collected experimental data for samples of untreated kangaroo tail tendon. They are:

(1) unit cell dimensions

(2) density

(3) chemical composition

All the experiments were carried out with a batch of tendon equilibrated with a saturated solution of K₂CO₃ (relative humidity about 45%).

(1) An X-ray photograph has been taken in the 10 cm. camera of an unstretched sample of the material. Humidity equilibrium was maintained throughout the exposure. The photograph was measured with a photometer.

Results:

(a) Meridional spacing: d = 2.88 Å.

(b) Equatorial spacing:

[Data Equations]

Since the 1. Order reflection is much sharper, double weight was given to it for the average.

The volume of the unit cell, assuming that the equatorial reflection is the (1100) of a hexagonal unit cell, is V = 2.88 x 11.54² x cos 30º = 443.3 A³0

(2) The density of the material was determined by flotation. The density of the liquid (bromo- and chlorobenzene) was adjusted approximately with some samples, and the final adjustment was made with new samples; thus a possible change in hydration was reduced to a minimum. In a second check, samples were evacuated for 15 minutes and the density liquid was added before breaking the vacuum in order to get rid of the possible air bubbles. No change in the density of the samples, however, was observed.

The density was found to be 1.32 gr. cm.-³. This is a lower limit, because air may be included in the samples.

From (1) and (2) one obtains as minimum weight of material in unit cells

M = V x d x N = 443.3 x 10-24 x 1.32 x 6.023 x 1023 = 352 gr.

(3) A part of the equilibrated material was sent to Dr. Elek for a microanalysis. He carried out duplicate determinations of C, N, H, ash and water. The values reported by him, and listed in column 1 of Table I, are referred to the equilibrium weight of the sample. Elek reports the ash as sulfate. The composition of the ash was not determined and for the calculations I assume that the amount of inorganic impurity in the sample is one half of the ash. The same amount of water is, within the accuracy of the analysis, lost by the sample at the three drying temperatures, 105˚, 125˚, and 150˚. This is undoubtedly the total amount of water bound in my collagen sample. This conclusion agrees with the observation of other investigators; samples used for the determination of the amino acid composition were in general dried at 105° and the amount of amino acid residues found added up to nearly 100% of the material. In column 2 Elek’s results are recalculated for the dry, ash-free basis, i. e., 83.4 gr. = 100%.

In column 3 the values for C, N, H, calculated from the amino acid composition as reported in the literature are given, in parentheses the observed nitrogen value cited by Tristram is added; it is appreciably higher than the calculated one because the latter does not include amide nitrogen. I took the data of Tristram (Adv. Prot. Chem. 15, 143 (1949)) which are based mainly on Bowes’ results because his collagen samples are probably least denatured of all samples investigated.

Our value for nitrogen compares well with the observed nitrogen of Tristram; our carbon value, however, is significantly higher than the calculated one. This high value may be caused by a non-collagenous component in our sample. One cannot, however, exclude the possibility that the calculated value does not represent the real composition; oddly enough, I was not able to find in the literature an experimental determination of the carbon content of collagen.

Average number of amino acid residues in the unit cell. I assumed in this calculation that all the organic material of the sample is collagen of an average residue weight of 93.3 as given by Tristram.

In our sample, 16.3% water + cations are associated with 83.7% protein; 93.3 gr. protein would hold 93.3 x 16.6 = 18.6 gr. H2O, 83.4

i. e., the apparent molecular weight = 93.3 + 18.6 == 111.9 gr. mol. Total weight of the unit cell = 352

[Data Equation]

The uncertainties in this value are

(1) average residue weight

(2) observed density may be low; this would increase n

(3) the presence of non-collagenous material would decrease n

I plan a series of experiments with gelatin designed to minimize the sources of error (2) and (3).

[Data Table]