Molecular Weight and Dimensions of Phycocyanin Monomer and Aggregates
I n recent piibiications1.2 in this journal, the size and shape of C-phycocyanin monomer and higher aggregates are considered in great detail. The authors suggest molecular weight values and shape fact.ors using various models in their calculations. Both papers use extensively data from our laboratory. The impression is given that a serious discrepancy exists in monomer molecular weights reported by 1 ~3 . 4 (28,000-30,000) and those reported by Hattori et Jennings,' and Bloomfield et aL2 (14,000 and 45,000). This apparent confusion also permeates the determination of the molecular weight and shape factors of higher aggregates.
Resolution of the apparent discrepancy can be accomplished best, by clarification of certain facts. The impression is given in t.he work of both Jennings' and Bloomfield et a1.2 that we suggested monomer molecular weights of both 14,000 and 30,000. I n no publication have we ever cited a monomer molecular weight ot,her than 28,0O(t30,0PO. Hattori et al.5 are solely responsible for siiggesting a minimal molecular weight of 14,000.
The calculation of all molecular weights by Jennings,] Bloomfield et a1.,2 and Hattori et aL6 is dependent upon the use of diffusion coefficients reported from this laboratory.
Our original assignment of the diffusion coefficients to each species was clearly substantiated.3s4r6 These assignments are considered incompatible by the authors ~i t e d . ~, ~, ~ However, no experiment,al data are given to support t,heir new assignments. Thus, the only self-consistent set of experimental data that can be used to determine t,he monomer and higher aggregate molecular weights is that reported by Scott and Berns.4 The difficulties in employing the available information for such calculations are enumerated in considerable detail.4 I t is t,heoretically iinsoinid to employ the Svedberg equat,ion for determination of molecular weights in a polydisperse system, except perhaps at the top of the centrifuge cell where after a time only one component is present. I n all other parts of the cell, sedimentation coefficients are affected by complex chemical and physical interact.ions of polymeric components.' The only reliable estimate is thought to be that for the molecular weight of the monomer, namely 30,000.