April 9, 1955
Dear Rosalind
Thanks very much for your latest manuscript. For the past several weeks I have been on the East Coast and so out of touch with TMV. Don Caspar had told me that he would write you, hence the reason I didn't. However upon my return, I find that he hasn't and so I'll communicate his latest returns.
His sign combination is [diagram], starting at the origin [diagram]. This I believe is quite different from your choice. It gives a sharp peak at 23 angstroms (which we believe are the phosphates) and another peak around 45 angstroms. It terminates at 85 angstroms and is almost zero from 0 angstroms to 15 angstroms which we believe is a hole filled with water. He furthermore has a Difference Fourier (using the same sign combination from his lead [symbol for a positive charge of two] substituted TMV. He finds that 2 lead [symbol for a positive charge of two] atoms per sub unit (30,000) combine (more [ . . . ] its TMV is precipate) specifically with the TMV, one at a radius of 25 angstroms, the other at 82 angstroms. The peaks are very sharp, which he believes is a good indication that the sign assignment is correct.
For the RNA core we favor a 10-12 stranded model in which the RNA chains follow the same helical grid as the protein - This seems necessary from the perfection of the TMV diagram [diagram]. We like this model since the Cohen and Stanley RNA appears to have a molecular weight approximately 1/10 that of the [mathematical equation see Lister Hill for tech solution] core. Under this picture the TMV length of 300 angstroms would be determined by the
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length of the RNA. The problem then becomes to assign a structure to the RNA strands. They must have a slow twist and would take up essentially a ribbon configuration. To fit these requirement [sic] I have constructed a double stranded ribbon 12 in which the two strands are linked together by pyrophosphate bonds between the phosphates. However it is all very speculative. Fortunately a chemical test is possible and this we are doing. The main thing in favor of the P-O-P model is that it is very pretty sterochemically. But does nature always like to be pretty?
I have made an application to the National Science Foundation for a grant to allow me to be in England next year. If it comes through I shall be back in Cambridge by the first of July. Just recently I have received an offer from Harvard (Asst. Professor in Biology) and its [sic] most likely that I'll accept. Fortunately they are being very nice and allow me leave of absence for the initial year. Hence in any eventuality I'll be leaving Caltech in the middle of June.
Hope to see you soon. I'll try to get Don to write you more details
Jim