For those that don't understand the use of the term "essentially pure spider silk", I will try to explain it as simply as possible:
First off, when a silkworm spins it's silk, there are multiple proteins at play. The silk fibroin itself is made up of at least three proteins(in this exercise we can leave out sericin because it is washed off and doesn't end up in the final silk product). These three proteins are essential for the structure of the silk.
The three proteins are heavy-chain fibroin (H-fib), light-chain fibroin (L-fib), and another small glycoprotein called P25. Now, quantitatively, these three proteins have a molar ratio of 6:6:1, respectively. So for every 6 molecules of H-fib, there are 6 molecules of L-fib, and one molecule of P25.
However, the molecular weight of these proteins differ greatly. In normal silk fibroin, the molecular weight of H-fib is ~390 kDa (kilodaltons, or Daltons, are a unit to measure atomic mass and molecular weight), L-fib is ~25 kDa, and P25 protein is ~30 kDa. This means that in normal silk fibroin, H-fib accounts for ~93%, L-fib is ~6%, and P25 is ~1%. Dr. Kane's job was to knock out the H-fib and replace it with spider silk protein. That is what he said he succeeded in doing during the webcast. It was also said that he was able to insert a protein that was 4 times larger than what had been previously disclosed in scientific literature.
If we were to assume that this new novel protein is 4 times the size of normal silkworm h-fib, then we can extrapolate (from the molar ratios and molecular weights) that the new silk fibroin is ~98% spider silk protein (replacing the H-fib), ~1.5% L-fib, and ~.5% P25. Dr. Kane probably understands that the silkworms need the L-Fib and P25 proteins to produce their silk and create a cocoon so it is useless to try and knock those proteins out, which is why they state it is "essentially pure spider silk". Without knowing the exact molecular weight of their new protein, I would say the new fiber is anywhere from 93-99% pure spider silk after the sericin is washed away (as is typical for silk reeling).
I hope this helps with the mass confusion on this board.
