Will the protein's phosphorylation state could be affected if the samples are sitting into the 4X SDS-PAGE Sample Loading Buffer for more than a year?
Sitting at what temperature? 4, possibly, -20, probably not. In theory if your samples were denatured, samples should be pretty stable since phosphatases should be inactive. In practice however all chemical bonds have a finite lifetime, some more labile than others; tread with care.
The concern will be the exchange of phosphate with sulphate in the sodium dodecyl sulphate to form sodium dodecyl phosphate. Un-catalysed this is a very slow exchange reaction. But given enough time it will occur until equilibrium
is reached. As previously stated temperature is an important factor. So you may have lost some phosphorylation of your proteins in high SDS buffer. The degree of loss in an uncatylsed reaction is dependant on time and free energy (Tempearture).
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